Synthesis of perovskite-related layered AnBnO3n+2 = ABOX type niobates and titanates and study of their structural, electric and magnetic properties

Progress in Solid State Chemistry 29 (2001) 1–70 www.elsevier.nl/locate/pssc

Synthesis of perovskite-related layered AnBnO3n+2 = ABOX type niobates and titanates and study of their structural, electric and magnetic properties F. Lichtenberg*, A. Herrnberger, K. Wiedenmann, J. Mannhart University of Augsburg, Institute of Physics, Center for Electronic Correlations and Magnetism, Experimentalphysik VI, D-86135 Augsburg, Germany

Abstract ABOX niobates and titanates belonging to the homologous series AnBnO3n+2 are a special group of perovskite-related layered materials. These oxides comprise the highest-Tc ferroelectrics such as CaNbO3.50 and LaTiO3.50, as well as thermodynamically stable bulk compounds involving well-ordered stacking sequences of layers with different thickness such as SrNbO3.45. An extensive overview on many ABOX compositions of the AnBnO3n+2 family and its properties is presented. The crystal structure type is given by n and can be tuned by adjusting the oxygen content X. The charge carrier concentration of the electrical conducting oxides can be varied by appropriate substitutions at the A or B site. To investigate the properties of these systems, more than 150 different compositions were prepared. Most of them were grown by floating zone melting, of which many were fabricated as single crystals with precise control of the oxygen content X. For these crystalline compounds, the synthesis, structural, electric and magnetic features are discussed. Attempts to prepare series members beyond the known structure types n=4, 4.33, 4.5, 5 and 6 were not successful. For some of the known structures types n, however, pronounced non-stoichiometric homogeneity ranges with respect to the oxygen content X and cation ratio A/B were found. Thus, these systems offer many possibilities to vary the compositional, structural, chemical and physical properties. Further, measurements of the resistivity as a function of temperature T are reported for crystals of the n=4 type Sr0.8La0.2NbO3.50, n=4.5 type Sr0.96Ba0.04NbO3.45 and n=5 types Sr1⫺YLaYNbO3.41 (Y=0, 0.035, 0.1), Sr0.95NbO3.37, CaNbO3.41 and LaTiO3.41. These measurements, which were performed in the temperature range 4 KⱕTⱕ290 K and along the a-, b- and c-axis, revealed a highly anisotropic conductivity and intricate behavior. In parts of the temperature range, these materials are quasi-1D metals which display temperature-driven metal-semiconductor transitions at lower temperatures. The niobates and titanates investigated represent a new group

* Corresponding author. E-mail address: [email protected] (F. Lichtenberg). 0079-6786/01/$ - see front matter  2001 Published by Elsevier Science Ltd. PII: S 0 0 7 9 - 6 7 8 6 ( 0 1 ) 0 0 0 0 2 - 4

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of quasi-1D metals which are in compositional, structural and electronical proximity to nonconducting layered (anti)ferroelectrics. Furthermore, measurements of the magnetic susceptibility as a function of temperature are reported for many compounds. As a typical property at elevated temperatures, it was observed that the magnetic susceptibility rises with increasing temperature.  2001 Published by Elsevier Science Ltd. Keywords: Niobates; Titanates; Oxygen content; Crystal growth; Floating zone melting; Layered structures; Mixed valence; Electrical conductors; Quasi-1D metals

1. Introduction and overview Perovskites and materials with perovskite-related structures display a very broad range of structural, chemical and physical properties. The properties of the perovskite-related title compounds of the AnBnO3n+2 type and their crystal structures are not well known, however. To introduce these materials in this work, first a more familiar and structurally somewhat simpler example will be briefly described, the so-called Ruddlesden–Popper phases Am+1BmO3m+1. Fig. 1 sketches the crystal structures of the members of this perovskite-related layered homologous series Am+1BmO3m+1, established by Ruddlesden and Popper in the Sr-Ti-O system [1,2]. The thickness of the layers rises with m, for m=⬁ the three-dimensional perovskite structure ABO3 is realized. The m=1 member is also known as the K2NiF4 crystal structure type. Fig. 1 shows compositional examples from the Sr-Ti-O, (La,Ba)-Cu-O and Sr-Ru-O systems. Due to their physical properties two of them are especially remarkable. The first is the m=1 cuprate (La,Ba)2CuO4, for which Bednorz and Mu¨ ller discovered high-Tc superconductivity with Tc⬇30 K [3]. It represents the parent compound of the high-Tc superconductors. The second noteworthy system, the m=1 ruthenate Sr2RuO4, is remarkable for several reasons. Single crystals fabricated by Lichtenberg et al. revealed a highly metallic conductivity along its layers and it was demonstrated that Sr2RuO4 was the first metallic substrate for the epitaxial growth of high-Tc thin films [4]. On the same crystals Maeno et al. found superconductivity with Tc=0.93 K [5]. Crystals grown with a small concentration of impurities showed a Tc of 1.3 K [6]. Sr2RuO4 is the only Cu-free superconductor known, that has the same crystal structure like (La,Ba)2CuO4. Furthermore, it is argued that Sr2RuO4 is an unconventional p-wave spin-triplet superconductor, see e.g. ref. [7]. Therefore, this compound is a subject of intensive research. Figs. 2–4 display the idealized (i.e. non-distorted) perovskite-related layered crystal structures of the members of the title series AnBnO3n+2. A comparison between the series AnBnO3n+2 and the Ruddlesden–Popper phases Am+1BmO3m+1 is given in Table 1. The Ruddlesden–Popper phases Am+1BmO3m+1 are known for many B cations, whereas the AnBnO3n+2 type oxides exist only for B=Ti, Nb or Ta (see Table 1). More specific compositions will be given later. Both series share a layered structure, in which the layers are n or m octahedra thick. For n=m=⬁ the three-dimensional perovskite structure ABO3 is realized. However, both systems differ in the arrangement of the corner-shared BO6 octahedra within the layers, as well as in the cation

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Fig. 1. Sketch of the crystal structure of the m=1, 2, 3 and ⬁ members of the perovskite-related layered homologous series Am+1BmO3m+1 projected along the a- (or b-) axis. Circles represent the A cations. The layers are formed by corner-shared BO6 octahedra within the ab-plane. The layers are m octahedra thick, thus the thickness of the layers rises with increasing m. For m=⬁ the three-dimensional perovskite structure ABO3 is realized. Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicate a height difference perpendicular to the drawing plane. Compositional examples are taken from the systems Sr-Ti-O, (La,Ba)-Cu-O and Sr-Ru-O.

ratio A/B. With respect to the a- and b-axis, the Am+1BmO3m+1 members are structurally (almost) isotropic, whereas the AnBnO3n+2 type compounds show a pronounced crystallographic anisotropy. In Figs. 2–4 only the idealized, i.e. non-distorted, AnBnO3n+2 type structures are sketched. The BO6 octahedra are usually strongly dis-

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Fig. 2. Sketch of the idealized (i.e. non-distorted) crystal structure of the n=2, 3, 4 and 4.33 members of the perovskite-related layered homologous series AnBnO3n+2 projected along the a-axis. The stoichiometries are also given as ABOX with its corresponding ideal oxygen content X=3+2/n. Circles represent the A cations. Within the layers the corner-shared BO6 octahedra extend zig-zag-like along the b-axis and chain-like along the a-axis (see also Fig. 4). The layers are n octahedra thick, thus the thickness of the layers rises with increasing n. The n=4.333 member represents the well-ordered stacking sequence n=5, 4, 4, 5, 4, 4,... Light and heavy drawing of the BO6 octahedra indicates a height difference perpendicular ˚ , the half of the octahedron body diagonal and B-O bond length. to the drawing plane of about 2 A Filled and open circles indicate A cations also differing in height by this distance perpendicular to the drawing plane.

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Fig. 3. Sketch of the idealized (i.e. non-distorted) crystal structure of the n=4.5, 5, 6 and ⬁ members of the perovskite-related layered homologous series AnBnO3n+2 projected along the a-axis. The stoichiometries are also given as ABOX with its corresponding ideal oxygen content X=3+2/n. Circles represent the A cations. Within the layers the corner-shared BO6 octahedra extend zig-zag-like along the b-axis and chain-like along the a-axis (see also Fig. 4). The layers are n octahedra thick, thus the thickness of the layers rises with increasing n. For n=⬁ the three-dimensional perovskite structure ABO3 is realized. The n=4.5 member represents the well-ordered stacking sequence n=5, 4, 5, 4,... Light and heavy drawing of ˚ , the half the BO6 octahedra indicates a height difference perpendicular to the drawing plane of about 2 A of the octahedron body diagonal and B-O bond length. Filled and open circles indicate A cations also differing in height by this distance perpendicular to the drawing plane.

torted. This structural feature will be discussed later in more detail regarding the electric properties and results from band structure calculations. The lattice constants of the idealized (i.e. non-distorted) AnBnO3n+2 type compounds are derived from a cubic perovskite with unit cell length a0:

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Fig. 4. Sketch of the idealized (i.e. non-distorted) crystal structure of the n=5 member of the perovskiterelated layered homologous series AnBnO3n+2 projected along the a- and b-axis. In contrast to Figs. 2 and 3 the projection along the b-axis clearly shows the chain-like array of the corner-shared BO6 octahedra along the a-axis using the n=5 member as a representative example. The circles represent the A cations. Light and heavy drawing of the BO6 octahedra as well as filled and open circles indicate a height difference perpendicular to the drawing plane.

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Table 1 Comparison between the perovskite-related layered homologous series Am+1BmO3m+1 and AnBnO3n+2 Am+1 Bm O3m+1 (Ruddlesden– Popper phases)

An Bn O3n+2

integer index m or n

number of BO6 octahedra within a layer along the c-axis → layer thickness rises with increasing m or n

cation ratio A/B

⬎1 for m⬍⬁ =1 for m=⬁

=1

crystal structure type for m=n=⬁ three-dimensional perovskite structure ABO3

series members for which bulk compounds are known

m=⬁

n=⬁ n=6 n=5 n=4.5 n=4.333 n=4

m=3 m=2 m=1

(n=2) (n=2 members are known at fluorides but not for oxides)

B cations for which compounds with m, n⬍⬁ are known

Al Ti V Cr Mn Fe Co Ni Cu Ga Zr Mo Ru Rh Sn Ir Pb U

Ti Nb Ta (for n=2 fluorides:Mg Mn Fe Co Ni Zn)

symmetry for m, n⬍⬁

tetragonal or orthorhombic

orthorhombic or monoclinic

a ⫽ a0 b ⫽ a0√2 c ⫽ 2s ⫹ na0√2 ˚ is the distance between neighbouring layers [8]. Compared to these where s⬇2.3 A cell parameters the real materials often display a doubled a-axis and/or a half or doubled c-axis. In the following the compositions of the AnBnO3n+2 type oxides are expressed as ABOX, thus normalized to one B cation, whereby the corresponding ideal oxygen content X of a structure type n is given by X=3+2/n. The existence of phases of the AnBnO3n+2 type was established by Carpy et al. in 1972 in the Na-Ca-Nb-O and Ca-La-Ti-O systems [9]. Later, related and more detailed studies were published by Galy and Carpy in 1974 [8]. In the following years Nanot et al. performed comprehensive crystallographic studies on these and other quaternary niobates and titanates, of which examples are given in references [10–14].

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Before 1991, only fully oxidized AnBnO3n+2=ABOX type titanates and niobates were prepared and studied, i.e. electrical insulators involving exclusively Ti4+ (3d0) and/or Nb5+ (4d0). To the best of our knowledge, the only exception was a crystallographic study on CaNbOX (3.4ⱕXⱕ3.5) by Hervieu et al. in 1977 [15]. This CaNbOX system involves a mixed valence Nb4+/Nb5+ for X⬍3.5 and therefore a high electrical conductivity can be expected. The physical properties were not investigated, however. Among the fully oxidized insulating titanates and niobates there are compounds with remarkable structural and physical properties. An interesting structural phenomenon was already been shown in Figs. 2 and 3, namely the structure types n=4.333 and n=4.5, which involve well-ordered stacking sequences of layers with different thickness. Examples are La0.923Ca0.077TiO3.462 (n=4.333) [10] and CaNb0.889Ti0.111O3.444 (n=4.5) [11]. Some fully oxidized n=4 type compounds such as CaNbO3.50, SrNbO3.50 and LaTiO3.50 display extraordinary dielectric properties. They are ferroelectric up to extremly high temperatures, as shown by Nanamatsu et al. in 1974 and 1975 [16– 18]. Their ferroelectric transition temperatures Tc lie in the range of 1615 K to 1850 K. No other ferroelectric materials with such high Tc are known. Fig. 5 shows the temperature dependence of the dielectric permittivity ⑀ of SrNbO3.50 along the a-, b- and c-axis. Below its ferroelectric transition temperature Tc=1615 K, several dielectric and structural (also incommensurate) phase transitions occur [19]. Since 1991 electrically conducting ABOX titanates and niobates of the AnBnO3n+2 type were synthesized and investigated with respect to their physical and structural properties [20–37], initiated by Lichtenberg et al. in the LaTiOX, SrNbOX and CaNbOX systems [20–22]. In the case of LaTiOX the oxygen content X was systematically varied between the two known end members LaTiO3.0 and LaTiO3.5 [21]. LaTiO3 (Ti3+, 3d1) has a three-dimensional orthorhombically distorted perovskite structure of the n=⬁ type, whereas LaTiO3.5 (Ti4+, 3d0) is a ferroelectric insulator with a layered n=4 type structure. A summary of this study is shown in Fig. 6, which reveals a surprising richness and complexity of structural and physical phenomena in the LaTiOX system. Recently, Becker studied in great detail the structural mechanisms of oxidation and reduction in the LaTiOX system [30]. For crystals of LaTiO3.42 (n=5) and SrNbO3.45 (n=4.5), semiconducting resistivity behavior along and perpendicular to the layers was reported in the temperature range between T=4 K and T=300 K [22]. However, within the layers the resistivity was measured along only one direction which was arbitrarily selected and not specified. The electrical conducting titanates and niobates were crystallographically explored by Williams et al. using high-resolution transmission electron microscopy [21–24]. An example is shown in Fig. 7, namely the well-ordered stacking sequence n=4, 5, 4, 5, … in the n=4.5 type compound SrNbO3.45 (see also Fig. 3, where the n=4.5 type is sketched). This well-ordered stacking sequence was found to be a bulk property. The thermodynamic stability of the n=4.5 type phases is remarkable. Among the electrical conductors there is up to now only one compound for which the atomic coordinates are precisely known. It is SrNbO3.4 (n=5) on which Schmalle et al. and Abrahams et al. carried out detailed crystallographic studies by means of single crystal x-ray diffraction [29]. A further step concerning the study of physical properties on these layered electri-

