Synthesis, structure and microstructure of the layered compounds Ln1−xSr1+xCoO4 (Ln: La, Nd and Gd)

Solid State Sciences 6 (2004) 21–27 www.elsevier.com/locate/ssscie

Synthesis, structure and microstructure of the layered compounds Ln1−x Sr1+x CoO4 (Ln: La, Nd and Gd) Manuel Sánchez-Andújar, María Antonia Señarís-Rodríguez ∗ Departamento de Química Fundamental, Universidad de A Coruña, 15071 A Coruña, Spain Received 7 March 2003; received in revised form 3 November 2003; accepted 17 November 2003

Abstract By using the nitrate decomposition method in the presence of KNO3 , we have been able to enlarge the compositional range of the solid solution of the layered compounds with K2 NiF4 structure Ln1−x Sr1+x CoO4 (Ln: La, Nd, Gd) to 0
x
0.40 in the case of Ln = La, to 0
x
0.30 for Ln = Nd and to 0
x
0.20 for Ln = Gd. In this paper we present the results of their structural characterization by means of powder X-ray diffraction and Rietveld analysis as a function of composition (x) and the rare earth (Ln), information that has been complemented by the results obtained from electron diffraction and transmission electron microscopy.  2003 Elsevier SAS. All rights reserved. Keywords: Ruddlesden–Popper phases; Cobalt mixed-oxides; Rare earth compounds; Structural characterization

1. Introduction Mixed-oxides with Ruddlesden–Popper (RP) structures have received considerable attention in recent years due to their interesting magnetic and electrical properties, and, particularly, due to the discovery of high temperature superconductivity [1] and colossal magnetoresistance [2,3] in phases belonging to or being closely related to this family. The Ruddlesden–Popper phases, that have the general formula (AO)(ABO3 )n , 1
n
∞ [4], consist of perovskite blocks, n octahedra thick, separated by rock–salt AO layers. The presence of these AO layers reduces the threedimensional character exhibited by the ending member with n = ∞, the well-known perovskite structure. For n = 1 the other limiting composition A2 BO4 , the so-called K2 NiF4 structure, results [5]. It is quasi-twodimensional and can be described as a sequence of BO2 layers in-between AO rock–salt planes, in which the B ions are in a usually distorted octahedral environment while the A cations are 9-coordinated (Fig. 1). While the RP phases of some transitions metals, such as copper, nickel and manganese, have been thoroughly

* Corresponding author.

E-mail address: [email protected] (M.A. Señarís-Rodríguez). 1293-2558/$ – see front matter  2003 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2003.11.005

Fig. 1. Schematic representation of the K2 NiF4 structure displayed by the n = 1 RP phases A2 BO4 . In the case of the Ln1−x Sr1+x CoO4 oxides: B = Co3+ /Co4+ and A = Ln3+ /Sr2+ .

studied, there are series of other elements, such as those of cobalt, that have been relatively much less investigated, and that could also show interesting properties. In this context, while 3D Co perovskites with Co ions in oxidation state 3+ and 4+, Ln1−x Mx CoO3 (Ln3+ : La3+ , rare earths; M2+ : Ca2+ , Sr2+ , Ba2+ ), have been prepared and studied in view of their interesting magnetic and electri-

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cal properties [6–11], high ionic (O2− ) conductivity [12,13], etc., little information is available about the equivalent systems of lower dimensionality. In the case of Co compounds with the bidimensional structure (n = 1 series) [14–17], LaSrCoO4 is the bettercharacterized oxide, even if its intrinsic magnetic behavior remains controversial [16,17]. Meanwhile, the literature about other Ln1−x M1+x CoO4 compounds (Ln3+ : La3+ , rare earths; M2+ : Ca2+ , Sr2+ , Ba2+ ) is very scarce and refers only to the x = 0 oxides [18,19]. Therefore, following our studies on 3D Co systems, we have focused in the 2D analogues Ln1−x Sr1+x CoO4 (Ln: La, Nd, Gd), whose synthesis, structure and microstructure we report in this paper.

