Metal–semiconductor transition, charge disproportionation, and low-temperature structure of Ca1–xSrxFeO3 synthesized under high-oxygen pressure

Solid State Sciences 2 (2000) 673– 687

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Metal–semiconductor transition, charge disproportionation, and low-temperature structure of Ca1 – x Srx FeO3 synthesized under high-oxygen pressure Takashi Takeda a, Ryoji Kanno a,*, Yoji Kawamoto a, Mikio Takano b,c, Syuji Kawasaki b, Takashi Kamiyama d, Fujio Izumi e a

Department of Chemistry, Faculty of Science, Kobe Uni6ersity, 1 -1 Rokkodai-cho, Nada, Kobe, Hyogo 657 -8501, Japan b Institute for Chemical Research, Kyoto Uni6ersity, Gokasho, Uji, Kyoto 611 -0011, Japan c CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332 -0012, Japan d Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1 -1 Oho, Tsukuba, Ibaraki 305 -8577, Japan e National Institute for Research in Inorganic Materials, 1 -1 Namiki, Tsukuba, Ibaraki 305 -0044, Japan Received 30 June 2000; accepted 18 August 2000

Abstract The solid solution Ca1 – x Srx FeO3 was synthesized under high-oxygen pressure, and its structural and electronic properties were investigated by means of X-ray and neutron Rietveld analyses, resistivity measurements, SQUID magnetometry, and Mo¨ssbauer spectroscopy. The system was found to be divided into an orthorhombic region for 0.0 5 x5 0.5, a cubic one for 0.8 5x 51.0, and possibly a mixed region at x :0.6. In the orthorhombic region, a well-defined metal– semiconductor transition took place and the transition temperature decreased with increasing x from 290 K for x = 0.0 to 200 K for x=0.4. The Mo¨ssbauer measurements on the low-temperature (LT) phase of Ca0.8Sr0.2FeO3 confirmed the occurrence of the well-known charge disproportionation (CD), 2Fe4 + “ Fe(4 − l) + + Fe(4 + l) + with l increasing toward unity with decreasing temperature. The neutron data on this composition and CaFeO3 both indicated an orthorhombic (Pnma) to monoclinic (P21/n) transition accompanying the electronic transition. The two crystallographically different FeO octahedra created in the LT phase were both almost regular in shape but varied their sizes in an inverse fashion as temperature decreased: the average FeO bond lengths were typically 1.941 (9) A, and 1.900 (9) A, for CaFeO3 at 130 K. These concomitant structural and electronic changes indicated that the CD and the breathing phonon mode were intimately coupled in such a way that the CD proceeded (l: 0“ 1) as the size difference between the two kinds of FeO octahedra increased. © 2000 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved. Keywords: Perovskite; Ca1-x Srx FeO3; Crystal structure; Metal– semiconductor transition; Neutron diffraction; Charge disproportionation; Breathing phonon mode

1. Introduction

* Correspondence and reprints: Tel.: + 81-78-803-5681; fax: + 81-78-803-5681. E-mail address: [email protected] (R. Kanno).

Recently, various types of interplay between spin, charge, and orbital degrees of freedom have been newly found or re-examined for 3d transition metal oxides [1]. From a chemical viewpoint, it is interesting to survey such electronic properties as a function

1293-2558/00/$ - see front matter © 2000 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved. PII: S1293-2558(00)01088-8

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of the number of d electrons, the depth of the d levels relative to O2p, and other relevant parameters by changing the valence state of the transition metal ion. Iron is known to usually take valence states of 2+ and 3 + in oxides, but there exists a small number of perovskites which contain Fe4 + (d 4), the d-levels of which must be considerably deeper than those of Fe2 + (d 6) and Fe3 + (d 5). The electronic configuration of this ion is basically a two-fold orbital-degenerate state of t 32ge 1g, but these oxides generally remain free from instabilities like the cooperative Jahn – Teller effect and orbital ordering. In this sense, Fe4 + is quite different from isoelectronic Mn3 + , which is known as a typical ‘‘Jahn –

Fig. 2. Composition dependence of the lattice parameters for Ca1 – x Srx FeO3 at room temperature determined by Rietveld refinement.