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Fig. 5. Log-linear plot of the dielectric permittivity ⑀ versus temperature of ferroelectric SrNbO3.50 along the a-, b- and c-axis measured at 1 MHz. The dielectric anomaly at 1342°C represents the ferroelectric transition temperature Tc. A second dielectric anomaly or transition occurs at ⫺156°C. After Nanamatsu et al. [17]. Note that in ref. [17] the longest axis is defined as the b-axis. The labels a, b and c shown in this figure correspond to the definition used in this work where c represents the longest axis.

cal conductors was recently carried out by exploring the electronic properties of some niobate crystals such as SrNbO3.45 (n =4.5), SrNbO3.4 (n =5) and Sr0.9La0.1NbO3.4 (n =5) [22]. The electronic structure of SrNbO3.45 was investigated by Lu et al. using angle-resolved photoemission (ARPES) [28]. The electronic structure of SrNbO3.45, SrNbO3.4 and Sr0.9La0.1NbO3.4 along the a- and b-axis was comprehensively explored by Kuntscher et al. using x-ray absorption (NEXAFS), ARPES and optical spectroscopy [31–34]. LDA band structural calculations by means of the pseudopotential and LMTO method were performed on SrNbO3.4 by Bohnen [31] and Winter [35], respectively, using the atomic coordinates determined by Schmalle et al. and Abrahams et al. [29]. The most surprising results obtained by ARPES on SrNbO3.45 at T=150 K, SrNbO3.4 at T=75 K and Sr0.9La0.1NbO3.4 at T=75 K and 100 K were strong indications of the presence of a quasi-1D metallic behavior, because discernible electronic dispersion of one of two bands near the Fermi energy EF and its apparent crossing of EF was observed only along the a-axis but not along the b-axis [31,32,34].

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Fig. 6. Phase diagram of the LaTiOX system. Informations about the crystal structure at room temperature are given at the top of the diagram. The index n indicates the structure type of the series LanTinO3n+2. After Lichtenberg et al. [21] and modified at X⬇3.41 according to results from this work (see Fig. 24).

These results were corroborated by optical spectroscopy experiments and band structure calculations [31–35]. The outcomes of the following very recent experiments will be mentioned later: High-resolution ARPES by Kuntscher et al. on SrNbO3.4 at low temperatures [32,33], dielectric response along the c-axis on Sr0.8La0.2NbO3.5 (n=4), SrNbO3.45 and SrNbO3.4 by Bobnar et al. [36] as well as nuclear magnetic

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Fig. 7. Transmission electron microscopy lattice image from the electrical conducting n=4.5 type compound SrNbO3.45. It reveals the well-ordered stacking sequence n=4, 5, 4, 5,... (see also Fig. 3 where this sequence is sketched). The dark spots represent the A cations. The electron diffraction pattern (inset) confirms the perfection of ordering. After Lichtenberg et al. [22] and Williams et al. [24].

resonance (NMR) and electron paramagnetic resonance (EPR) by Weber et al. on SrNbO3.4 at low temperatures [37]. This work mainly reports on the materials synthesis of AnBnO3n+2=ABOX type niobates and titanates including crystal growth by floating zone melting and structural, electric and magnetic features. To start, a comprehensive overview on many ABOX compositions with a AnBnO3n+2 type structure and their properties is presented in Tables 2–18, which lists literature data and results obtained in this work. Tables 2– 15 are classified according to the structure type n. Specified are the composition, electron configuration N of the B cation based on charge neutrality, lattice constants with z as number of formula units ABOX per unit cell, references, special properties and remarks. Almost all materials from this work which are presented in Tables 4–15, 17 and 18 were prepared by floating zone melting. If samples were not synthesized in

Ba Ba Ba Ba Ba Ba

Mn F4 Fe F4 Co F4 Ni F4 Zn F4 Mg F4

3d5 3d6 3d7 3d8 3d0

Composition ABOx N

381 367 360 348 358 348

˚ 3) V (A 4.22 4.24 4.21 4.15 4.21 4.13

˚) a(A

Lattice constants

5.98 5.83 5.85 5.80 5.84 5.81

˚) b (A 15.10 14.84 14.63 14.46 14.56 14.51

˚) c (A 90 90 90 90 90 90

b (°) 4 4 4 4 4 4

z [38] [38] [38] [38] [38] [38]

References

Table 2 Overview on n =2 type compounds which are only known for fluorides. Lattice constants are given as rounded numbers

ferroelectric ferroelectric ferroelectric ferroelectric

Special properties, remarks

12 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

[39]

611

5d0

Sr Ta0.88 Nb0.12 O3.50

3.96

3.86

5.69

5.49

13.56×2=27.11

13.19×2=26.38

90

90

8

8

[40]

30–1304, [17]

558

8

5d0

90

Sr0.9 Na0.05 Nd0.05 Ta O3.50

13.60×2=27.19

z

[17]

5.69

b (°)

5d0

3.94

˚) c (A

Sr1-W CaW Ta O3.50

609

˚) b (A

5d0

˚) a(A

References

Sr Ta O3.50

˚ 3) V (A

Lattice constants

N

Composition ABOX

ferroelectric, TC=675 K

ferroelectric, TC=363 K

0ⱕWⱕ0.8, ferroelectric properties of this solid solution studied on polycrystalline samples

ferroelectric, TC=166 K [17]

Special properties, remarks

Table 3 Overview on n =4 type tantalates. Lattice constants are given as rounded numbers. The reference 30–1304 is related to the ICDD (former JCPDS) data base

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 13

604 604

598

4d0

4d0.2

Sr Nb O3.50

Sr0.8 La0.2 Nb O3.50

˚ 3) V (A

3.99

3.96 3.96

˚) a (A

Lattice constants

N

Composition ABOx

5.65

5.70 5.71

˚) b (A

13.28×2=26.56

13.39×2=26.78 13.38×2=26.76

˚) c (A

90

90 90

b (°)

8

8 8

z

this work [24,32,34,36]

28–1246, [17] this work [19,41–43]

References

(continued on next page)

for 90 KⱕTⱕ150 K quasi-1D metal along aaxis according to resistivity measurements (this work); electron diffraction displays doubled a-axis, photoemission at T=75 K shows along a-axis weak and along b-axis no electronic dispersion [32,34]; along the c-axis: dielectric measurements indicate charge transport by hopping of localized charge carriers [36]

preparation of thin films [43]

ferroelectric, TC=1615 K [17], several ferroelectric and structural (also incommensurate) phase transitions below 1615 K [19,41,42]

Special properties, remarks

Table 4 Overview on n=4 type niobates (part 1). Lattice constants are given as rounded numbers. References such as 28–1246 are related to the ICDD (former JCPDS) data base

14 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

606

605

601

591

583

4d0

4d0.03

4d0.2

4d0.2

4d0

Sr0.9 Ba0.1 Nb O3.50

Sr0.85 Ba0.1 La0.05 Nb O3.51

Sr0.7 Ba0.1 La0.2 Nb O3.50

Sr0.6 Ca0.2 La0.2 Nb O3.50

Sr0.5 Ca0.5 Nb O3.50

˚ 3) V (A

3.90

3.96

4.00

3.97

3.96

˚) a (A

Lattice constants

N

Composition ABOx

Table 4 Continued

5.62

5.63

5.67

5.70

5.71

˚) b (A

13.31×2=26.61

13.25×2=26.51

13.28×2=26.56

13.36×2=26.72

13.39×2=26.78

˚) c (A

90

90

90

90

90

b (°)

8

8

8

8

8

z

  47–492

this work  [32,34]

this work

this work

References

electron diffraction displays doubled a-axis, photoemission at T=75 K shows no electronic dispersion along a- and baxis [32,34]

Special properties, remarks

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 15

601

599

602

561

4d0

4d0.11

4d0.11

4d0

Sr0.86 La0.14 Nb O3.57 Sr0.75 La0.25 Nb O3.57 Sr0.65 Ba0.1 La0.25 Nb O3.57 Ca0.86 La0.14 Nb O3.57

602

3.85 3.85 x 2=7.70 3.86 x 2=7.72

558 560 564

Sr0.8 La0.2 Nb O3.60 4d0

3.84

558

3.88

3.99

3.98

3.96

3.96

3.90

3.85

˚) a (A

559

563

4d0

˚ 3) V (A

Lattice constants

Ca0.8 La0.2 Nb O3.51 4d0.18

Ca Nb O3.50

Composition ABOX N

5.50

5.68

5.66

5.69

5.69

5.50

5.49 5.50 5.51

5.49

5.49

˚) b (A

13.15×2=26.30

13.30×2=26.60

13.30×2=26.59

13.35×2=26.71

13.34×2=26.69

13.13×2=26.25

13.22×2=26.43 13.36 13.40

13.23×2=26.45

13.23×2=26.46

˚) c (A

90

90

90

90

90

90

90 98.4 98.2

90

90

b (°)

8

8

8

8

8

8

8 8 8

8

8

z

this work

this work

this work

this work

this work

  



this work 

[39] 18–301 [16,44]

this work

23–122

References

(continued on next page)

significantly overstoichiometric with respect to the oxygen content x

probably a phase transition into an incommensurate structure at T=750 K [44]

K=melting point [16],

ferroelectric,TC⬎1850

Special properties, remarks

Table 5 Overview on n=4 type niobates (part 2). Lattice constants are given as rounded numbers. References such as 23–122 are related to the ICDD (former JCPDS) data base

16 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

8

this work

this work



  [17] 

90

8

z

Sr1-Z BaZ Nb O3.50 4d0

13.33×2=26.67

90

b (°)

[17]

5.68

13.36×2=26.72

˚) c (A

Sr1-Y PbY Nb O3.50 4d0

3.97

5.70

˚) b (A

[17] 

601

Sr0.85 La0.1 Nb O3.45 4d0.1

3.96

˚) a (A

References

Sr1-W CaW Nb O3.50 4d0

603

˚ 3) V (A

Lattice constants

Sr0.85 La0.1 Nb O3.50 4d0

Composition ABOX N

Table 5 Continued

0ⱕZⱕ0.6

0ⱕYⱕ0.4, ferroelectric properties of these solid solutions studied on polycrystalline samples

0ⱕWⱕ1

significantly understoichiometric with respect to the A site (cation ratio A / B =0.95) and with respect to the oxygen content x

significantly understoichiometric with respect to the A site (cation ratio A / B =0.95)

Special properties, remarks

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 17

3d0

La Ti0.9 Zr0.1 O3.50

3.91×2=7.81

558×2=1115

554

548

3d0

3d0

Ce Ti O3.50

560

3.88×2=7.75

3.89×2=7.79

3.91×2=7.83

3.91×2=7.82

3.90×2=7.81

557

562

3.91×2=7.82

˚) a (A

558

˚ 3) V (A

Lattice constants

La0.67 Ce0.33 Ti O3.50

La Ti0.8 Nb0.2 O3.51 3d0.18

3d0

La Ti O3.50

Composition ABOX N

5.50

5.53

5.55

5.56

5.55

5.55

5.55

˚) b (A

12.98

13.00

13.02

13.05

12.87×2=25.75

13.00

13.01

˚) c (A

98.6

98.6

98.3

98.5

90

98.6

98.6

b (°)

8

8

8

8

16

8

8

z

Special properties, remarks

47–667

this work

this work

this work

(continued on next page)

prepared at 1400°C under Argon using the mixture CeO2 + 0.25 TiN + 0.75 TiO2

see text for peculiarities of Ce oxides

in contrast to this work, ref. [26] says that no substitution of Ti by Nb could be achieved

28–517, [18] ferroelectric, TC=1770 K [18], several structural this work (also incommensurate) phase transitions below [45,46] 1770 K [47,48], [47–49] structural study on thin films [49]

References

Table 6 Overview on n=4 type titanates. Lattice constants are given as rounded numbers. References such as 28–517 are related to the ICDD (former JCPDS) data base

18 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

546×2=1092

544×2=1089

540×2=1081 538×2=1076 536×2=1072 534×2=1069 533×2=1066 529×2=1057 528×2=1056

3d0

La0.5 Nd0.5 Ti O3.50 3d0

3d0 3d0 3d0 3d0 3d0 3d0 3d0

La0.5 Pr0.5 Ti O3.50

La0.5 Sm0.5 Ti O3.50 La0.5 Eu0.5 Ti O3.50 Pr0.5 Nd0.5 Ti O3.50 Pr0.5 Sm0.5 Ti O3.50 Pr0.5 Eu0.5 Ti O3.50 Nd0.5 Sm0.5 Ti O3.50 Nd0.5 Eu0.5 Ti O3.50

540

3d0

3.85×2=7.70 3.84×2=7.67 3.84×2=7.67 3.82×2=7.63 3.82×2=7.64 3.81×2=7.62 3.80×2=7.60