2. Experimental To obtain highly doped Ln1−x Sr1+x CoO4 compounds we tried different synthetic methods, both at ambient pressure and under high pressure [20]. Finally, the best results were obtained by decomposing the corresponding mixture of nitrates in the presence of KNO3 . This method, which uses low melting alkali metal nitrates to provide a powerful liquid medium for the precipitation of metal salts [21] had been previously successfully used for the synthesis of YCoO3 [22], superconducting YBa2 Cu4 O8 [23], as well as other mixed oxides [21]. The procedure was as follows: stoichiometric amounts of dry Ln2 O3 , SrCO3 and Co(NO3 )2 ·6H2 O were initially dissolved in HNO3 (≈ 30%). The resulting solution was gently warmed up so as to slowly evaporate the solvent. The so-obtained mixture of nitrates was then mixed with KNO3 , that was added in a mole ratio 1 KNO3 :1 Co(NO3 )2 ·6H2 O. The obtained product was subsequently heated at 450 ◦ C/ 1 h followed by a treatment at 650 ◦ C/48 h. The resulting powder was ground, heated in air at 900 ◦ C/24 h, and after another regrinding it was pressed into pellets and finally annealed in air at 975 ◦ C/24 h. The morphology and size of the particles were studied in a scanning electron microscope (SEM) Jeol 6400. X-ray microanalysis was also carried out in this microscope. Powder X-ray diffraction (PXD) data of the samples were collected with a Siemens D-5000 diffractometer with CuKα radiation. For the determination of accurate unit-cell parameters and structural analysis, PXD data were collected with a step of 0.015◦ from 20 to 80◦ 2θ , over a period of 12 h. The lattice parameters and the structure were refined with the Rietveld program Rietica [24]. Electron diffraction (ED) and transmission electron microscopy (TEM) studies were carried out in a JEOL 2010 microscope, working at 200 kV. For this purpose the samples were finely ground, dispersed in propanol and deposited on a carbon-coated copper grid. In the case of La1−x Sr1+x CoO4 samples a Mettler Toledo Star system was used to determine their thermogravimetric

behavior. The measurements were performed on samples around 50 mg which were loaded into platinum crucibles and heated at 15 ◦ C min−1 under flowing dried 15% H2 in N2 up to 900 ◦ C.

3. Results 3.1. Synthesis, morphology and oxygen content According to the X-ray powder diffraction results, we have obtained single-phase Ln1−x Sr1+x CoO4 materials with 0
x
0.40 for Ln = La, with 0
x
0.30 in the case of Ln = Nd, and with 0
x
0.20 for Ln = Gd. The presence of perovskite-related phases, that often appear as impurities in the n = 1 Ruddlesden–Popper phases, is not detected in the analysis of the PXD data. Also, the as-obtained microcrystals are K+ -free as seen by X-ray microanalysis. This means that by the time the reaction is complete all K2 O, coming from the decomposition of KNO3 has evaporated from the material, as also observed by other authors [21–23]. On the other hand, SEM micrographs show that these polycrystalline materials consist of homogeneous platelets 1–2 µm long. In the case of the La-samples thermal reduction experiments were carried out on a TGA equipment. In this context, the obtained results (Fig. 2) show that the reduction of the La1−x Sr1+x CoO4 samples takes place in two steps giving rise to two “plateaus”: one for ≈ 450
T
500 ◦ C, that corresponds to a mass loss of ≈ 2.3%. A second plateau at T  590 ◦ C that corresponds to the complete reduction to La2 O3 , SrO and Co metal. From the total weight loss we have calculated the oxygen content of these samples (Fig. 2). These results show that while the x = 0 compound is stoichiometric (LaSrCoO4), as the Sr content gets higher (x > 0) a small but increasing oxygen-deficiency (δ) appears, so that δmax (x = 0.4) = 0.06.

Fig. 2. Thermogravimetric data for the reduction of the La1−x Sr1+x CoO4−δ compounds with 15% H2 in N2 .