Fig. 1. X-ray diffraction patterns for Ca1 – x Srx FeO3 at room temperature (a). Enlargement of the patterns showing the change in symmetry from orthorhombic (Pnma) to cubic (Pm3( m) with increasing Sr content (b).

Teller ion’’. SrFeO3 (SFO), which is cubic, and CaFeO3 (CFO), distorted to the orthorhombic GdFeO3-type structure, take the following different ways to avoid these instabilities: in SFO, a metallic band is formed, whereas in CFO, a charge disproportionation (CD) to a pair of exchange-stabilized, highly symmetric orbital-singlet states, i.e. 2Fe4 + “ Fe3 + (d 5)+ Fe5 + (d 3), takes place at 290 K [2]. It is possible to assign the difference between Fe4 + - and Mn3 + -oxides by the depth of the d levels relative to O2p; i.e. the difference in effective chargetransfer energy is −3 eV for SFO [3] but is +1.8 eV for LaMnO3 [4]. The negative sign for the Fe4 + -oxide implies that the ‘‘Fe4 + ’’ ion is actually a Fe3 + ion accompanied by an oxygen hole, i.e. d 5L6 (S=2), and that the ‘‘Fe5 + ’’ ion would be a ferric ion accompanied by a pair of holes, d 5L6 2 (S=3/2), where the O2p spins are coupled antiparallel to the Fe spins. The link between the above mentioned CD picture and this oxygen-hole picture is such that ‘‘charge’’ corresponds to oxygen holes and ‘‘disproportionation’’ corresponds to confinement and triplet pairing of the oxygen holes in half of the FeO6 octahedra [3,5]. The electronic properties of Fe4 + oxides must thus be strongly dominated by oxygenhole character. It is evident that the oxygen-hole band is broader in SFO, a cubic metal, than in orthorhombic CFO,

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Table 1 Rietveld refinement results for Ca1–x Srx FeO3 using X-ray diffraction data at room temperature. Ca/Sr

x = 0.0

x= 0.1

x= 0.2

x =0.4

Space group

Pnma

Pnma

Pnma

Pnma

a (A, ) b (A, ) c (A, ) Ca/Sr (4c) x y z B (A, 2)

5.34967(9) 7.53558(14) 5.32356(10)

5.35326(15) 7.5548(3) 5.34146(16)

5.3602(5) 7.5762(5) 5.3605(5)

5.39295(19) 7.6062(3) 5.3824(2) 0.0092(13) 1/4 0.001(8) 1.49(9)

0.0329(7) 1/4 −0.004(2) 1.45(10)

0.0285(8) 1/4 −0.005(3) 1.72(12)

0.0233(11) 1/4 −0.003(5) 1.87(13)

Fe (4b) x y z B (A, 2)

0 0 1/2 1.06(8)

0 0 1/2 1.65(10)

0 0 1/2 1.48(11)

0 0 1/2 0.96(10)

O1 (8d) x y z B (A, 2)

0.285(2) 0.029(2) 0.716(2) 0.94(17)

0.284(4) 0.024(3) 0.716(4) 1.7(5)

0.276(11) 0.019(6) 0.722(11) 1.6(7)

0.268(8) 0.027(5) 0.729(7) 1.5

O2 (4c) x y z B (A, 2)

0.493(2) 1/4 0.071(4) 0.94

0.503(3) 1/4 0.072(3) 1.1(8)

0.503(4) 1/4 0.060(10) 1.0(12)

0.493(6) 1/4 0.031(19) 1.49

Rwp Re RI S

11.14 8.30 6.37 1.34

12.56 10.02 3.32 1.25

Ca/Sr

x= 0.8

x= 1.0

Space group

Pm3m

Pm3m

a (A, )

3.83670(5)

3.85086(4)

Ca/Sr (1a) x y z B (A, 2)

0 0 0 1.3(3)

0 0 0 1.4

Fe (1b) x y z B (A, 2)

1/2 1/2 1/2 1.3

1/2 1/2 1/2 0.6(4)

O (3c) x y z B (A, 2)

1/2 1/2 0 1.3

1/2 1/2 0 0.8(8)

Rwp Re RI S

14.6 9.97 2.12 1.47

15.41 11.49 2.83 1.34

14.21 10.63 7.04 1.34

13.40 10.59 4.01 1.27

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T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