3.86×2=7.72

3.87×2=7.74

3.84×2=7.68

3.87×2=7.73

547×2=1093

Nd Ti O3.50

3.86×2=7.71

˚) a (A

546

3d0

˚ 3) V (A

Lattice constants

Pr Ti O3.50

Composition ABOX N

Table 6 Continued

5.47 5.46 5.45 5.44 5.44 5.42 5.42

5.51

5.51

5.46

5.49

5.50

˚) b (A

12.83×2=25.66 12.85×2=25.70 12.83×2=25.65 12.88×2=25.75 12.83×2=25.65 12.80×2=25.60 12.82×2=25.63

12.80×2=25.60

12.81×2=25.61

13.00

12.88×2=25.76

13.02

˚) c (A

90 90 90 90 90 90 90

90

90

98.6

90

98.5

b (°)

16 16 16 16 16 16 16

16

16

8

16

8

z





 

[46] 

33–942, [51]

35–224 [46,50]

35–267

References

prepared by coprecipitation

unstable in air (35–267), prepared by coprecipitation [46,50] ferroelectric, TC ⬎1770 K [51]

Special properties, remarks

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 19

d0

572×2=1144

d0

Sr0.5 Nd0.5 Ta0.5

Ti0.5 O3.50

Ca0.5 La0.5 Nb0.5

Ti0.5 O3.50

Sr0.5 Pr0.5 Nb0.5

Ti0.5 O3.50

d0

d0

559×2=1118

561×2=1121

576×2=1151

582×2=1164

d0

Sr0.5 La0.5 Nb0.5

571×2=1142

Sr0.5 Pr0.5 Ta0.5 Ti0.5 d0 O3.50

Ti0.5 O3.50

580×2=1159

d0

d0

˚ 3) V (A

Lattice constants

Sr0.5 La0.5 Ta0.5 Ti0.5 O3.50

TiW O3.50

Sr1-W LaW Ta1-W

Sr Ta1-W NbW O3.50 d0

NbW O3.50

Sr1-W CaW Ta1-W

Composition ABOX N

3.89×2=7.78

3.89×2=7.78

3.92×2=7.83

3.94×2=7.88

3.91×2=7.81

3.91×2=7.81

3.94×2=7.88

˚) a (A

5.54

5.54

5.60

5.63

5.57

5.57

5.60

˚) b (A

12.98×2=25.97

13.01×2=26.02

13.13×2=26.25

13.12×2=26.24

13.12×2=26.24

13.15×2=26,30

13.14×2=26.27

˚) c (A

90

90

90

90

90

90

90

b (°)

16

16

16

16

16

16

16

z

 

[50]  this work

[50]

[50] 

[50]

[50] 

35–225 [50]

[17]  [18]







[17] 

References

these compositions are individual compounds and no solid solutions [50]

prepared by coprecipitation

0ⱕWⱕ1, ferroelectric properties of these solid solutions studied on polycrystalline samples

Special properties, remarks

Table 7 Overview on miscellaneous n=4 type compounds. Lattice constants are given as rounded numbers. The reference 35–225 is related to the ICDD (former JCPDS) data base

20 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

O3.46

3.92×2=7.84

3551

5.52

5.53 82.8

83.6 97.7

98

50

50

[10]

3.90×2=7.80

3d0

La0.92 Ca0.08 Ti

3571

[26,27]

this work

this work

3d0.06

82.7

50

[24]

Nd Ti O3.47

5.50

97.6

z

3.88×2=7.76

3499

b (°)

3d0.06

85

˚) c (A

Ce Ti O3.47

5.5

˚) b (A

3.9×2=7.8

˚) a (A

References

3d0.06

˚ 3) V (A

Lattice constants

La Ti O3.47

Composition ABOX N

Table 8 Overview on n=4.33 type titanates. Lattice constants are given as rounded numbers

see text for peculiarities of Ce oxides

Special properties, remarks

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 21

1196×2=2392

1238×2=2477

1233×2=2465

1223×2=2446

3d0

3d0.12

La Ti O3.44

La0.98 Sr0.02 Ti O3.45 3d0.08

3d0.17

Nd0.89 Ca0.11 Ti O3.44

La Ti0.95 Nb0.05 O3.44

3.90×2=7.81

1239×2=2478

3.95×2=7.90

3.95×2=7.91

3.90×2=7.81

3.84×2=7.68

3.90×2=7.81

1234×2=2468

3d0

3.91×2=7.81

˚) a (A

La0.89 Ca0.11 Ti O3.44

˚ 3) V (A

Lattice constants

1232×2=2464

N

La0.89 Sr0.11 Ti O3.44 3d0

Composition ABOX

5.49

5.52

5.54

5.46

5.54

5.54

5.53

˚) b (A

56.9

56.9

57.8

57.5

57.8

57.6

57.0

˚) c (A

97.5

97.5

98.0

97.7

97.8

97.8

90

b (°)

36

36

36

36

36

36

36

z

this work

this work

[30]

[11]

[30]

27–1058, [11]

[52]

References

Special properties, remarks

Table 9 Overview on n=4.5 type titanates. Lattice constants are given as rounded numbers. The reference 27-1058 is related to the ICDD (former JCPDS) data base

22 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

1330×2=2660

1330×2=2661

1332×2=2665

4d0.1

4d0.1

4d0.12

Sr Nb O3.45

Sr0.96 Ba0.04 Nb O3.45

Sr0.96 Ba0.04 Nb0.95 Ta0.05 O3.44

˚ 3) V (A

Lattice constants

N

Composition ABOX

3.96×2=7.92

3.94×2=7.88

3.95×2=7.90

˚) a (A

5.70

5.68

5.68

˚) b (A

59.1

59.4

59.3 59

˚) c (A

90

90

90 90

b (°)

36

36

36

z

this work

this work

this work [24] [22,28,31] [32,34,36]

References

(continued on next page)

for 120 KⱕTⱕ240 K quasi-1D metal along aaxis according to resistivity measurements

quasi-1D metal along aaxis according to photoemission at T=150 K and optical spectroscopy [28,31,32,34], along the c-axis: according to dielectric measurements phase transition at T=290 K, relatively high values of the high frequency dielectric permittivity and charge transport by hopping of localized charge carriers [36]

Special properties, remarks

Table 10 Overview on n=4.5 type niobates. Lattice constants are given as rounded numbers. The reference 27–1412 is related to the ICDD (former JCPDS) data base

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 23

N

1242

1244

1242×2=2484

1245

1239×2=2478

4d0.12

4d0.1

4d0.13

4d0

d0

Ca Nb O3.45

Ca0.95 La0.05 Nb O3.46

Ca0.89 Na0.11 Nb O3.44

Ca Nb0.89 Ti0.11 O3.44

1334×2=2667

˚ 3) V (A

Lattice constants

Ca Nb O3.44

Sr0.96 Ba0.04 Nb0.9 Ta0.1 4d0.11 O3.45

Composition ABOX

Table 10 Continued

3.84×2=7.68

3.85

3.87×2=7.74

3.86

3.85

3.96×2=7.92

˚) a (A

5.48

5.50

5.51

5.51

5.49

5.70

˚) b (A

59.4

58.9

58.4

58.4

58.8

59.0

˚) c (A

97.4

90

90

90

90

90

b (°)

36

18

36

18

18

36

z

[11]

27–1412, [12]

this work

this work

[15]

this work

References

crystallographic study by electron microscopy and diffraction

Special properties, remarks

24 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

674

683

682

680

687

683

680

4d0.2

4d0.22

4d0.18

4d0.18

4d0.18

4d0.25

4d0

Ca Nb O3.4

Ca Nb O3.39

Ca Nb O3.41

Ca0.95 Nb O3.36

Ca0.9 Sr0.1 Nb O3.41

Ca0.93 La0.07 Nb O3.41

Ca0.8 Na0.2 Nb O3.40

˚ 3) V (A

3.85

3.89

3.89

3.87

3.88

3.88

3.86

˚) a (A

Lattice constants

N

Composition ABOX

5.50

5.50

5.51

5.50

5.49

5.49

5.47

˚) b (A

16.07×2=32.14

15.96×2=31.91

16.03×2=32.06

16.11×2=32.23

16.01×2=32.03

16.02×2=32.04

15.96×2=31.92

˚) c (A

90

90

90

96.7

90

90

90

b (°)

10

10

10

10

10

10

10

z

27–1411, [12]

this work

this work

this work

this work

this work

[15]

References

significantly understoichiometric with respect to the A site (cation ratio A / B =0.95) and with respect to the oxygen content x

for 185 KⱕTⱕ290 K quasi-1D metal along a-axis according to resistivity measurements

polycrystalline sample, not prepared by floating zone melting

crystallographic study by electron microscopy and diffraction

Special properties, remarks

Table 11 Overview on n=5 type Ca-based niobates. Lattice constants are given as rounded numbers. The reference 27–1411 is related to the ICDD (former JCPDS) data base

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 25

N

4d0.18

Composition ABOX

Sr Nb O3.41

734 736

˚ 3) V (A 3.99 4.00

˚) a (A

Lattice constants

5.67 5.67

˚) b (A 16.23×2=32.45 16.23×2=32.46

˚) c (A 90 90

b (°)

Table 12 Overview on n=5 type Sr-based niobates. Lattice constants are given as rounded numbers

10 10

z this work [29] [22,31–33] [34–37]

References

(continued on next page)

quasi-1D metal along a-axis according to photoemission at T=75 K, optical spectroscopy and band structure calculations [31–35], along the a-axis at low temperatures T⬍50 K: semiconducting state with very small energy gap of few meV according to resistivity measurements (this work), photoemission [32,33], optical spectroscopy [33] and nuclear magnetic resonance (NMR) [37], NMR hints to a charge density wave [37] along the c-axis: according to dielectric measurements phase transition at T=300 K, relatively high values of the high frequency dielectric permittivity and charge transport by hopping of localized charge carriers [36]

for 60 KⱕTⱕ130 K quasi-1D metal along a-axis according to resistivity measurements (this work),

Special properties, remarks

26 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

this work

732

733

4d0.16

4d0.14

Sr0.95 Nb O3.37

Sr0.93 Nb O3.36 3.98

3.98

5.67

5.67

16.24×2=32.47

16.22×2=32.44

16.13×2=32.27

90

90

90

10

10

10

this work

this work

this work

[22,31,32]

5.66

10

4d0.32

4.00

90

Sr0.9 La0.1 Nb O3.39

this work

731

16.16×2=32.32

10

z

4d0.28

5.66

90

b (°)

Sr0.9 La0.1 Nb O3.41

4.00

16.19×2=32.37

˚) c (A

732

5.67

˚) b (A

4d0.25

3.99

˚) a (A

References

Sr0.93 La0.07 Nb O3.41

˚ 3) V (A

Lattice constants

732

N

Sr0.965 La0.035 Nb O3.41 4d0.22

Composition ABOX

Table 12 Continued

significantly understoichiometric with respect to the A site (cation ratio A / B=0.93) and with respect to the oxygen content x

significantly understoichiometric with respect to the A site (cation ratio A / B=0.95) and with respect to the oxygen content x

for 60 KⱕTⱕ160 K quasi-1D metal along a-axis according to resistivity measurements

quasi-1D metal along a-axis according to photoemission at T=75 K and T=100 K [31,32]

for 80 KⱕTⱕ290 K quasi-1D metal along a-axis according to resistivity measurements

for 105 KⱕTⱕ290 K quasi-1D metal along a-axis according to resistivity measurements

Special properties, remarks

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 27

3d0

3d0

Nd0.8 Sr0.2 Ti O3.40

La0.8 Ca0.2 Ti O3.40

3d

Nd0.8 Ca0.2 Ti O3.40 3d0

La0.89 Ca0.11 Ti O3.36

675×2=1350

3d0

La0.8 Sr0.2 Ti O3.40

0.17

[53] 27-1059, [11]

673

3d0.2

Nd Ti O3.4

652×2=1304

3.84×2=7.68

3.92×2=7.84

3.89×2=7.78

673×2=1346

669×2=1337

3.89×2=7.78

673×2=1346

3.91×2=7.81

3.9×2=7.8

3.93×2=7.85

676×2=1353

5.44

5.52

5.52

5.52

5.53

5.5

5.52

15.73×2=31.45

15.55×2=31.09

15.78×2=31.57

15.78×2=31.56

15.76×2=31.51

15.80

15.72×2=31.44

97.0

96.1

97.0

97.1

97.1

96.5

97.0

97.1

20

20

20

20

20

10

20

20

[11]

this work

[30]

[52]

[25]

this work

[30]

[21–23]

?