M. Sánchez-Andújar, M.A. Señarís-Rodríguez / Solid State Sciences 6 (2004) 21–27

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Table 1 Structural parameters for La1−x Sr1+x CoO4 with 0
x
0.4 according to Rietveld refinements of PXD data (space group I 4/mmm with Ln/Sr ions in the 4(e) Wyckoff sites with fractional coordinates (0, 0, z), Co ions at 2(a) sites (0, 0, 0), O(1) in 4(e) (0, 0, z) and O(2) in 4(c) (0, 0.5, 0)). Calculated standard deviations are given in parentheses

a (Å) c (Å) V (Å3 ) ZLa/Sr ZO(1) Rwp Rexp G.O.F.

x = 0.0

x = 0.1

x = 0.2

x = 0.3

x = 0.4

3.8052(6) 12.489(2) 180.84 0.3604(1) 0.1630(4) 13.45 12.13 1.23

3.8023(6) 12.511(2) 180.88 0.3593(3) 0.1626(4) 13.87 11.96 1.35

3.8005(5) 12.521(2) 180.84 0.3593(4) 0.1620(4) 13.27 11.90 1.24

3.8003(7) 12.520(3) 180.82 0.3592(4) 0.1610(5) 15.59 11.94 1.71

3.8002(7) 12.523(3) 180.84 0.3594(3) 0.1605(5) 15.64 12.52 1.56

Table 2 Refined bond lengths (Å) for the refined phases La1−x Sr1+x CoO4 (0
x
0.4) Co–O(1)×2 Co–O(2)×4
Co–O(1)  ap Co–O(2)eq La/Sr–O(1b )×1 La/Sr–O(1a )×4 La/Sr–O(2)×4

x = 0.0

x = 0.1

x = 0.2

x = 0.3

x = 0.4

2.035(1) 1.902(1)

2.034(1) 1.901(1)

2.028(1) 1.900(1)

2.015(1) 1.900(1)

2.010(1) 1.900(1)

1.069

1.069

1.067

1.061

1.057

2.458(1) 2.707(1) 2.581(1)

2.467(1) 2.703(1) 2.586(1)

2.470(1) 2.700(1) 2.591(1)

2.482(1) 2.699(1) 2.591(1)

2.495(1) 2.698(1) 2.591(1)

Table 3 Structural parameters for Nd1−x Sr1+x CoO4 with 0
x
0.3 as obtained from Rietveld refinements of PXD data. Calculated standard deviations are given in parentheses

a (Å) c (Å) V (Å3 ) ZNd/Sr ZO(1) Rwp Rexp G.O.F. Fig. 3. Typical powder X-ray profile for Ln1−x Sr1+x CoO4 samples. Key: observed data (+) and calculated profile (solid line); the difference plot is drawn below the profile and tick marks represent the allowed reflections.

3.2. X-ray structure refinement The Rietveld refinements were carried out in the space group I 4/mmm, using the typical coordinates for the tetragonal K2 NiF4 -type structure. Final steps of the refinement included all atomic coordinates and isotropic temperature factors for all atoms. The background was modeled with a 5-term polynomial; the peak shapes were described by a pseudo-Voigt function. A preferential orientation along the [001] direction was included in the refinement. An example of the profile fit is shown in Fig. 3. The obtained lattice parameters, refined atomic coordinates and derived interatomic distances are summarized in Tables 1–4. As it can be seen in Tables 1, 3 and 5, in all three series the lattice parameter a changes very little with the doping degree; meanwhile the cell parameter c and the cell volume

x = 0.0

x = 0.1

x = 0.2

x = 0.3

3.7728(5) 12.304(1) 175.15 0.3604(1) 0.1638(4) 14.40 13.14 1.20

3.7718(5) 12.359(3) 175.83 0.3594(1) 0.1629(5) 14.79 12.13 1.49

3.7727(8) 12.381(3) 176.25 0.3591(1) 0.1623(5) 16.29 14.18 1.32

3.7739(1) 12.404(5) 176.63 0.3606(2) 0.1618(5) 12.75 10.16 1.58

Table 4 Refined bond lengths (Å) for the refined phases of Nd1−x Sr1+x CoO4 (0
x
0.30) Co–O(1)×2 Co–O(2)×4
Co–O(1)  ap Co–O(2)eq Nd/Sr–O(1b )×1 Nd/Sr–O(1a )×4 Nd/Sr–O(2)×4

x = 0.0

x = 0.1

x = 0.2

x = 0.3

2.018(1) 1.885(1)

2.013(1) 1.885(1)