Table 2 Interatomic distances (A, ) and angles (°) for Ca1–x Srx FeO3 at room temperaturea Ca/Sr

x= 0.0

x= 0.1

x = 0.2

x = 0.4

Ca (Sr)O1i (×2) Ca (Sr)O1ii (×2) Ca (Sr)O1iii (×2) Ca (Sr)O1iv (×2) Ca (Sr)O2i Ca (Sr)O2 Ca (Sr)O2v Ca (Sr)O2vi FeO1ii (×2) FeO1 (×2) FeO2i (×2) O1iiFeO2i O1FeO2i O1FeO1ii FeO1Fevii FeviiiO2Feix

2.413(14) 2.595(15) 2.609(14) 3.106(14) 2.32(3) 2.488(13) 2.920(13) 3.02(3) 1.913(14) 1.920(14) 1.921(5) 88.3(10) 91.3(9) 90.48(16) 159.8(8) 157.5(14)

2.432(19) 2.58(2) 2.65(2) 3.073(19) 2.32(4) 2.573(17) 2.842(16) 3.073(19) 1.91(3) 1.92(3) 1.927(7) 86.0(14) 91.2(13) 90.4(3) 161.2(11) 157(2)

2.49(3) 2.60(13) 2.66(3) 3.00(3) 2.38(6) 2.59(2) 2.81(2) 3.00(3) 1.91(8) 1.92(8) 1.921(9) 87(2) 91(2) 90.3(5) 164.8(17) 161(3)

2.47(2) 2.72(2) 2.64(2) 2.96(2) 2.52(6) 2.61(3) 2.79(3) 2.87(6) 1.93(5) 1.91(5) 1.910(9) 93(3) 88(3) 90.5(3) 165(2) 170(6)

Ca/Sr

x = 0.8

x=1.0

Ca (Sr)O (×12) FeO (×6) OFeO FeOFe

2.7130(1) 1.9183(1) 90 180

2.7230(1) 1.9254(1) 90 180

a Coordinate triplets: i) x−1/2, −y+1/2, −z+1/2; ii) −x+1/2, −y, z−1/2; iii) x, y, z−1; iv) −x, −y, −z+1; v) x−1, y, z; vi) x−1/2, y, −z−1/2 vii) −x+1/2, −y, z+1/2; viii) x+1/2, −y+1/2, −z+1/2; ix) x+1/2, y, −z+1/2.

the metallicity of which ceases at 290 K. The confinement of hole pairs in a half of the FeO6 octahedra is a narrow oxygen-hole band phenomenon. Recently, partial substitution of Co for Fe was found to broaden the band and stabilize a metallic, non-disproportionated, ferromagnetic (FM) state for CFO as well as for SFO, although both are antiferromagnets. For example, Sr2FeCoO6 (SFCO) is a better metal than SFO and the Curie temperature of SFCO is 340 K while SFO has a TN of 134 K [5,6]. This is another interesting feature which adds to the richness of the electronic phases in the Fe4 + -oxide system dominated by oxygen hole character. Concerning the mechanism of the CD, or the hole confinement, it is interesting to note that the phase transition occurs as a function of temperature and Sr/Ca ratio: the hyperfine parameters measured by Mo¨ssbauer spectroscopy change continuously with temperature suggesting that Fe(4 − l) + and Fe(4 + l) + coexist in CFO below 290 K with l increasing continuously from 0 toward 1, and the hyperfine parameters for Ca1 – x Srx FeO3 (CSFO) at a fixed temperature of 4 K also change in a similar way as

Ca content increases [7]. It is therefore tempting to assume that the CD is intimately coupled to the breathing phonon mode, an alternate expansion and contraction of the FeO6 octahedra, and that the Fe(4 − l) + ions are contained, or the oxygen holes are confined, in the smaller octahedra, as supported by a model calculation [8]. The oxygen-hole band must be coupled to the breathing mode in such a way that a gap opens in the band when the breathing mode is frozen, while both the gap width and the oxygen displacement increase smoothly as temperature decreases and as Ca content increases. The crystal and magnetic structures of the low temperature phase of Sr1 – x Lax FeO3 has been focused on because it shows a first-order transition such as 3Fe11/3 + (paramagnetic) l 2Fe3 + +Fe5 + [antiferromagnetic (AF)] for x: 1/3 [9] and because a nearly oxygen-stoichiometric sample can be obtained even in an atmospheric pressure of oxygen. First, Battle et al. [10] found a 2:1 ordering along the B 111\ direction of the two kinds of Fe ions with different magnetic moments of 3.61 and 2.72vB for Sr2/3La1/3FeO2.97, although no corresponding struc-