15.73×2=31.46

20

La0.5 Ce0.5 Ti OX

5.53

97.1

3.93×2=7.86

15.73×2=31.47

this work [22]

678×2=1357

5.53

20

[21,22]

3.93×2=7.86

97.1

20

678×2=1357

15.74×2=31.48

97.2

z

3d0.16

5.53

15.75×2=31.5

b (°)

La Ti O3.42

3.93×2=7.86

5.53

˚) c (A

679×2=1357

3.93×2=7.86

˚) b (A

3d

679×2=1358

˚) a (A

References

La Ti O3.41

3d0.2

˚ 3) V (A

Lattice constants

0.18

La Ti O3.40

Composition ABOX N

significantly understoichiometric with respect to the oxygen content x

the presence of a n=5 type structure suggests an oxygen content of x=3.4

see text for peculiarities of Ce oxides,

for 60 KⱕTⱕ205 K quasi-1D metal along aaxis according to resistivity measurements (this work)

Special properties, remarks

Table 13 Overview on n=5 type titanates. Lattice constants are given as rounded numbers. The reference 27–1059 is related to the ICDD (former JCPDS) data base

28 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

3.94×2=7.88

5.55

15.81×2=31.62

97.1

20

20

[11]

[54] this work [55]

this work

this work

this work

687×2=1373

96.8

10

10

10

z

3d0.17

16.17×2=32.33

90

90

90

b (°)

La Ti0.9 Zr0.1 O3.42

5.48

16.3×2=32.6

16.22×2=32.44

16.12×2=32.23

˚) c (A

[26]

3.84×2=7.69

5.59

5.65

5.60

˚) b (A

from 3d0.2 to 3d0.3

676×2=1353

3.95

3.97

3.94

˚) a (A

References

La Ti1-Y NbY O3.4

Ca Nb0.8 Ti0.2 O3.40 d0

Sr Nb0.8 Ti0.2 O3.40 d

720

729

0

Sr Nb0.9 Ti0.1 O3.39 4d

711

0.12

˚ 3) V (A

Lattice constants

Sr0.5 Ca0.5 Nb O3.41 4d0.18

Composition ABOX N

Table 14 Overview on miscellaneous n=5 type compounds. Lattice constants are given as rounded number

0ⱕYⱕ0.1, in-plane resistivity measurements (direction not specified) revealed thermally activated behavior, activation energy rises with increasing Nb content Y from 9 to 51 meV

antiferroelectric, TCⱖ860 K [54], preparation by floating zone melting was practically impossible because the feed material showed an extremely strong tendency to grow out of the molten zone (this work)

Special properties, remarks F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 29

3.89×2=7.78 3.89×2=7.77 3.87×2=7.74

790×2=1580 789×2=1578

781×2=1563

763×2=1526 764

789

3d0

3d0

d0

La0.67 Ca0.33 Ti O3.33

Nd0.67 Ca0.33 Ti O3.33

Ca Nb0.67 Ti0.33 O3.33

3.84×2=7.67

3.83×2=7.66 3.83×2=7.66

3.91×2=7.83

798×2=1596

La0.73 Sr0.27 Ti O3.36 3d0

3.89×2=7.79

˚) a (A

794×2=1588

˚ 3) V (A

Lattice constants

La0.67 Sr0.33 Ti O3.33 3d0

Composition ABOX N

5.46

5.44 5.44

5.50

5.52 5.52

5.52

5.54

˚) b (A

18.91

18.32×2=36.64 18.42

18.34×2=36.67

18.40×2=36.80 18.40×2=36.80

18.57×2=37.14

18.42×2=36.84

˚) c (A

95.8

90 95.7

90

90 90

95.8

90

b (°)

12

24 12

24

24 24

24

24

z

 

significantly overstoichiometric with respect to the oxygen content x  also incommensurate modulated phases

Special properties, remarks

[11,13]

[11,13,56] [57]



(continued on next page)

non-centrosymmetric space group and results from second harmonic generation experiments, however dielectric measurements are not presented [13,57]

most probably ferroelectric because of

this work prepared and [11,13,14, crystallographically 56,57]  investigated [13,14,56]

27–1057 [30]

this work

this work

References

Table 15 Overview on n=6 type compounds. Lattice constants are given as rounded numbers. References such as 27–1057 are related to the ICDD (former JCPDS) data base

30 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

d0

Ca0.67 Na0.33 Nb O3.33

4d0

Ca Nb0.8 Ti0.2 O3.33 4d0.14

Sr Nb0.67 Ti0.33 O3.33

Composition ABOX N

Table 15 Continued

800

792×2=1583

854

˚ 3) V (A

Lattice constants

3.86

3.86×2=7.72

3.93

˚) a (A

5.50

5.47

5.66

˚) b (A

18.86×2=37.71

18.74×2=37.48

19.2×2=38.4

˚) c (A

90

90

90

b (°)

12

24

12

z

27-1413, [12]

this work

[55,58]

References

ferroelectric, TC =900 K [58]

Special properties, remarks

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 31

n=4

n=4

n=4

n=4

4d0

0

4d

3d0

3d0

Sr Nb O3.50

Ca Nb O3.50

La Ti O3.50

Nd Ti O3.50

4d

3d0

Sr Nb O3.41

Nd0.67 Ca0.33 Ti O3.33

n=6

n=5

n=4

d0

Sr Nb0.12 Ta0.88 O3.50

0.2

n=4

5d0

Sr Ta O3.50

3.954 7.810

orthorhombic orthorhombic

orthorhombic

orthorhombic

7.664

3.995

7.677

7.812

monoclinic

monoclinic

7.800

7.697

monoclinic monoclinic

7.692

3.933

3.961

3.937

˚) a (A

5.436

5.674

5.465

5.547

5.607

5.544

5.546

5.502

5.501

5.683

5.687

5.692

˚) b (A

Lattice constants

orthorhombic

orthorhombic

orthorhombic

orthorhombic

Structure Crystal symmetry type

N

Composition ABOX

36.64

32.46

26.01

25.75

25.95

13.01

13.01

13.39

26.46

26.73

27.11

27.20

˚) c (A

90

90

98.4

90

90

98.7

98.6

98.3

90

90

90

90

b (°)

[67]

[29]

[66]

[45]

[65]

[64]

[63]

[62]

[61]

[60]

[40]

[59]

References

the atomic coordinates were used for band structure calculations which indicated a quasi-1D metallic system along the a-axis [31,32,35]

T=1053 K

results for T=573 K, 773 K and 1073 K are also presented

results for T=123 K are given in ref. [40]

Special properties, remarks

Table 16 Overview on compounds whose atomic coordinates are precisely known from single crystal x-ray diffraction studies. If the temperature T is not specified the data refer to room temperature

32 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

n=3 (x=3.667)

n=3 (x=3.667) n=3 (x=3.667) n=3 (x=3.667) n=4 beyond x=3.57 n=3.5 (x=3.571) n=4 beyond x=3.57 n=3.5 (x=3.571)

4d0

0

d0

4d

4d0

4d0

d0

4d0

4d0

La Ti0.67 Nb0.33 O3.67

Sr0.67 La0.33 Nb O3.67

Ca0.67 La0.33 Nb O3.67

Ba0.67 La0.33 Nb O3.67

Ca0.8 La0.2 Nb O3.60

La Ti0.86 Nb0.14 O3.57

Sr0.8 La0.2 Nb O3.60

Sr0.86 La0.14 Nb O3.57

n=3.5 (x=3.571) n=5.5 (x=3.364)

n=5.5 (x=3.364)

n=5.5 (x=3.364)

4d

3d0.17

3d0

d0

La0.89 Ca0.11 Ti O3.36

La0.73 Sr0.27 Ti O3.36

Ca Nb0.73 Ti0.27 O3.36

4d0.11

Sr0.65 Ba0.1 La0.25 Nb O3.57

Ca0.86 La0.14 Nb O3.57

n=3.5 (x=3.571)

4d

Sr0.75 La0.25 Nb O3.57

0

n=3.5 (x=3.571)

0.11

Aim

Structure type

N

Composition ABOX

multiphase

n=6

n=5

n=4

n=4

n=4

n=4

n=4

multiphase

multiphase

multiphase

multiphase

multiphase

multiphase

Actual result found by XRD

this work

this work

this work

this work

this work

this work

this work

this work

this work

this work

this work

this work

this work

[24]

Reference

 



main phase is of n=6 type

significantly overstoichiometric with respect to the oxygen content x (the ideal x of the n=6 type is x=3.33), see also Table 15

significantly understoichiometric with respect to the oxygen content x (the ideal x of the n=5 type is x=3.40), see also Table 13

significantly overstoichiometric with respect to the oxygen content x (the ideal x of the n=4 type is x=3.50), see also Table 5

incongruent melting

main phase is of n=4 type

incongruent melting

incongruent melting

incongruent melting

incongruent melting; transmission electron microscopy revealed minority phases with disordered as well as ordered intergrowth of n=3 and n=4 type layers

Special properties, remarks

Table 17 Results of studies aimed at the preparation of compounds with n⬍4, 5⬍n⬍6 and oxygen overstoichiometry by floating zone melting. Structural investigations were done by powder x-ray diffraction (XRD)

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70 33

Y

0.95

0.95

0.95

0.95

0.93

0.933

0.90

0.90

0.90

0.75–0.95

0.67–1.00

Composition AYBOX

Sr0.85 La0.1 Nb O3.50

Sr0.85 La0.1 Nb O3.45

Ca0.95 Nb O3.36

Sr0.95 Nb O3.37

Sr0.93 Nb O3.36

La0.933 Ti O3.40

Sr0.7 La0.2 Nb O3.50

Sr0.7 La0.2 Nb O3.42

Sr0.9 Nb O3.40

SrY Nb O3

LaY Ti O3

n=4 n=5 n=5

n=5

multiphase

multiphase

4d0.1 4d0.18 4d0.16

4d0.14

3d0

4d0

this work

[68] [69–71]

multiphase

n= ⬁ n= ⬁

4d0.5 - 4d0.9 3d0 - 3d1

this work

this work

this work

this work

this work

this work

this work

this work

References

4d0

4d

multiphase

n=4

4d0

0.16

Structure type found by XRD

N

change of solidified composition during floating zone melting: at first SrNbO3.50 did grow, but later a multiphase composition did appear

2 phases: n=4 type (main phase) + Sr0.6NbO3 (45– 295)

main phase is SrNbO3.50

change of solidified composition during floating zone melting: at first LaTiO3.50 did grow, but later a multiphase composition did appear

significantly understoichiometric also with respect to x (the ideal x of the n=5 type is x=3.40), thermogravimetry hints to the presence of a second phase in small amounts (see text) →Y=0.93 could be the stability limit

significantly understoichiometric also with respect to x (the ideal x of the n=5 type is x=3.40), for 60 KⱕTⱕ160 K quasi-1D metal along a-axis according to resistivity measurements

significantly understoichiometric also with respect to x (the ideal x of the n=5 type is x=3.40)

significantly understoichiometric also with respect to x (the ideal x of the n=4 type is x=3.50)

Special properties, remarks

Table 18 Results of studies aimed at the preparation of compounds with understoichiometry at the A site by floating zone melting. Structural investigations were done by powder x-ray diffraction (XRD). The number 45–295 refers to Sr0.6NbO3 in the ICDD (former JCPDS) data base. Also mentioned are n=⬁ types which are known as polycrystalline materials

34 F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

35

this way, it is explicitly mentioned. Table 16 shows an overview about compounds whose atomic coordinates are precisely known from single crystal x-ray diffraction studies.

2. Experimental 2.1. Sample preparation Starting materials were CaCO3 (99.9%, MaTeck), SrCO3 (99+ %, MaTeck), BaCO3 (99.8% Alfa), La2O3 (99.99%, MaTeck), CeO2 (99.99%, Alfa), TiO2 (99.9%, Alfa), ZrO2 (99.98%, Alfa), Nb2O5 (99.9%, MaTeck), Ta2O5 (99.99%, MaTeck), Nb powder (99.9%, MaTeck) and TiO (99.9%, MaTeck). The powders were weighed with an accuracy of 0.3 mg and mixed in an agate mortar. Special care was taken to prepare nearly moisture-free powders. The carbonates were heated for several hours in vacuum at 110°C and subsequently stored in a dry atmosphere. The oxides (apart from TiO) were heated at higher temperatures (700–1100°C) in air, then kept in a dry ambience, too. Thermogravimetry (NETZSCH TG 209) was used in order to check the oxygen content of the Nb powder and of the TiO. Small amounts of powders were oxidized in air at temperatures up to 1000°C. Assuming that the uptake of oxygen leads to 100% Nb and 100% Ti, the actual compositions of the powders were found to be NbO0.02 and TiO1.03. These formulas were used for stoichiometric calculations. The synthesis of electrical conducting ABOX type niobates and titanates, i.e. reduced mixed-valence compositions, involved the following steps. 1. Preparation of a nearly carbonate-free fully oxidized Ti4+ or Nb5+ composition AB1⫺VOY-W. This was done by using a mixture of oxides and carbonates of total composition AB1⫺VOY-W(CO2)Z with 0ⱕZⱕ1 which was heated in air for at least 6 h at temperatures of typically 1250°C. The carbonate loss was traced by weighing the powder mixture before and after this process. 2. The nearly carbonate-free and fully oxidized composition AB1⫺VOY-W was mixed with a reduced powder BVOW, resulting in a composition ABOY according to the following equation AB1⫺VOY⫺W ⫹ BVOW ⫽ ABOY (powder, total amount about 6 g)

(1)

The oxygen content Y of the powder (1) was checked thermogravimetrically by oxidizing a small amount at temperatures up to 1000°C in air. Fig. 8 displays the measured thermogravimetric behavior using an example from the SrNbOX system. The difference between the thermogravimetrically determined oxygen content, YM, and the theoretical value based on the corresponding stoichiometric calculation, YC, was typically found to be |YM⫺YC|ⱕ0.005. 3. The powder mixture (1) was pressed into two rectangular rods and then heated at temperatures in the range of 1200–1400°C under argon (purity 5.0) in a molybdenum furnace (GERO HTK8MO) for at least 6 h. Fig. 9 shows the sintered rods

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F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

Fig. 8. Thermogravimetric behavior of the oxidation of reduced ABOX compositions by means of an example from the SrNbOX system. The specimen were heated in air with a rate of 10°C/min from room temperature to 1000°C. Shown is the relative specimen mass, (m+⌬m)/m, as function of temperature T. The saturation at higher temperatures indicates the presence of fully oxidized compositions involving Nb5+ . The total mass increase was used to calculate the oxygen content X of the specimen. For this a constant cation ratio A/B=Sr/Nb=1 was assumed. Numbers of X are given with three digits behind the comma to display the tendency. The materials were prepared to grow crystals of n=5 type SrNbO3.4, which has a homogeneity range from SrNbO3.40 to SrNbO3.42. The resulting niobate had the composition SrNbO3.41.

as obtained after this heating process. In most cases this heat treatment resulted in a small oxidation of the pressed powder mixture (1), however for some titanates a decrease of the oxygen content was observed. The small change of the oxygen content is described by the following equation ABOY ⫹ O⌬ ⫽ ABOY ⫹ ⌬ (polycrystalline sintered rods)

(2)

The causes of a small oxidation ⌬⬎0 are probably small concentrations of moisture, carbonates and/or hydroxides in the powder mixture (1), but may also be associated with the degree of the purity of the inert gas atmosphere in the furnace. A small piece of the rods (2) was used to check the oxygen content Y+⌬ by thermogravimetric oxidation at temperatures up to 1000°C in air. Fig. 8 displays the measured thermogravimetric behavior using an example from the SrNbOX system. In most cases, the amount of absorbed oxygen was found to be ⌬⬍0.02. 4. The sintered rods (see Fig. 9) were subjected to a floating zone melting process under argon (purity 5.0) whereby the long rod acted as feed material and the small rod as seed part. An optically heated floating zone melting furnace (GERO) was used. The zone speed and the rotation frequency of the seed part were chosen within the range 5–15 mm/h and 10–15 rpm, respectively. By this, single crystals

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

37

Fig. 9. Photograph of two sintered rectangular rods after heating at high temperatures under argon. The length of the long rod is 8 cm.

can be grown if the compositions melt congruently or almost congruently. The as-prepared samples have a cylindrical shape. Fig. 10 displays a part of such a cylinder with composition SrNbO3.41. Because of the layered structure the cylindrical samples can easily be cleaved. By crushing the sample in an agate mortar, single crystalline platelets can be obtained as shown in Fig. 10. Also the as-grown samples were inspected with respect to a change d of the oxygen content according to the equation ABOY ⫹ ⌬ ⫹ Od ⫽ ABOX with X ⫽ Y ⫹ ⌬ ⫹ d (as–grown samples)

(3)

Small pieces of the as-grown samples (3) were used to check the oxygen content

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F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

Fig. 10. (a) Photograph of a 15 mm long cylindrical part of as-grown crystalline SrNbO3.41. Its layered structure is revealed by shining faces which easily cleave. (b) Photograph of a plate-like single crystal obtained by crushing the cylindrical sample.