2.010(1) 1.886(1)

2.007(1) 1.886(1)

1.071

1.067

1.065

1.063

2.418(1) 2.684(1) 2.551(1)

2.429(1) 2.681(1) 2.564(1)

2.445(1) 2.682(1) 2.568(1)

2.451(1) 2.680(1) 2.568(1)

V clearly increase with x, and the rise is more pronounced in the series with a smaller rare earth ion (IX rLa3+ = 1.216 Å > IX r IX r Nd3+ = 1.163 Å > Gd3+ = 1.107 Å) [25] (see also Fig. 4). Also for a given x, all cell parameters a, c and the cell volume decrease linearly as the size of the rare earth ion diminishes. On the other hand, even if X-ray data do not allow a very accurate location of the oxygen atoms, we can

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M. Sánchez-Andújar, M.A. Señarís-Rodríguez / Solid State Sciences 6 (2004) 21–27

Table 5 Structural parameters for Gd1−x Sr1+x CoO4 with 0
x
0.2 as obtained from Rietveld refinements of PXD data. Calculated standard deviations are given in parentheses

a (Å) c (Å) V (Å3 ) ZGd/Sr ZO(1) Rwp Rexp G.O.F.

x = 0.0

x = 0.1

x = 0.2

3.7481(4) 12.160(1) 170.845 0.3606(1) 0.1656(3) 10.04 8.04 1.56

3.7490(6) 12.230(2) 171.906 0.3597(1) 0.1652(4) 11.71 8.11 2.09

3.7502(7) 12.270(2) 172.569 0.3592(1) 0.1637(4) 11.44 8.58 1.77

Table 6 Refined bond lengths (Å) for the refined phases of Gd1−x Sr1+x CoO4 (0
x
0.2) Co–O(1)×2 Co–O(2)×4
Co–O(1)  ap Co–O(2)eq Gd/Sr–O(1b )×1 Gd/Sr–O(1a )×4 Gd/Sr–O(2)×4

x = 0.0

x = 0.1

x = 0.2

2.013(1) 1.874(1)

2.013(1) 1.874(1)

2.009(1) 1.875(1)

1.074

1.074

1.071

2.371(1) 2.669(1) 2.527(1)

2.386(1) 2.667(1) 2.541(1)

2.398(1) 2.666(1) 2.549(1)

deduce interesting structural trends from the evolution of the obtained interatomic distances, as a function of x and the rare earth ion (see Tables 2, 4 and 6). In this context it is worth noting that in all these n = 1 RP Co compounds, the Co ions are in a tetragonally-distorted octahedral site within the perovskite blocks, so that there are two different Co–O bond lengths: a shorter Co–O(2) in the ab plane and a longer Co–O(1) along the c-axis (Fig. 1). As x increases the Co–O(1) distance decreases while the Co–O(2) distance hardly changes. On the other hand, both distances decrease as the size of the rare earth gets smaller (Tables 2, 4 and 6). Also very interestingly, the extent of the tetragonal distortion from the regular [CoO6 ] octahedral coordination, that can be estimated from the ratio [Co–O(1)/Co–O(2)], decreases as the doping degree and/or the size of the rare earth increases (see Tables 2, 4 and 6). It is also worth noting that, for the studied doping degrees, in all three Ln1−x Sr1+x CoO4 series the value of the Co–O– Co angle always remains 180◦ . In the Ln/Sr–O layers the Ln/Sr ions are randomly distributed over the 9-coordinated sites and display three different Ln/Sr–O bond lengths, whose values increase as the mean size of the Ln/Sr cations gets bigger (Tables 2, 4 and 6): the shortest distance is to the apical oxygen ion just above or below it along the c-axis, that we have distinguished as O(1b ) in Tables 2, 4 and 6; four longer correspond to bonds to the O(2) oxygen ions from the neighboring Ln/Sr–O layer and the longest ones are to four oxygen ions O(1a ) in its own Ln/Sr–O layer. As a result,

Fig. 4. Variation of the unit-cell volume of the Ln1−x Sr1+x CoO4 compounds (Ln = La, Nd and Gd) as a function of the Sr content.