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677

tural change was observed within experimental error. More recently, however, Li et al. [11] detected a structural modulation along the B 111 \ direction by transmission electron microscopy. Photoemission measurements and Hartree – Fock band structure calculations led Matsuno et al. to a ground state hole picture such as 2Fe3 + +Fe3 + L6 2 [12]. Detailed neutron studies are thus needed to clarify the structural details.

Fig. 4. Temperature dependence of the resistivity of Ca1 – xSrx FeO3.

Fig. 5. Temperature dependence of the inverse magnetic susceptibility of Ca1 – x Srx FeO3.

Fig. 3. Composition dependences of the interatomic distances (a) and bond angles (b) for Ca1 – x Srx FeO3 at room temperature. The values determined by the neutron diffraction are also marked with .

With regard to CFO, we reported a proper screw spin structure with k // B111\ and k =0.161a* at 4 K [5]. Here the angle between the moments of a pair of nearest-neighboring (n.n.) Fe ions is ca. 60°, which is considerably larger than that of 40° for SFO [13]. Considering that this type of spin structure results from a competition of the n.n. FM interaction with AF interactions over long distances [13], it

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T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

is possible to interpret the difference between SFO and CFO as indicating that the delocalization of oxygen holes occurs parallel to the enhancement of the FM Fe( )O (¡)Fe( ) interaction, finally lead-

ing to the metallic ferromagnetism found in the Co-substituted phases. Here we report mainly the structural data for CSFO obtained by means of X-ray and Neutron Rietveld analyses.

2. Experimental

Fig. 6. Temperature dependence of the difference in isomer shift between the Fe (4 − l) + and Fe (4 + l) + ions for Ca0.8Sr0.2FeO3.

Fig. 7. Rietveld refinement patterns of the neutron diffraction data of CaFeO3 at 130 K and 305 K. The solid lines, the overlying crosses, yi, and bars indicate the calculated intensities, the observed intensities, the difference between these, and Bragg peak locations, respectively.

Oxygen-deficient samples of Ca1 – x Srx FeO2.5 + l were first synthesized from CaCO3, SrCO3, and hFe2O3 (CaCO3, SrCO3: Nakarai Chemicals, aFe2O3: Merck, all \ 99% pure). These were weighed, mixed, pressed into pellets, and calcined at 900°C for 24 h. After being reground, these were pressed into pellets again and fired at 1150 – 1350°C for 24 h in air or Ar. The products were put into a gold capsule with an oxidizing agent, KClO4, and treated under a pressure of 2 GPa at 750 –900°C for 30 min using a piston-cylinder-type high-pressure apparatus. The samples thus obtained were examined at room temperature by X-Ray diffraction (XRD) (Rigaku RAD-C, 12 kW) using CuKh radiation over a 2q range from 10 to 100° in 0.04° steps and 1-s counting times. The structural parameters were refined by Rietveld analysis of the data with RIETAN-94 [14]. Neutron diffraction (ND) data of CFO and Ca0.8Sr0.2FeO3 were taken over a temperature range of 1305 T5 325 K using a time-of-flight (TOF) powder diffractometer, VEGA, at the KENS pulsed spallation neutron source of the National Laboratory for High Energy Physics (KEK) [15]. The specimens (ca. 2 g each) were packed in cylindrical vanadium cells with a radius of 5 mm, a height of 55 mm, and a thickness of 0.2 mm. The structural parameters were refined with RIETAN96T. Electrical resistivity was measured with a dc four-probe method with silver-paste contacts over a temperature range of 155 T5 300 K using a lowtemperature electrical conductivity measurement unit (Toyo-Sanso). Magnetization was measured with a SQUID magnetometer (Quantum Design, MPMS2) between 5 and 300 K in a field of 1 kOe. Mo¨ssbauer spectra were obtained over a temperature range of 1505 T5 300 K using a 57Co/Rh source, the velocity of which was calibrated using pure iron metal as a standard, and were computerfitted using a Lorentzian function.