X=Y+⌬+δ by thermogravimetric oxidation at temperatures up to 1000°C in air. Fig. 8 displays the measured thermogravimetric behavior using an example from the SrNbOX system. For samples which melt (almost) congruently and/or which are compositionally more or less homogeneous, the extent of this small change was typically found to be 0⬍δ⬍0.01. A few reduced samples were also prepared by using the following approach. Rods

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

39

were pressed from powders with a fully oxidized composition ABOF and heated in air at temperatures of 1200–1400°C. The sintered, fully oxidized rods were then subjected to a floating zone melting process under a reducing atmosphere consisting of 98% argon (purity 5.0) and 2% hydrogen (purity 5.0). If the materials are reduced under these conditions, the resulting as-grown samples have a composition ABOF⫺E with E⬎0. The oxygen content X=F⫺E was determined thermogravimetrically by oxidizing small pieces from the samples at temperatures up to 1000°C in air. Compared to the steps 1–4 described above, this way of preparation is relatively simple. However, for a given composition of A and B the final oxygen content X=F⫺E is fixed and cannot be varied systematically. Fully oxidized samples were grown in a similar manner, the only difference being that the atmosphere during the floating zone melting consisted of artificial air. In the course of previous [21,22] and this work, the accuracy of the thermogravimetrically measured oxygen content X of samples with reduced composition was found to be about 0.3% (i.e. two digits behind the comma). In addition a relative check was done by comparing the compositional status (1), (2) and (3) of the sample, as described above in the steps 1–4 and shown in Fig. 8 using an example from the SrNbOX system. 2.2. Structural, magnetic and electric measurements Bulk structural analysis of powdered samples was performed by powder x-ray diffraction (XRD) with Cu Ka radiation using a PHILIPS X’Pert diffractometer. Lattice constant refinement of peaks located in the diffraction angle range 4°ⱕ2⌰ⱕ60° was done with the PHILIPS software X’Pert Plus. Laue diffraction was used to check the quality and orientation of selected (single) crystalline platelets, which were considered as candidates for resistivity measurements. Magnetic measurements were taken on a SQUID magnetometer (QUANTUM DESIGN MPMS-5S) in a temperature range 2 KⱕTⱕ390 K and in small magnetic fields H, i.e. 50 GⱕHⱕ1000 G. For these experiments, pieces of the samples with relatively large size or mass were selected, typical masses were in the range 300– 1000 mg. Resistivity measurements between room temperature and T=4 K were done on rectangular plate-like crystals obtained by crushing the melt-grown samples. Frequently the as-crushed crystals were additionally cleaved and/or cut by means of a razor blade to obtain a rectangular shape with appropriate size. In a few cases the crystals were cut with a wire saw. Laue diffraction was used to check the quality and orientation of the (single) crystalline platelets. Typically, the platelets were 0.2–0.8 mm thick and 2– 4 mm long and wide. The resistivity r was measured in a four-point configuration along the a-, b- and c-axis in a way as shown in Fig. 11. The voltage contacts along the c-axis, i.e. perpendicular to the layers, and the current contacts along the a-, b- and c-axis were made by gold wires which were attached to the sample with silver paint. The voltage contacts along the a- and b-axis, i.e. along the layers, were made on the crystal surface by ultrasonically bonded aluminum wires.

40

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

Fig. 11. Sketch of the arrangement of electrical contacts for resistivity measurements in a four-point configuration on a rectangular plate-like crystal along the a-, b- and c-axis. V and I denote voltage and current contacts, respectively.

3. Results and discussion More than 100 compositionally different samples were prepared by floating zone melting. Additionally, more than 50 exclusively polycrystalline specimen were synthesized by heating pressed powder mixtures at high temperatures according to the steps 1–3 as described in the experimental section. Apart from a few exceptions, the following sections focus on compounds grown by floating zone melting. 3.1. Sample growth by floating zone melting and structural features Those compounds or crystals which were identified by powder x-ray diffraction as single phase are presented in Tables 4–15 together with materials reported in literature. Although every composition shows its own characteristic properties, the crystals share the following common features. In most cases, the layers grow parallel to the cylinder axis. In some cases, however, they grow with an inclination relative to the cylinder axis, but never perpendicular to it. This behavior may be explained by the crystal growth rates, which probably differ strongly along and perpendicular to the layers. Furthermore, it was often observed that crystals of reduced composition preserve their shape when they are fully oxidized during thermogravimetry at high temperatures in air (note that in many cases this oxidation results in a change of the structure type n). This observation is in accordance with the results of a detailed

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

41

study of the structural mechanisms of oxidation and reduction in the LaTiOX system, which was recently performed by Becker [30]. For layered LaTiOX compounds, it was revealed that the oxidation proceeds in a topotactic manner [30]. 3.1.1. Compositions based on SrNbOX Most of the following refers to several of the compounds presented in Tables 4, 10, 12 and 14. All n=4 and n=5 type plate-like crystals which were subjected to Laue diffraction were found to be oriented with the a-axis parallel to the cylinder axis of the asgrown sample. Fig. 12 shows the powder x-ray diffraction pattern of several n=4, 4.5 and 5 type (Sr,La)NbOX niobates. From the position of the (0 0 L) peak at small diffraction angles the structure type n can be inferred. The n=4 type niobates Sr1⫺YLaYNbO3.5 remain single phase at least up to Y=0.2. For Y=0.25, the powder x-ray diffraction pattern shows additional peaks which do not belong to the n=4 type structure. This statement about the homogeneity range refers to samples prepared by floating zone melting. According to the refinement of powder x-ray diffraction data, the n=4.5 type ˚ , whereas the n=4 and n=5 types materials show a doubled a-axis, i.e. a ⬇7.9 A ˚ . The latter is in contrast to the results from display a simple a-axis of a ⬇3.95 A low energy electron diffraction (LEED), which revealed weak superstructure reflec-

Fig. 12. Square root-linear plot of powder x-ray diffraction pattern of some (Sr,La)NbOX compounds. For clarity only selected peaks are indexed, namely those with the highest intensity and those in the range of small diffraction angles 4°⬍2q⬍7°. The position of the latter indicates the structure type n. Note the diffraction pattern of Sr0.8La0.2NbO3.60, one of several examples of n=4 type phases which are significantly overstoichiometric with respect to the oxygen content X (see Section 3.1.4 and Table 17).

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F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

tions and therefore a doubled a-axis [31,32]. Even for SrNbO3.41, LEED as well as neutron scattering experiments indicated a doubled a-axis [32], whereas a simple aaxis was inferred from single crystal x-ray diffraction studies [29]. This discrepancy suggests that these materials contain some sophisticated structural details which are possibly difficult to verify. Maybe these details depend also on the preparation process. Subtle crystallographic features like phase transitions and incommensurate phases are already known for fully oxidized compounds, e.g. for SrNbO3.50 [19,41]. Dielectric measurements by Bobnar et al. along the c-axis on the electrical conductors SrNbO3.45 (n=4.5) and SrNbO3.41 (n=5) revealed a phase transition for both niobates at T⬇300 K [36]. Due to its relatively low resistivity, however, it was impossible to conclude whether these are polar or structural phase transitions [36]. Laue diffraction performed on many different plate-like crystals of the n=4.5 type SrNbO3.45 always displayed an unclear orientation of the a- and b-axis. In contrast to that, plate-like crystals with clear orientation could readily be found from the corresponding Ba-containing compound Sr0.96Ba0.04NbO3.45. Therefore attempts to measure the resistivity along the a-, b- and c-axis were done on Sr0.96Ba0.04NbO3.45. Note that for both niobates the conditions during floating zone melting were the same (argon atmosphere, zone speed 15 mm/h). The n=5 type compositions Sr1⫺YLaYNbO3.4 (0ⱕYⱕ0.1) grown by floating zone melting under argon atmosphere with a zone speed of 15 mm/h revealed a tendency to form a second phase with a flashy purple-like color. Depending on the La content Y, the second phase appears in the whole as-grown sample or only within the first few grown millimeter. Perhaps this second phase is the Sr-deficient cubic perovskite compound SrWNbO3 because for W ⬇0.8 a purple color was reported [68]. By using lower zone speeds and an atmosphere containing 98% argon and 2% hydrogen the formation of the second phase was completely suppressed, or its formation was restricted to the first 1 or 2 mm of the as-grown sample. For the fully oxidized n=5 type material SrNb0.8Ti0.2O3.4, the presence of antiferroelectricity with Tcⱖ860 K was reported by Isupov et al. [54]. Attempts to grow crystals of this composition turned out to be practically impossible because the feed material showed an extremely strong tendency to grow out of the molten zone. A similar composition, the electrical conducting compound SrNb0.9Ti0.1O3.4, displayed the same tendency but in a less pronounced manner and it was possible to grow small amounts of SrNb0.9Ti0.1O3.4. The Sr-based niobates SrNbOX are those with the largest unit cell volume V compared to CaNbOX and LaTiOX. Regarding the same structure type n, CaNbOX and LaTiOX have approximately the same unit cell volume V. Fig. 13 displays the evolution of the lattice constants a, b, c and V of n=5 type Sr1⫺YCaYNbO3.41 as a function of the Ca content Y (see also Tables 11, 12 and 14). One may expect that the lattice constants and the unit cell volume also influence the physical properties. For example, the resistivity and its temperature dependence of the both n=5 type niobates SrNbO3.41 (Y=0) and CaNbO3.41 (Y=1) are quite different as shown in Figs. 19 and 23.

F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

43

Fig. 13. Lattice constants a, b, c and the corresponding unit cell volume V of orthorhombic n=5 type Sr1⫺YCaYNbO3.41 as function of the Ca content Y.

3.1.2. Compositions based on CaNbOX Attempts to prepare single phase n=4.333 type compositions (ideal oxygen content X=3.462), which are known for titanates, were unsuccessful. Examples are the following. CaNbO3.47 displays a powder x-ray diffraction (XRD) pattern, which suggests the concurrent presence of n=4.333 and n=4.5 type phases. Ca0.95La0.05NbO3.46 (see Table 10) was identified as a single phase n=4.5 type compound by means of XRD. The following refers to several of the compounds presented in Tables 5, 10, 11, 14 and 15. In contrast to the SrNbOX based compositions, all plate-like crystals which were

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F. Lichtenberg et al. / Progress in Solid State Chemistry 29 (2001) 1–70

Fig. 14. Powder x-ray diffraction pattern of some compounds related to (La,Ce)TiOX and CaNbOX. It shows the position of the (0 0 L) peak in the range of small diffraction angles 4°⬍2q⬍7° for different structure types n. The indexing of the peaks is focussed on the small-angle peaks whose (H K L) are given in large numbers, the other peaks are indexed with numbers of smaller size. The c-axes of the compounds are provided in relationship to L of (0 0 L).

subjected to Laue diffraction revealed an inclination of the a-axis with respect to the cylinder axis of the as-grown sample. The n=4 type niobates Ca1⫺YLaYNbO3.5 remain single phase at least up to Y=0.2. For Y=0.25, the powder x-ray diffraction pattern displays additional peaks which do not belong to the n=4 type structure. This statement about the homogeneity range refers to samples prepared by floating zone melting. For the composition with Y=0.2, the following phenomenon was observed during the floating zone melting process. As the crystallized material passed slowly through the temperature gradient, large cracks arose. These cracks seemed to work through from the inner to the outer part of the sample resulting in large cavities and poor quality of the grown crystals. Maybe a phase transition at high temperatures was responsible for this phenomenon. However, a similar behavior was not observed for any other compositions shown in Tables 4–15, even for compounds where structural phase transitions at high temperatures are known, like for LaTiO3.50. In a previous work, the composition CaNbO3.35 was grown by floating zone melting [22]. Structural investigation by transmission electron microscopy (TEM) indicated a n=5 type structure with monoclinic symmetry and doubled a-axis, i.e. ˚ =7.8 A ˚ [22]. The n=5 type homogeneity range of CaNbOX was suggested a⬇2×3.9 A to be 3.35ⱕXⱕ3.42, whereas the corresponding ranges for SrNbOX and LaTiOX were established as 3.40ⱕXⱕ3.42 for samples synthesized by floating zone melting

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45

Fig. 15. Thermogravimetric (TG) behavior of the oxidation of reduced (La,Ce)-based titanates. The specimen were heated in air with a rate of 10°C/min from room temperature to 1000°C. Shown is the relative specimen mass, (m+⌬m)/m, as function of temperature T. The saturation at higher temperatures indicates the presence of fully oxidized compositions involving Ti4+ and Ce4+. The total mass increase was used to calculate the oxygen content X of the specimen. For this a constant cation ratio A/B=(La,Ce)/Ti=1 was assumed. Consistent interpretation of the TG and powder x-ray diffraction data were obtained for CeTiO3.47 and LaTiO3.41. For Ce0.5La0.5TiOX, however, the oxygen content results in X=3.55 (3.30) assuming a Ce oxidation state of Ce4+ (Ce3+) in the range where the saturation takes place. This is in contrast to its n=5 type structure which has an ideal oxygen content of X=3.40 (see text for a discussion of this discrepancy).