Fig. 5. Typical electron diffraction patterns of the Ln1−x Sr1+x CoO4 compounds taken along the [001] (a) and [010] (b) zone axis.

these Ln/Sr–O layers are not flat, but slightly corrugated, especially as the mean size of the Ln/Sr ions decreases. 3.3. Microstructural characterization Electron diffraction (ED) and transmission electron microscopy (TEM) were used to further characterize the samples and to try to detect the presence of any microstructural defects in these samples. In this context, the ED patterns obtained on different microcrystals in various orientations can all be indexed on the basis of the tetragonal cell (space group I 4/mmm) seen by PXD; and the presence of extra spots, or “streaking” of the diffraction maxima or any indication of additional microstructural complexity is not detected. Two typical ED patterns are shown in Fig. 5. On the other hand, for the TEM studies we mainly concentrated in crystal orientations in which the c-axis appears on the plane of projection, as in this type of structure intergrowths of perovskite or other RP phases typically occur along the long axis [26]. Nevertheless, these studies were constrained by the fact that when the microcrystals

M. Sánchez-Andújar, M.A. Señarís-Rodríguez / Solid State Sciences 6 (2004) 21–27

Fig. 6. Medium resolution image of a microcrystal of the La0.6 Sr1.4 CoO4 sample, where the regular d001 spacing gets locally disrupted by the presence, in well-localized areas, of stacking defects along the c-axis. White arrows indicate the location of these defective areas.

were oriented along [uv0] zone axes they became too thick and it was very difficult to obtain good images. In any case, the obtained low and medium resolution images show that the regular d001 spacing expected for these K2 NiF4 -type of materials predominates in all cases. Nevertheless, in the case of samples with x > 0, it is also observed that in some microcrystals this regular periodicity gets locally disrupted by the presence of layer stacking defects along the c-axis (Fig. 6).

4. Discussion By using an appropriate synthetic method, we have been able to enlarge the compositional range of the solid solution of the n = 1 RP series of compounds Ln1−x Sr1+x CoO4 (Ln: La, Nd, Gd) that now becomes 0
x
0.40 for Ln = La, 0
x
0.30 for Ln = Nd and 0
x
0.20 for Ln = Gd. Nevertheless, it is observed that the upper limit for the formation of the solid solution decreases as the size of the rare earth gets smaller. Also, in the case of the studied La compounds, as the strontium content gets higher (x > 0), the samples become oxygen-deficient. This result that is also found in the corresponding 3D perovskites La1−x Srx CoO3−δ [27], is due to the reluctance of the cobalt ions to achieve a high oxidation state; and as upon substitution of Sr2+ for La3+ we are simultaneously forcing the oxidation of Co3+ to Co4+ , the system is reacting by releasing oxygen so as to reduce that oxidation state.

25

Also, the thermal reduction experiments carried out in the La compounds reveal that partial reduction of these materials leads to the stabilization of intermediate nonstoichiometric phases with oxygen content ∼ 3.5, La1−x Sr1+x CoO3.5 in the temperature range ∼ 450
T
500. These intermediate phases, that we have not structurally characterized, are probably similar to that described by Hayward et al. for the LaSrCoO4 compound, that at that temperature reduces to LaSrCoO3.5 in which the oxygen vacancies are concentrated in the [CoO2 ] plane [28]. From PXD data all these materials show a tetragonal K2 NiF4 -type structure (space group I 4/mmm) where the Co ions are in a tetragonally-distorted octahedral environment, the Co–O–Co angles are of 180◦ , and the Ln/Sr–O layers are slightly buckled. In intergrown structures, these distortions from the ideal structure come from the adjustment to the bond-length mismatch that exists across the interface between the perovskite blocks and the rock–salt (Ln/Sr–O) layers along the c-axis [29], and they √ can be estimated by a tolerance factor t = (Ln/Sr–O)/ 2(Co–O). In this context, the tolerance factor of these Co-based materials lie in the range 0.946 < t < 1, for which the tetragonal distortion is favored [30]. Under these circumstances the metal–oxygen bonds are strained, and the extensions of these strains can be estimated using the bond valence method (BVM), based in the socalled valence sum rule (VSR), which establishes that the sum of the bond valences around an ionmust be equal to the formal valence (charge) of this ions ( ij Sij = Vi ) [31]. The deviations observed between these two values can be attributed to instabilities of the structure and the root mean square of these deviations for all atoms involved in the unit cell is called global instability index, defined as [32]:   N 2 i=1 {( j sij − Vi ) } . GII = N The variation of this GII values has been analyzed for the different Ln1−x Sr1+x CoO4 compounds finding that increasing the size of the rare earth ion and/or the doping level results in a decrease of strain, as indicated by the GII values (see Fig. 7). These results correlate well with the observed diminution of the tetragonal distortion as the size of the rare earth increases (IX rGd3+ = 1.107 Å < IX rNd3+ = 1.163 Å < IX r 2+ cation La3+ = 1.216 Å) [25] and/or the bigger Sr (IX rSr2+ = 1.30 Å) [25] replaces the considerable smaller rare earth ions, as both factors increase the tolerance factor and allow the structure to evolve towards the ideal one. It is worthnoting that the presence and extent of this tetragonal distortion could greatly affect the spin state of the cobalt ions, and even stabilize unusual spin-state configurations, such as the intermediate spin state [33]. This, in turn, will strongly influence the magnetic and electrical transport of these compounds, as it is well known to occur in the corresponding 3D perovskites [27].