Rwp Rp S = Rwp/Re RI RF

O3 (4e)

O2 (4e)

O1 (4e)

Fe2 (2c)

0.716(2) 0.290(3) 0.966(2) 0.5(2) 0.710(2) 0.284(3) 0.5352(19) 0.4(2) 0.0667(10) 0.4886(8) 0.750(3) 0.37(7) 7.89 5.82 0.807 2.34 1.52

0.7158(15) 0.2903(19) 0.9644(13) 0.31(13) 0.7099(15) 0.285(2) 0.5337(12) 0.47(14) 0.0670(6) 0.4889(5) 0.7525(16) 0.42(4) 5.10 3.85 0.9129 1.69 0.93

x Y z B (A, 2) x y z B (A, 2) x y z B (A, 2)

x 0 y 1/2 z 0 B (A, 2) =Fe1

0 1/2 0 =Fe1

1/2 0 0 0.216 (17)

1/2 0 0 0.181 (11)

Fe1 (2d)

x y z B (A, 2)

−0.0061(16) 0.0365(7) 0.753(4) 0.59(7)

x −0.0066(10) y 0.0370(4) z 0.751(3) B (A, 2) 0.57(4)

Ca (4e)

5.31418(8) 5.34717(8) 7.52509(10) 90.063(3)

5.31301(5) 5.34719(5) 7.52351(6) 90.063(2)

a (A, ) b (A, ) c (A, ) i (°)

160

130

T (K)

8.47 6.08 0.7634 2.13 1.29

0.0657(10) 0.4886(9) 0.754(3) 0.41(8)

0.712(3) 0.287(5) 0.5325(19) 0.4(2)

0.714(3) 0.287(5) 0.963(2) 0.6(3)

0 1/2 0 = Fe1

1/2 0 0 0.230 (19)

−0.0050(18) 0.0357(8) 0.748(5) 0.61(7)

5.31550(9) 5.34747(9) 7.52680(11) 90.059(4)

190

9.42 6.82 0.7831 2.30 1.48

0.0652(12) 0.4883(11) 0.752(3) 0.51(9)

0.709(3) 0.284(4) 0.535(3) 0.4(3)

0.716(3) 0.290(4) 0.965(3) 0.6(3)

0 1/2 0 = Fe1

1/2 0 0 0.21 (2)

−0.006(2) 0.0352(9) 0.750(7) 0.74(7)

5.31707(10) 5.34791(10) 7.52907(13) 90.054(4)

220

4.59 3.50 0.8452 1.81 1.10

0.0659(6) 0.4890(5) 0.7526(18) 0.57(3)

0.7114(19) 0.285(3) 0.5337(15) 0.52(18)

0.7151(19) 0.289(2) 0.9648(15) 0.43(18)

0 1/2 0 = Fe1

1/2 0 0 0.243 (10)

−0.0060(11) 0.0343(5) 0.750(4) 0.74(4)

5.31960(5) 5.34840(5) 7.53210(6) 90.050(2)

250

8.22 6.03 0.7704 2.23 1.36

0.0654(11) 0.4895(9) 0.7542(15) 0.49(8)

0.713(3) 0.288(5) 0.533(2) 0.4(3)

0.715(3) 0.288(5) 0.964(2) 0.6(3)

0 1/2 0 = Fe1

1/2 0 0 0.258(19)

−0.005(2) 0.0343(9) 0.748(16) 0.76(7)

5.32054(9) 5.34875(9) 7.53345(12) 90.051(4)

265

8.02 5.92 0.7760 1.86 1.36

0.0656(11) 0.4902(9) 0.752(4) 0.53(8)

0.710(3) 0.288(7) 0.532(2) 0.5(4)

0.716(3) 0.286(7) 0.964(3) 0.5(4)

0 1/2 0 = Fe1

1/2 0 0 0.249(18)

−0.0087(19) 0.0344(8) 0.750(7) 0.75(7)

5.32113(9) 5.34893(9) 7.53413(11) 90.037(6)

270

8.05 5.84 0.7830 2.21 1.43

0.0657(11) 0.4890(9) 0.751(16) 0.51(8)