[21,22]. During this work, several polycrystalline and melt-grown CaNbOX samples with 3.33ⱕXⱕ3.39 were prepared in order to investigate the n=5 type homogeneity range in more detail. The polycrystalline specimen were synthesized by heating pressed powder mixtures according to the steps 1–3 as described in Section 2.1. Structural analysis by powder x-ray diffraction (XRD) indicated the presence of ˚ ). For X=3.39 a single orthorhombic n=5 type structures with simple a-axis (a =3.88 A phase n=5 type compound was observed. For Xⱕ3.38 additional peaks appeared, which could be assigned to the perovskite CaNbO3. With decreasing oxygen content X the intensity of the CaNbO3 peaks increased, but the peak positions and lattice constants of the n=5 type phase did not show any significant change. These results on polycrystalline samples with orthorhombic symmetry and simple a-axis suggest a lower bound of X=3.39 for the n=5 type homogeneity range. Also the melt-grown samples showed indications for a multiphase composition for X⬍3.39, but this was not so clearly visible in the XRD pattern as for the polycrystalline specimen. Thermogravimetric analysis and magnetic measurements, however, revealed an inhomogenous distribution of the oxygen content X over the sample and the presence of Nb. Therefore a lower bound of X=3.39 or X=3.40 for the n=5 type homogeneity range is also suggested for the melt-grown samples. This is in contrast to the melt-grown sample with X=3.35 from the previous work mentioned, where the structural examination by TEM indicated a monoclinic symmetry and a doubled a-axis. Most probably the TEM results were not representative for the bulk properties regarding the lower bound of the n=5 type homogeneity range.

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Fig. 16. Sketch of non-stoichiometric homogeneity ranges with respect to the oxygen content X of AnBnO3n+2=ABOX type niobates and titanates as suggested from structural analysis by powder x-ray diffraction.

3.1.3. Compositions based on (La,Ce)TiOX The following refers to several of the compounds presented in Tables 6, 8, 9, 13 and 15. Plate-like crystals of n=5 type LaTiO3.4 subjected to Laue diffraction did not reveal a clear tendency regarding the orientation of the a-axis. Some samples showed an a-axis parallel to the cylinder axis of the as-grown sample, whereas some other platelets displayed an a-axis inclined to that axis. The majority of the titanates crystallize in a monoclinic form, in contrast to the niobates which are orthorhombic in the most cases. Fig. 14 shows the powder x-ray diffraction pattern of n=4, 4.33, 4.5 and 5 type titanates and n=6 type CaNb0.8Ti0.2O3.33 in the range of small diffraction angles. The position of the (0 0 L) peak indicates the structure type n. It was possible to prepare the n=4 type compound LaTi0.8Nb0.2O3.51, whereas in ref. [26] it is said that a substitution of Ti by Nb in LaTiO3.5 could not be achieved. Titanates involving Ce display some peculiarities, because Ce may appear in the two different oxidation states Ce3+ and Ce4+. In most ternary or higher oxides Ce3+

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47

Fig. 17. Log-linear plot of the resistivity r versus temperature T of n=4 type Sr0.8La0.2NbO3.50 along the a-, b- and c-axis. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided.

is preferred, whereas for synthesis purposes CeO2 (Ce4+) is the most common starting material. For the preparation of polycrystalline n=4 type CeTiO3.5 involving Ce3+ and Ti4+, a heat treatment at 1400°C under argon using a mixture of CeO2, TiN and TiO2 was reported (see Table 6). During this work, Ce containing titanates were synthesized by floating zone melting using air as oxidizing atmosphere, or 98% argon +2% hydrogen as reducing ambience. Only rods with fully oxidized compositions involving TiO2 (Ti4+) and CeO2 (Ce4+) were used. The oxidation state of Ce in the samples grown in this way was suggested to be Ce3+, as indicated by the crystal structure type and/or by the oxygen content X. Thus Ce4+ (CeO2) converts to Ce3+ even under an oxidizing atmosphere like air, if the compositions are molten at high temperatures. The n =4 type compound La0.67Ce0.33TiO3.50 involving Ce3+ and Ti4+, for example, was obtained in this way (see Table 6). Under reducing ambience a single phase n=4.33 type material, CeTiO3.47, involving Ce3+ and mixed-valence Ti could be grown. Only a few n=4.33 type bulk compounds are known, all of them being titanates (see Table 8). Also a sample with composition Ce0.5La0.5TiOX was obtained under the reducing

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Fig. 18. Log-linear plot of the resistivity r versus temperature T of n=4.5 type Sr0.96Ba0.04NbO3.45 along the a-, b- and c-axis. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided. The linear-linear type inset displays the presence of metallic behavior (i.e. dr/dT⬎0) along the a-axis which is not so clearly visible at the log-linear scale.

atmosphere. Powder x-ray diffraction clearly revealed a single phase n=5 type titanate. However, there was no straightforward way to determine its oxygen content X by thermogravimetric analysis for this composition. Fig. 15 shows the thermogravimetric behavior of the oxidation of Ce0.5La0.5TiOX in comparison to CeTiO3.47 and LaTiO3.41. It was assumed that Ce reaches an oxidation state of Ce4+ at high temperatures where the saturation appears. For CeTiO3.47 and LaTiO3.41 this leads to a consistent interpretation of the results of powder x-ray diffraction and thermogravimetric analysis. For Ce0.5La0.5TiOX, however, the oxygen content results in X=3.55 (3.30) assuming a Ce oxidation state of Ce4+ (Ce3+) in the range where the saturation is reached. This is in contrast to its n=5 type structure with an ideal oxygen content of X=3.40. Concerning this, the following interpretations are conceivable:

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1. The n=5 type sample Ce0.5La0.5TiOX contains Ce3+, but its thermogravimetric oxidation up to 1000°C was not sufficient to reach the highest Ce valence of Ce4+. A separate themogravimetric experiment up to temperatures of 1200°C, however, revelead a decreasing specimen mass above 1000°C. This suggests that the highest possible oxidation was already reached around 1000°C. Therefore this interpretation seems to be unlikely. 2. The n=5 type sample Ce0.5La0.5TiOX contains Ce3+ but its thermogravimetric oxidation up to temperatures of 1000°C results in a composition involving mixed valence Ce. A formal oxidation state of about Ce3.5+ in the oxidized sample leads to an oxygen content of X⬇3.4 which is in agreement with the n=5 type structure. This seems to be the most likely interpretation. 3. The n=5 type sample Ce0.5La0.5TiOX contains Ce in a mixed valence state Ce3+/Ce4+ and the thermogravimetric oxidation leads to a composition in which all Ce cations are in the Ce4+ state. In this case, the gain of the specimen mass due to the oxidation is smaller compared to the assumption that the sample Ce0.5La0.5TiOX includes only Ce3+. This interpretation agrees qualitatively with the thermogravimetric results. On the other hand, the temperature dependence of the magnetic susceptibility c of Ce0.5La0.5TiOX is similar to other titanates involving Ce3+ and the magnitude of c lies between that of compounds with 100% Ce3+ and 33% Ce3+ at the A site (see Fig. 33). This might indicate the exclusive presence of Ce3+ in Ce0.5La0.5TiOX, which supports interpretation 2. Note that c of the Ce containing titanates (see Fig. 33) is governed by the large paramagnetic contribution of Ce3+, otherwise the magnitude of c would be much smaller. Further studies are necessary to clarify this subject. 3.1.4. Attempts to prepare series members with n⬍4, 5⬍n⬍6 and nonstoichiometric materials ABOX bulk compounds of the AnBnO3n+2 type are known for n=4, 4.33, 4.5, 5 and 6 (see also Tables 1–15). In a previous work a hypothetical n=3 type composition LaTi0.67Nb0.33O3.67 was prepared by floating zone melting [24]. It melted incongruently and was multiphase. Nevertheless, transmission electron microscopy revealed a minority phase which showed disorded and ordered intergrowths of n=3 and n=4 type layers [24]. Table 17 shows the results of several attempts to synthesize n=3, 3.5 and 5.5 type compounds by floating zone melting. All of the hypothetical n=3 type compositions ABO3.67 melted incongruently and were found to be multiphase. The floating zone melting of compositions with a hypothetical n=3.5 type structure (X=3.57) and with even higher oxygen content (X=3.60), however, resulted in samples which were often identified as single phase n =4 type compounds by powder x-ray diffraction. Fig. 12 displays the example Sr0.8La0.2NbO3.60 in comparison to niobates having an ideal oxygen content of X=3.50. These observations suggest a large overstoichiometric homogeneity range for some n=4 type phases with respect to the oxygen content X. It seems to resemble to the n=⬁ type LaTiOX with a three-dimensional perovskite structure. Its homogeneity range was found to be extraordinarily large, namely

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Fig. 19. Log-linear plot of the resistivity r versus temperature T of n=5 type SrNbO3.41 along the a-, b- and c-axis. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided.

3.00ⱕXⱕ3.20 as displayed in Fig. 6 whereby the unit cell volume V diminishes with increasing oxygen content X [21]: n ⫽ ⬁ type LaTiOX V ⫽ 250 A˚ 3 for X ⫽ 3.00 (ideal oxygen content) V ⫽ 243 A˚ 3 for X ⫽ 3.20 (maximum overstoichiometry) This volume change hints to a structural implementation of the overstoichiometry by cation deficiency, which can be expressed by normalizing the formula LaTiOX with 3.00ⱕXⱕ3.20 to its ideal oxygen content X=3.00, i.e. LaYTiYO3 with 1ⱕYⱕ0.94. This has to be compared with the n=4 type niobates. For Sr0.8La0.2NbOX with X=3.50 and X=3.60 the unit cell volume V is directly available (see Tables 4 and 5). The value for X=3.57 can be also obtained from Table 5 by using the average ˚ 3) and Sr0.86La0.14NbO3.57 V of the adjacent compositions Sr0.75La0.25NbO3.57 (V=599 A 3 ˚ (V=601 A ), thus

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n ⫽ 4 type Sr0.8La0.2NbOX V ⫽ 598 A˚ 3 for X ⫽ 3.50 (ideal oxygen content) V ⫽ 600 A˚ 3 for X ⫽ 3.57 (overstoichiometric) V ⫽ 602 A˚ 3 for X ⫽ 3.60 (overstoichiometric) Compared to LaTiOX the relative volume change is smaller and of opposite direction, i.e. the lattice expands upon oxidation. This suggests that the excess oxygen is located at interstitial sites or is intercalated between the layers. In the case of (Ca,La)NbOX there are indications for a smaller homogeneity range, because a sample with X=3.57 was identified as single phase, whereas a composition with X=3.60 consists of two phases (see Table 17). The Ca-based niobates possess smaller unit cell volumes than their Sr-based counterparts. Therefore the Ca-based materials could have less capacities to incorporate the excess oxygen. Also the n=5 type phases SrNbOX, LaTiOX and CaNbOX have an overstoichiometric homogeneity range, even though its extent 3.40ⱕXⱕ3.42 is smaller than that of the n=4 and n=⬁ types just mentioned [21,22]. The reason for that is certainly the following. The oxidation of a n=5 type compound (ideal X=3.40) leads quickly to a formation of a n=4.5 type phase (ideal X=3.44) because the difference between these both oxygen contents X is relatively small. Therefore it is impossible for n=5 type structures to have a large overstoichiometric homogeneity range. The oxidation of a n=4 type compound (ideal X=3.50), however, may take place within the same structure type up to an unknown limit because there were no indications for a formation of a n=3.5 type phase (ideal X=3.57) found. Therefore the n=4 type phases may have a larger overstoichiometric homogeneity range compared to its n=5 type counterparts. For the hypothetical n=5.5 type composition La0.73Sr0.27TiO3.36 powder x-ray diffraction indicates a n=6 type structure (see Table 17). The ideal oxygen content of the n=6 type structure is X=3.33. Therefore the existence of n=6 type phases having a significant overstoichiometry with respect to its ideal oxygen content is suggested. For the hypothetical n=5.5 type composition La0.89Ca0.11TiO3.36 powder x-ray diffraction indicates a n=5 type structure (see Table 17). The ideal oxygen content of the n=5 type structure is X=3.40. Thus the existence of n=5 type phases having a significant understoichiometry with respect to its ideal oxygen content is suggested. As yet, non-stoichiometric compositions with respect to the oxygen content X were discussed. Materials which are non-stoichiometric with respect to the A site are known for n=⬁ types, namely SrYNbO3 with 0.75ⱕYⱕ0.95 [68] and LaYTiO3 with 0.67ⱕYⱕ1 [69–71]. That raises the question if also n⬍⬁ types with a cation deficiency at the A site exist. Several of such AYBOX compositions with Y⬍1 were synthesized during this work. The results are summarized in Table 18. Indeed, several single phase compounds with a cation deficiency at the A site were obtained. Note that some of these single phase oxides are simultaneously understoichiometric with respect to the oxygen content X (see Table 18). That hints to the existence of many materials with a great variety of non-stoichiometric composition. Sr0.93NbO3.36 was identified as a single phase n=5 type niobate by powder x-ray diffraction, however the results of thermogravimetric oxidation hint to a presence of a second phase in small amounts. The thermogravimetric curves show a weak increase in the region of saturation, which indicates that