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mentioning that they remind of those detected in other RP phases such as Ca1.92 Pr0.08 MnO4 [35] and Ln2−y Ay Mn2 O7 [36], and that have also been studied by high-resolution electron microscopy (HREM). According to those studies [35], those so-called “pancake” defects, consist of defective perovskite slices of different widths that are intergrown with the predominant RP structure along the c-axis, even if they are confined in small regions and run only a few Angstroms along the direction perpendicular to c. This could be also the situation present in the defective areas of these Co compounds and more work is in progress to clarify this point. Fig. 7. Global instability index (GII) versus rare earth ionic radii for LnSrCoO4 (Ln: La, Nd and Gd).

Another interesting structural feature is which part of the structure accommodates, on an atomic level, the variation of the cell parameters upon doping and as the size of the rare earth changes. In this context, it is observed that the increase of the c parameter comes from the expansion of the |Ln/Sr–O– Ln/Sr–O| layers between the perovskite blocks as the larger size Sr2+ ion (IX rSr2+ = 1.30 Å) replaces the considerable smaller Ln3+ ions; meanwhile, in the perovskite blocks the Co–O(1) bond length and the [CoO6 ] distortion get smaller as Co3+ (VI r(Co3+ )l.s. = 0.55 Å, VI r(Co3+ )h.s. = 0.61 Å) oxidizes to the smaller Co4+ ions (VI r(Co4+ )h.s. = 0.53 Å) [25] upon doping. Meanwhile, the a parameter changes very little with x, probably due to the rigidity imposed by the Ln/Sr–O slabs. On the other hand, as the size of the rare earth gets smaller, the whole structure shrinks both in the ab plane and along c, even if the bigger effect comes again from the contraction of |Ln/Sr–O–Ln/Sr–O| layers as the size of the rare earth decreases leading to a smaller separation between the perovskite blocks along the c-axis. Nevertheless, diminutions in the Co–O bond lengths are also observed and are probably due to size effects imposed by these rather rigid intergrown structures even if they could also be related to changes in the spin state of the Co ions [7]. In this context, it is known that in Co perovskites Ln1−x Srx CoO3 (Ln: rare earth) an increase in the acidity of the rare earth ion results in stabilization of lower spin-states and smaller bond distances [34]. Also, the increasing buckling of the Ln/Sr–O planes as the rare earth ion gets smaller and its acidity, Φ, higher (Φ: charge/radius) can be related to the tendency of such highly acidic ions to form a strong covalent bonding with their apical oxygen ions. Finally, even if according to PXD these samples are single-phase materials, low and medium resolution transmission electron microscopy studies reveal the presence of microstructural defects in some microcrystals. Although the resolution of the available electron micrographs do not allow an unambiguous interpretation of such defects, it is worth

Acknowledgements We acknowledge the financial support from DGICYT, Ministerio de Ciencia y Tecnología, Spain, under Project FEDER-MAT2001-3749 and M.S.-A. his F.P.I. grant.

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