0.713(6) 0.288(8) 0.532(3) 0.4(3)

0.713(6) 0.288(8) 0.965(3) 0.4(3)

0 1/2 0 = Fe1

1/2 0 0 0.25(2)

−0.0065(19) 0.0348(8) 0.748(8) 0.70(7)

5.32148(9) 5.34893(9) 7.53482(12) 90.036(6)

275

6.69 4.88 0.7830 1.74 1.10

0.0662(9) 0.4894(8) 0.753(3) 0.59(7)

0.714(4) 0.284(4) 0.535(3) 0.5(3)

0.712(4) 0.289(3) 0.967(3) 0.5(3)

0 1/2 0 =Fe1

1/2 0 0 0.252(15)

−0.0051(17) 0.0337(7) 0.750(6) 0.81(5)

5.32206(8) 5.34918(8) 7.53511(10) 90.040(4)

280

6.59 4.84 0.7890 1.64 1.04

0.0658(10) 0.4900(8) 0.750(5) 0.62(7)

0.711(4) 0.286(5) 0.536(2) 0.6(3)

0.715(4) 0.287(5) 0.968(2) 0.5(3)

0 1/2 0 =Fe1

1/2 0 0 0.239(15)

−0.0057(16) 0.0343(7) 0.754(3) 0.74(8)

5.32264(7) 5.34948(8) 7.53552(9) 90.036(5)

285

6.55 4.82 0.7848 1.97 1.36

0.0662(9) 0.4898(8) 0.751(6) 0.62(7)

0.711(5) 0.288(6) 0.532(2) 0.6(3)

0.714(5) 0.285(6) 0.964(2) 0.4(3)

0 1/2 0 =Fe1

1/2 0 0 0.260(15)

−0.0069(16) 0.0340(7) 0.748(7) 0.74(7)

5.32297(7) 5.34981(8) 7.53605(9) 90.023(7)

290

Table 3 Rietveld refinement results of CaFeO3 using neutron diffraction data for 1305T5290 K (space group P21/n) (a), for 2955T5325 K (space group Pnma) (b), and anisotropic thermal parameters (c) (a)

T. Takeda et al. / Solid State Sciences 2 (2000) 673–687 679

U11 (A, 2) 0.014(3) 0.0037(11) 0.0081(15) 0.009(3)

Atom

Ca Fe O1 O2

300 K

0.013(3) 0.0029(10) 0.0070(15) 0.009(3)

0.006(4) 0.002(2) 0.008(2) 0.006(3)

U22 (A, 2)

0.006(4) 0.002(2) 0.0061(18) 0.009(3)

6.88 5.04 0.7831 2.02 1.33

6.77 4.99 0.7864 2.05 1.17

Ca Fe O1 O2

0.4886(11) 1/4 0.0673(13) 0.557

0.4891(11) 1/4 0.0666(13) 0.693

x y z Beq (A, 2)

U22 (A, 2)

0.2878(6) 0.0335(6) 0.7141(7) 0.540

0.2867(6) 0.0336(6) 0.7133(7) 0.498

x y z Beq (A, 2)

U11 (A, 2)a

1/2 0 0 0.271

0.0337(10) 1/4 −0.004(3) 0.806

5.35082(8) 7.53833(10) 5.32464(8)

300

1/2 0 0 0.258

0.0339(10) 1/4 −0.006(3) 0.823

5.35009(8) 7.53668(10) 5.32359(8)

296

x y z Beq (A, 2)

x y z Beq (A, 2)

Atom

(c) 296 K

Rwp Rp S= Rwp/Re RI RF

O2 (8c)

O1 (8c)

Fe (4b)

Ca (4c)

a (A, ) b (A, ) c (A, )

T (K)

(b)

Table 3 (Continued)

0.010(4) 0.0049(19) 0.0045(14) 0.006(3)

U33 (A, 2)

0.012(4) 0.0050(19) 0.0059(14) 0.009(4)

U33 (A, 2)

4.72 3.55 0.8095 1.79 0.99

0.4893(8) 1/4 0.0656(10) 0.659

0.2867(4) 0.0337(5) 0.7134(5) 0.507

1/2 0 0 0.245

0.0336(7) 1/4 −0.0046(19) 0.815

5.35115(6) 7.53902(7) 5.32547(5)