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small partitions of the samples were not completely oxidized (the specimen were heated in air with a rate of 10°C/min up to a temperature of 1000°C). This is in accordance with the observation that small partitions of the oxidized specimen display a black color. Therefore it seems that among the prepared AYBOX compositions Y⬇0.93 is the smallest value which ensures a single phase material. All these results show that the synthesis of bulk compounds having a structure beyond the known types n=4, 4.33, 4.5, 5 and 6 seems to be difficult. However, for the known structure types n a further possibility to modify its composition was found, namely the creation of a distinct non-stoichiometry with respect to its ideal oxygen content X =3+2/n and ideal cation ratio A/B=1. In Fig. 16 an overview of suggested non-stoichiometric homogeneity ranges is sketched. 3.2. Electric properties Already in a previous work resistivity measurements performed on LaTiO3.42, SrNbO3.45 and CaNbO3.35 crystals were reported [22]. A semiconducting behavior along and perpendicular to the layers was found, but the direction of the current along the layers was arbitrarily selected and not specified [22]. In that previous work, the electrical contacts were prepared using indium or gold-beryllium wires and silver paint [22]. In the present study the same kind of electrical contacts were used for the first attempts of measuring the resistivity along the a- and b-axis. However, only semiconducting behavior and no anisotropy with respect to the a- and b-axis was observed. This outcome stood in contrast to the results from photoemission, optical spectroscopy and band structure calculations, which revealed a behavior differing strongly between the a- and b-axis, as already mentioned above [31–34]. Therefore, the voltage contacts were improved in two ways. First, the wires were attached with a conductive silver epoxy kit instead of with silver paint. Second, aluminum wires were bonded ultrasonically. Resistivity measurements using the first technique indeed revealed a clear anisotropy with respect to the a- and b-axis and even indications for a metallic behavior along the a-axis. However, these contacts were difficult to prepare and showed a poor quality concerning scattering and reproducibility of the measured data. The second method turned out to be the best, and was therefore used for all measurements. Depending on the composition of the crystals (see Figs. 17– 24), the upper bounds of the contact resistance of the voltage contacts were found to be in the range from 5 ⍀ to 250 ⍀. The contact resistances were obtained from resistance measurements in a two-point configuration. Resistivity measurements were performed on rectangular plate-like crystals of 8 different compounds: Sr0.8La0.2NbO3.50 (n=4), Sr0.96Ba0.04NbO3.45 (n=4.5), SrNbO3.41 (n=5), Sr0.965La0.035NbO3.41 (n=5), Sr0.9La0.1NbO3.41 (n=5), Sr0.95NbO3.37 (n=5), CaNbO3.41 (n=5) and LaTiO3.41 (n=5). Figs. 17–24 show the measured resistivity r as function of temperature T between T=300 K and T=4 K along the a-, b- and caxis. Obviously, these oxides are highly anisotropic conductors. Along the b- and c-axis they behave like semiconductors. Along the a-axis the temperature dependence of the resistivity is quite complicated. In parts of the temperature range the compounds investigated are quasi-1D metals, whereas at lower temperatures a metal-

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Fig. 20. Log-linear plot of the resistivity r versus temperature T of n=5 type Sr0.965La0.035NbO3.41 along the a-, b- and c-axis. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided.

semiconductor transition appears. SrNbO3.41 deviates somewhat from this picture because a weak metallic temperature dependence of the resistivity is also observed along the b-axis (see Fig. 19). Angle-resolved photoemission at T=75 K and optical spectroscopy at T=5, 50, 100, 150, 200, 250 and 300 K on SrNbO3.41 by Kuntscher et al., however, revealed quasi-1D metallic features along the a-axis and semiconductor-like properties along the b-axis [32–34]. Figs. 17–24 display that the temperature of the metal-semiconductor transition ranges from ⬇50 K for SrNbO3.41, Sr0.95NbO3.37 and LaTiO3.41 to ⬇180 K for CaNbO3.41. An increasing La content in the n=5 type niobates (Sr,La)NbO3.41 leads to a decreasing resistivity r (see Figs. 19–21) and for Sr0.9La0.1NbO3.41 the metalsemiconductor transition is almost completely suppressed (see Fig. 21). Among the 8 different compounds on which resistivity measurements were carried out, CaNbO3.41 displays a decreasing resistivity below T⬇17 K along the a-axis and below T⬇11 K along the b- and c-axis (see Fig. 23). As can be seen in Fig. 27, the magnetic susceptibility of CaNbO3.41 does not show any evidence for anomalous properties in

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Fig. 21. Log-linear plot of the resistivity r versus temperature T of n=5 type Sr0.9La0.1NbO3.41 along the a-, b- and c-axis. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided.

this temperature range. This strange resistivity behavior is at least partly in agreement with results from a previous work on CaNbO3.35, for which a saturation of the resistivity along the layers was found at low temperatures [22]. The semiconducting behavior of the resistivity r along the a-, b- and c-axis was inspected if it can be fitted to a thermally activated behavior r⬀exp(⌬/T). In parts of the temperature range a good fit was found. The corresponding activation energies ⌬ are often very small, especially for temperatures below that of the metal-semiconductor transition, and vary from 0.5 to 58 meV as shown in Figs. 17–24. In these figures, the activation energies ⌬ are provided with the corresponding temperature range for which the fit r ⬀ exp(⌬/T) was used. A small activation energy or energy gap along the a-axis at temperatures below that of the metal-semiconductor transition was not only observed in resistivity measurements. This was shown for SrNbO3.41 which is presently the most intensively studied AnBnO3n+2 type compound. High-resolution angle-resolved photoemission along the a-axis at T=25 K and optical spectroscopy between T=5 K and T=300 K

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Fig. 22. Log-linear plot of the resistivity r versus temperature T of significantly non-stoichiometric n=5 type Sr0.95NbO3.37 along the a-, b- and c-axis. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided.

with polarization of the incident light along the a- and b-axis by Kuntscher et al. revealed a small energy gap of ⬇5–8 meV [33]. This is similar to ⌬a=3.3 meV obtained from the resistivity ra⬀exp(⌬a/T) in the tempearture range 20 KⱕTⱕ40 K (see Fig. 19). In the same temperature range, nuclear magnetic resonance (NMR) measurements by Weber et al. also indicated an activated behavior of the spin-lattice relaxation rate with an energy gap of 6.5 meV [37]. During this work the semiconducting resistivity behavior along the a-, b- and caxis was exclusively considered with respect to a thermal activation r⬀exp(⌬/T). However, dielectric measurements along the c-axis on Sr0.8La0.2NbO3.50, SrNbO3.45 and SrNbO3.41 by Bobnar et al. [36] as well as NMR and electron paramagnetic resonance (EPR) experiments on SrNbO3.41 by Weber et al. [37] point to a further process of charge transport. Among other things, the dielectric measurements on Sr0.8La0.2NbO3.50 and SrNbO3.41 indicate that the resistivity rc along the c-axis behaves rather like rc⬀exp[(T0/T)1/2] than rc⬀exp(⌬/T), at least for temperatures

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Fig. 23. Log-linear plot of the resistivity r versus temperature T of n=5 type CaNbO3.41 along the a-, b- and c-axis. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided. The linear-linear type inset displays the presence of metallic behavior (i.e. dr/dT⬎0) along the a-axis which is not so clearly visible at the log-linear scale.

T⬍100 K [36]. For T⬍20 K the NMR spin-lattice relaxation rate and EPR line width of SrNbO3.41 displays the same temperature dependence (for T⬎20 K the NMR spinlattice relaxation rate shows an activated behavior as already mentioned above) [37]. The results of the dielectric measurements along the c-axis and the NMR and EPR experiments suggest a charge transport by hopping of localized charge carriers like small polarons, at least at low temperatures [36,37]. The most striking electronic property of the materials investigated is the presence of a quasi-1D metallic conductivity along the a-axis (see Figs. 17–24). This is in accordance with the results from angle-resolved photoemission (ARPES) along the a- and b-axis by Kuntscher et al. on SrNbO3.45 at T=150 K, SrNbO3.41 at T=75 K, Sr0.9La0.1NbO3.39 at T=75 K and 100 K [31,32,34]. It is furthermore in agreement with the outcomes of optical spectroscopy with polarization of the incident light along the a- and b-axis by

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Fig. 24. Log-linear plot of the resistivity r versus temperature T of monoclinic n=5 type LaTiO3.41 along the a- and b- axis as well as perpendicular to the layers. The activation energies ⌬ in meV indicate that parts of r(T) can be fitted to a thermally activated behavior r⬀exp(⌬/T), the corresponding temperature ranges are also provided.

Kuntscher et al. on SrNbO3.41 at T=5, 50, 100, 150, 200, 250 and 300 K [33]. Also band structure calculations on SrNbO3.4 revealed significant electronic dispersion near the Fermi energy only along the a-axis but not along the b-axis [31,32,35]. ARPES on Sr0.8La0.2NbO3.50 at T=75 K revealed only a weak electronic dispersion along the a-axis [32,34]. This is in qualitative agreement with the resistivity behavior of Sr0.8La0.2NbO3.50 which shows only a weak metallic character along the a-axis (see Fig. 17). Is is known that quasi-1D metals are inclined to structural instabilities (Peierls transition, charge density wave) or electronic instabilities. At these transitions an energy gap arises and the electronic state changes from metallic to semiconducting, which resembles the temperature-driven metal-semiconductor transitions along the a-axis shown in Figs. 17–24. Among the materials on which resistivity measurements were carried out (see Figs. 17–24) there are some compounds like SrNbO3.41 where the metal-semiconductor transition takes place at very low temperatures (T⬇50 K)

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Fig. 25. Distortions of the BO6 octahedra of n=4, 5 and 6 type compounds whose atomic coordinates are known. The distortion is defined as [(largest–smallest) B-O distance]/[average B-O distance]. The percentage values of these distortions are shown as bold numbers. They were calculated from the atomic coordinates at room temperature presented in the references of Table 16. The numbers of different B-O bond lengths are given in parenthesis. The layers consisting of corner-shared BO6 octahedra are sketched at the left side. The simple sketch of the BO6 octahedra does not reveal any distortion and for clarity the A cations are omitted. A bold number in a line displays the distortion of the octahedra located in the same line. Bold numbers which are presented in two columns indicate different distortions for adjacent octahedra along the a-axis. This Figure clearly reveals that octahedra located at the boundary of the layers possess larger distortions compared to those residing inside. Note that for the n=4 type SrTaO3.50 the distortions refer to the paraelectric phase, whereas all other n=4 type compounds are ferroelectrics at room temperature.

and the energy gap in the semiconduting state is very small (a few meV), compared to most of the other known quasi-1D metals. The intriguing electronic features of Sr-based niobates are discussed in detail by Kuntscher et al. [31–34]. NMR experiments on SrNbO3.41 by Weber et al. at low temperatures hint to the presence of a charge density wave [37]. The present results from the investigations of the electronic and magnetic properties call for further studies, especially for the clarification of the nature of the metal-semiconductor transitions. A further remarkable feature of the layered AnBnO3n+2=ABOX type niobates and titanates is the structural, compositional and electronical proximity between quasi-1D metals and non-conducting (anti)ferroelectric series members. Some of these layered (anti)ferroelectrics can be directly transformed into quasi-1D metals by creating charge carriers using appropriate substitutions at the A or B site. The most obvious examples are (see Tables 4, 12 and 14 and Figs. 5, 17, 19–21):

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Fig. 26. Molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of several SrNbOX type compounds. The layers of the crystalline samples were aligned parallel to the magnetic field.

layered n=4 type: high-Tc ferroelectric insulator SrNbO3.5 (4d0) → weakly metallic quasi-1D-like conductor Sr0.8La0.2NbO3.5 (4d0.2) layered n=5 type: antiferroelectric insulator (Sr,La)NbO3.4 (4d0.2⫺4d0.3)

SrNb0.8Ti0.2O3.4

(4d0)

→

quasi-1D

metal

Usually (anti)ferroelectrics display a high dielectric permittivity or polarizability. An example can be found in Fig. 5 where the dielectric permittivity of ferroelectric SrNbO3.50 along the a-, b- and c-axis is shown over a large temperature range. As already mentioned, these (anti)ferroelectrics are structurally, compositionally and electronically in proximity to the quasi-1D metals. Therefore, the structurally and electronically low-dimensional AnBnO3n+2=ABOX type materials could allow the possibility to create systems with an intrinsic coexistence of high dielectric polarizability and metallic conductivity. Such systems were considered as candidates for

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excitonic superconductivity [72]. An experimental hint for the realization of a simultaneous existence of metallic conductivity and high polarizability can be found in SrNbO3.41. Dielectric measurements along the c-axis by Bobnar et al. revealed the high-frequency dielectric permittivity ⑀c ⬁ in the temperature range 2 KⱕTⱕ70 K and relatively large values of ⑀c ⬁⬇100 were found [36]. In a part of this temperature range, namely 60 KⱕTⱕ70 K, a metallic resistivity behavior was measured along the a-axis (see Fig. 19). For temperatures T⬎70 K it was impossible to determine ⑀c ⬁ because of a high inductance contribution in the circuit [36]. However, it may be expected that the relatively large ⑀c ⬁ persists also at higher temperatures where the resistivity along the a-axis is metallic up to T=130 K (see Fig. 19). Band structure calculations by Bohnen [31] as well as by Winter [35] on the n=5 type niobate SrNbO3.4 suggested that the quasi-1D metallic features are related to the subtleties of the distortions of the NbO6 octahedra [31,32,35]. Some of these calculations were done on the real structure involving the distorted octahedra and on a hypothetical structure in which the distortions had been removed artificially [31,32]. Two results of these calculations are especially remarkable. First, the density of electronic states at the Fermi energy is mainly given by those Nb atoms belonging to the less distorted octahedra located in the middle of the layers [31,32,35]. Second, the distortion-free system is more of a quasi-2D type, indicating a close connection between the quasi-1D behavior and the characteristic distortions of the NbO6 octahedra [31,32]. This outcome calls for a detailed inspection of the BO6 octahedra distortions in several AnBnO3n+2=ABOX type materials. Such an inspection requires the atomic positions and B-O bond lengths to be known exactly. This is the case for the compounds shown in Table 16. The corresponding atomic coordinates and B-O bond lengths can be found in the references given in Table 16. These data and the definition BO6 octahedra distortion = [(largest - smallest) B-O distance]/[average B-O distance] were used to establish Fig. 25 which displays the distortions in several niobates and titanates. For the limited number of compounds listed, Fig. 25 reveals that the BO6 octahedra in the middle of the layers are less distorted than those located at the boundary. Furthermore, it is conspicuous that the n=5 and n=6 type materials display a relative large variation of the distortion along the c-axis compared to their n=4 type counterparts (the only exception is SrTaO3.50 which is paraelectric, whereas all other n=4 types are ferroelectrics at room temperature). Especially for the n=5 type SrNbO3.41 the “distortion gradient” is very large involving nearly non-distorted NbO6 octahedra in the middle of the layers (see Fig. 25). Figs. 17 and 19 present the resistivity r(T) of the n=4 type Sr0.8La0.2NbO3.50 and n=5 type SrNbO3.41, respectively. Both of these niobates have a formal electron configuration of 4d0.2 per Nb. Therefore these compounds may be used to compare the electric properties between the structure types n=4 and n=5. Figs. 17 and 19 reveal that the n=5 type niobate shows a larger anisotropy with respect to the a- and baxis, a lower resistivity and a more pronounced metallic character in the resistivity behavior than its n=4 type counterpart. Maybe these differences are related to the characteristic distortions of the NbO6 octahedra. The n=5 type niobate involves a

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relatively strong “distortion gradient” with nearly non-distorted NbO6 octahedra in the middle of the layers (see Fig. 25). Assuming that the NbO6 octahedra distortions of the insulating SrNbO3.50 (see Fig. 25) are also representative for the electrical conductor Sr0.8La0.2NbO3.50 , then the n=4 type niobate involves a relatively small “distortion gradient” with relatively large distortions (see Fig. 25). 3.3. Magnetic properties Figs. 26–32 show the molar magnetic susceptibility c of many compounds in low magnetic fields (50 GⱕHⱕ1000 G) as a function of temperature T. The layers of the crystalline samples were aligned parallel to the magnetic field. Some attempts were performed to measure the susceptibility for different orientations of the layers relative to the magnetic field. No significant influence of the orientation with respect to the temperature dependence was observed. Therefore it seems that the orientation does not play a crucial role. As expected, the insulating ferroelectrics SrNbO3.50, CaNbO3.50 and LaTiO3.50

Fig. 27. Molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of several CaNbOX type compounds. The layers of the crystalline samples were aligned parallel to the magnetic field.

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Fig. 28. Molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of several LaTiOX type compounds. The layers of the crystalline samples were aligned parallel to the magnetic field.

exhibit a nearly temperature-independent diamagnetism of the ionic cores (see Figs. 26–28). The increase of c(T) at low temperatures is indicative for a Curie- or CurieWeiss-like behavior of impurities and/or defects. For the niobates this effect is small compared to LaTiO3.50, which shows a relatively strong increase. The latter could result from the use of TiO2 (La2O3) as starting material which may have contained small concentrations of strongly paramagnetic impurities like Fe3+ (rare earths like Ce3+, Pr3+ and Nd3+). Compared to the ferroelectric insulators, the electrical conducting niobates and titanates exhibit a distinct temperature dependence in c(T). As expected, the susceptibility rises with an increase of the formal charge carrier concentration obtained from charge neutrality (see Figs. 26–32), especially for compounds with the same structure type and similar composition like the n=5 type Sr-based niobates shown in Fig. 32. The titanates with their 3d electrons have a higher magnetic susceptibility than the niobates, for which the charge carriers are given by 4d electrons. This is in accordance with the fact that 3d electrons display stronger electronic correlations than the

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Fig. 29. Molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of miscellaneous materials. Note that La0.89Ca0.11TiO3.36 and Sr0.75La0.25NbO3.57 are significantly non-stoichiometric compounds. The layers of the crystalline samples were aligned parallel to the magnetic field.

spatially more extended 4d electrons. The enhanced correlations result in a higher magnetic susceptibility. A characteristic property of the majority of the electrical conducting niobates and titanates is the presence of a minimum in c(T) (see Figs. 26–32). The increase of c(T) at low temperatures is indicative for a Curie- or Curie-Weiss-like behavior of impurities, defects and/or paramagnetic moments located at regular lattice sites like Ti3+ and Nb4+ which result from the mixed-valence composition of the compounds. The latter possibility was ruled out for SrNbO3.41, because a Curie-like behavior belonging to Nb4+ was not observed in electron paramagnetic resonance (EPR) experiments by Weber et al. [37]. This could imply, for example, a dimerization into pairs of Nb4+ with opposite spin or that every Nb4+ has transferred its 4d electron into bands leaving all Nb in a Nb5+ state. Some of the starting materials, Nb2O5 and SrCO3, were investigated by EPR and traces of Fe3+ and Mn2+ impurities were detected [37]. Note that the valence and therefore the magnetic moment of the impurities may change during the preparation process. For most of the compounds

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Fig. 30. Molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of n=4 type niobates. Note that Sr0.85La0.1NbO3.45 and Sr0.65Ba0.1La0.25NbO3.57 are significantly non-stoichiometric compounds. The layers of the crystalline samples were aligned parallel to the magnetic field.

shown in Figs. 26–32 it seems likely that the low-temperature behavior of c(T) is not related to the intrinsic features of the samples, but is due to impurities or defects. The special case of electrical conducting materials which do not display a minimum in c(T) will be briefly discussed later. In a simple approach the following contributions to the magnetic susceptibility c(T) may be assumed: 1. temperature independent diamagnetism of the ionic cores 2. Curie- or Curie-Weiss-like behavior c⬀1/(T⫺⌰) from defects, impurities and/or paramagnetic moments loacted at regular lattice sites → c diminishes with increasing T 3. (nearly temperature independent) paramagnetism of itinerant electrons in the conduction band 4. thermal activation of electrons into the conduction band, i.e. c⬀exp(⫺⌬/T) → c rises with increasing T

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Fig. 31. Molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of n=4.5 type niobates. The layers of the crystalline samples were aligned parallel to the magnetic field.

A combination of (2) and (4) can lead to a minimum in c(T). For the n=5 type SrNbO3.41 this simple model is discussed by Weber et al. in comparison to the Bonner–Fisher model for the magnetic susceptibility of a quasi-1D spin-chain, including its extension for delocalized electrons [37]. For temperatures T above ⬇100 K a well description of the magnetic susceptibility c(T) of SrNbO3.41 in terms of a quasi-1D spin chain was found [37]. This suggests that the enlargement of c(T) with increasing T, which is observed in many compounds at elevated temperatures (see Figs. 26–32), is related to their quasi-1D electronic features. In contrast to the majority of the conducting AnBnO3n+2=ABOX type materials investigated, there are also compounds for which c(T) diminishes with increasing T in the whole temperature range (see Figs. 28, 29, 31 and 33). Fig. 33 displays the inverse of c(T) of some of these compounds with a relatively large magnitude of c(T), like Ce-containing titanates and oxides involving the concurrent presence of Ti and Nb at the B site. Also in parts of the high-temperature range a linear Curieor Curie–Weiss-like behavior is observed. This indicates the presence of localized paramagnetic moments like Nb4+, Ti3+ or Ce3+ at high temperatures (see Fig. 33)

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Fig. 32. Molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of n=5 type niobates. Note that Sr0.95NbO3.37 is a significantly non-stoichiometric compound. The layers of the crystalline samples were aligned parallel to the magnetic field.

which suggests the two following possibilities. First, the quasi-1D metallic features coexist with the presence of localized paramagnetic moments which dominate the magnitude and temperature behavior of c(T). Second, the quasi-1D metallic conductivity is totally lost and all charge carriers are either localized or occupy a completely filled band. For the ABOX compounds with B=(Ti,Nb) the second possibility could result from a statistical distribution of Ti and Nb at the electronically active B sites, which may lead to a disordered-induced localization of the charge carriers. Further studies are required to clarify this subject. 3.4. Special features referring to Tables 1–15 It can be inferred from Tables 2–15 that ferroelectricity and/or non-centrosymmetric space groups occur only for series members having an even number, i.e. for the n=2 type fluorides, the n=4 and n=6 type oxides. The presence of antiferroelectricity was reported only for a member having an uneven number, namely for the n=5

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Fig. 33. Inverse molar magnetic susceptibility c in low fields (Hⱕ1000 G) versus temperature T of miscellaneous compounds. To fit into this scale, the inverse susceptibilities of CaNb0.8Ti0.2O3.33 and LaTi0.8Nb0.2O3.51 were multiplied with a factor of 0.07 and 0.3, respectively. The calculated molar susceptibility of La0.5Ce0.5TiOX refers to X=3.40. The layers of the crystalline samples were aligned parallel to the magnetic field. In parts of the temperature range a linear Curie- or Curie–Weiss-like behavior appears, which suggests the presence of localized paramagnetic moments like Ti3+, Nb4+ or Ce3+.

type compound SrNb0.8Ti0.2O3.40 (see Table 14). This different dielectric behavior of the even and uneven series members seems to be a general feature. It calls for more dielectric investigations on fully oxidized materials of the uneven n=5 type. In this context, the question is raised for the dielectric properties of the fully oxidized non-integer n=4.33 and n=4.5 type compounds. To the best of our knowledge they are so far not known. It is conspicuous that bulk compounds of the n=6 type are only known for quaternary oxides (see Table 15), whereas bulk materials of the n=2, 4, 4.33, 4.5 and 5 type occur also in ternary systems (see Tables 2–13). Looking at Table 1 it is remarkable that bulk compounds of the Ruddlesden–Popper series Am+1BmO3m+1 are known up to m=3, whereas its AnBnO3n+2 type counterpart begins its existence just at the next higher integer n=4 (m or n is the number of BO6 octahedra within a layer along the c-axis). Both series differ in the arrangement of the corner-shared BO6 octahedra within the layers. Possibly, the particular arrangement in compounds of the AnBnO3n+2 type allows the stability of thicker layers. 4. Summary and conclusion In the work presented, perovskite-related layered AnBnO3n+2=ABOX type niobates and titanates, in particular electrical conducting compounds, have been investigated. It was known that these materials involve the following structural features:

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앫 앫 앫 앫

the framework of the layers consists of distorted corner-shared BO6 octahedra along the c-axis the layers are n BO6 octahedra thick along the a-axis the BO6 octahedra are connected like a chain there are non-integer n=4.5 and 4.33 type phases which represent thermodynamically stable bulk compounds in which a well-ordered stacking sequence of layers with different thickness is realized 앫 the structure type n can be tuned by adjusting the oxygen content X During this work more than 150 different compositions were prepared, many of them as single crystals and with tight control of the oxygen content X. Hereby it was found that single phase materials with many substitutions at the A and/or B site are possible. Attempts to synthesize compounds with a structure type n beyond the known types n=4, 4.33, 4.5, 5 and 6 were unsuccessful. For some of the known structure types n, however, distinct non-stoichiometric homogeneity ranges with respect to the oxygen content X and cation ratio A/B were found. Therefore, many possibilities exist to vary the structural, physical and chemical properties in these materials. In the course of this study, mixed-valence electrical conductors with a formal charge carrier concentration of ⱕ0.3 electrons per B cation were prepared. Resistivity measurements on single crystals of 8 different compositions along the a-, b- and c-axis between T=300 K and T=4 K revealed 앫 a highly anisotropic resistivity 앫 a semiconductor-like temperature dependence of the resistivity along the b- and c-axis 앫 a complex behavior of the resistivity along the a-axis involving quasi-1D metallic conductivity and, at lower temperature, a metal-semiconductor transition 앫 the temperature TMS at which the metal-semiconductor transition appears was found to be in range from TMS⬇50 K to TMS⬇180 K 앫 in parts of the temperature range the semiconductor-like resistivity seems to show a thermally activated behavior, whereby the corresponding activation energies are often very small, especially in the semiconducting state for T⬍TMS, and vary from 0.5 to 58 meV

These compounds represent a new group of quasi-1D metals which offer many possibilities to vary the structural, compositional and electronic properties. The complex electronic features of these materials call for further studies, e.g. the clarification of the origin and nature of the metal-semiconductor transition along the a-axis. Of particular interest is the structural, compositional and electronical proximity between the quasi-1D metals and (anti)ferroelectric compounds. Therefore, these structurally and electronically low-dimensional oxides may have the potential to create systems in which an intrinsic coexistence of high dielectric polarizability and metallic conductivity can be realized.

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Acknowledgements We thank R. Schulz, C. Schneider, G. Hammerl, B. Goetz and S. Weigel for their help, A. Reller and O. Becker for many interesting discussions and C. A. Kuntscher for critically reading the manuscript. Furthermore we acknowledge fruitful collaborations with C. A. Kuntscher, S. Schuppler, D.-H. Lu, B. Gorshunov, M. Dressel, H. Schmalle, J. Hanss, N. Bu¨ ttgen, H.-A. Krug von Nidda, J. Hemberger, V. Bobnar, J.-E. Weber, C. Kegler and A. Loidl. This work was supported by the BMBF (project number 13N6918/1).

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