305

0 0.0010(13) 0.0020(16) 0

U12 (A, 2)

0 0.0019(13) −0.0001(15) 0

U12 (A, 2)

7.85 5.84 0.7524 2.39 1.56

0.4895(13) 1/4 0.0681(15) 0.664

0.2866(8) 0.0329(8) 0.7133(8) 0.541

1/2 0 0 0.257

0.0346(12) 1/4 −0.006(3) 0.751

5.35121(10) 7.53969(12) 5.32580(9)

310

8.35 6.11 0.7688 2.51 1.70

−0.03(4) 0.0002(18) −0.0002(17) −0.001(3)

U13 (A, 2)

−0.002(4) 0.0004(18) −0.0002(8) 0.000(3)

U13 (A, 2)

0.4896(15) 1/4 0.0665(17) 0.724

0.2866(8) 0.0334(8) 0.7137(8) 0.526

1/2 0 0 0.242

0.0335(13) 1/4 −0.006(3) 0.838

5.35168(11) 7.54004(12) 5.32613(9)

315

7.83 5.76 0.7463 2.29 1.43

0.4890(14) 1/4 0.0672(16) 0.751

0.2866(8) 0.0330(8) 0.7139(8) 0.505

1/2 0 0 0.267

0.0340(12) 1/4 −0.003(4) 0.862

5.35158(10) 7.54037(12) 5.32658(9)

320

0 0.002(4) −0.0006(16) 0

U23 (A, 2)

0 0.003(3) −0.0007(15) 0

U23 (A, 2)

12.80 9.35 0.7671 3.82 2.70

0.489(2) 1/4 0.067(3) 0.773

0.2868(13) 0.0334(12) 0.7128(13) 0.548

1/2 0 0 0.284

0.033(2) 1/4 −0.008(5) 0.795

5.35161(16) 7.54057(19) 5.32700(15)

325

680 T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

0.011(2) 0.0036(8) 0.0064(10) 0.009(2)

Ca Fe O1 O2

0.012(4) 0.0037(13) 0.0077(19) 0.007(3)

Ca Fe O1 O2

0.011(4) 0.0037(14) 0.0075(19) 0.011(4)

Ca Fe O1 O2

0.011(4) 0.0031(13) 0.0074(18) 0.010(4)

Ca Fe O1 O2

0.005(8) 0.003(4) 0.008(4) 0.013(7)

U22 (A, 2)

0.008(5) 0.002(2) 0.006(2) 0.0011(4)

U22 (A, 2)

0.013(6) 0.002(3) 0.008(2) 0.008(4)

U22 (A, 2)

0.009(5) 0.001(2) 0.007(2) 0.012(4)

U22 (A, 2)

0.008(3) 0.0021(15) 0.0065(13) 0.008(2)

U22 (A, 2)

0.011(7) 0.004(4) 0.006(3) 0.011(7)

U33 (A, 2)

0.013(5) 0.006(2) 0.0061(17) 0.008(4)

U33 (A, 2)

0.008(5) 0.004(2) 0.0050(17) 0.009(4)

U33 (A, 2)

0.008(4) 0.005(2) 0.0056(17) 0.007(4)

U33 (A, 2)

0.012(3) 0.0036(13) 0.0064(10) 0.008(3)

U33 (A, 2)

0 0.001(3) 0.001(3) 0

U12 (A, 2)

0 −0.0006(16) −0.0008(19) 0

U12 (A, 2)

0 0.0010(16) 0.000(2) 0

U12 (A, 2)

0 −0.0010(16) −0.0007(19) 0

U12 (A, 2)

0 0.0001(9) 0.0010(11) 0

U12 (A, 2)

The form of the anisotropic temperature factor is exp[−2y 2(h 2a 2U11+k 2b*U22+l 2c 2U33+2hka*b*U11+2hla*c*U13+2klb*c*U23)].

0.015(6) 0.004(2) 0.008(4) 0.005(6)

Ca Fe O1 O2

a

U11 (A, 2)

Atom

325 K

U11 (A, 2)

Atom

320 K

U11 (A, 2)

Atom

315 K

U11 (A, 2)

Atom

310 K

U11 (A, 2)

Atom

305 K

Table 3 (Continued)

−0.001(7) 0.000(4) −0.001(3) 0.000(5)

U13 (A, 2)

−0.004(5) 0.001(2) −0.002(2) −0.004(3)

U13 (A, 2)

0.001(4) − 0.002(2) −0.002(2) −0.001(3)

U13 (A, 2)

−0.001(4) − 0.000(2) −0.002(2) 0.000(3)

U13 (A, 2)

−0.03(3) − 0.0004(13) −0.0006(12) 0.0004(19)

U13 (A, 2)

0 0.006(4) 0.000(3) 0

U23 (A, 2)

0 0.002(4) −0.0012(19) 0

U23 (A, 2)

0 0.002(4) 0.001(2) 0

U23 (A, 2)

0 0.004(3) −0.0007(19) 0

U23 (A, 2)

0 0.0029(19) 0.0006(11) 0

U23 (A, 2)

T. Takeda et al. / Solid State Sciences 2 (2000) 673–687 681

682

T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

Table 4 Interatomic distances (A, ) for CaFeO3 at low temperatures Temperature T (K) 290 285 280 275 270 265 250 220 190 160 150 130 Temperature T (K) 290 285 280 275 270 265 250 220 190 160 150 130

Fe1–O1

Fe1–O2

Fe1–O3

Average Fe1–O

1.92(3) 1.93(3) 1.933(17) 1.93(4) 1.93(3) 1.94(2) 1.942(10) 1.947(16) 1.93(2) 1.948(13) 1.950(6) 1.948(8)

1.92(3) 1.94(2) 1.928(18) 1.92(4) 1.93(3) 1.92(2) 1.934(9) 1.947(15) 1.92(2) 1.946(12) 1.944(6) 1.940(8)

1.93(4) 1.91(3) 1.94(2) 1.92(4) 1.93(3) 1.947(19) 1.935(13) 1.93(3) 1.941(19) 1.92(2) 1.931(9) 1.933(12)

1.92(4) 1.93(3) 1.933(19) 1.92(4) 1.93(3) 1.94(2) 1.937(11) 1.940(19) 1.93(2) 1.937(15) 1.942(7) 1.941(9)

Fe2–O1

Fe2–O2

Fe2–O3

Average Fe2–O

1.93(3) 1.91(2) 1.918(18) 1.92(4) 1.92(3) 1.91(2) 1.907(9) 1.900(15) 1.92(2) 1.897(12) 1.898(6) 1.900(7)

1.92(3) 1.92(3) 1.917(18) 1.93(4) 1.92(3) 1.93(2) 1.912(10) 1.901(17) 1.92(2) 1.902(13) 1.902(7) 1.904(8)

1.91(4) 1.92(3) 1.90(2) 1.91(4) 1.90(3) 1.89(2) 1.898(13) 1.90(3) 1.888(19) 1.913(12) 1.899(9) 1.897(11)

1.92(4) 1.92(3) 1.91(2) 1.92(4) 1.91(3) 1.91(2) 1.906(11) 1.901(19) 1.91(2) 1.904(13) 1.900(7) 1.900(9)

3. Results and discussion

3.1. Synthesis The oxygen-deficient samples prepared at ambient pressure crystallize in either the brownmillerite structure, the perovskite structure, or in mixed structures, depending on the atmosphere and temperature used for the preparation and the Ca/Sr ratio. Monophasic brownmillerite samples could be obtained for the whole range of x by conducting the final ambient pressure treatment in flowing Ar above 1300°C, and it was only when we started from these that we could obtain the desired monophasic perovskites by the high-pressure treatment. It is known that the brownmillerite structure can be derived from the perovskite structure by ordering the oxygen vacancies in rows along the B110 \ direction in alternate FeO planes. Most likely, rapid oxygen diffusion along these rows was necessary to form fully oxidized perovskites. The oxygen stoichiometry of CFO was

confirmed by neutron powder diffraction analysis (see below). It is known that the oxidation of CaFeO2.5 to CFO needs much higher oxygen pressure (2 GPa at least) than the oxidation of SrFeO2.5 to SFO (50 MPa). Therefore, it is safe to assume here that we successfully obtained fully oxidized samples for the whole range of x. This was further confirmed by comparing their electrical and magnetic properties with those previously reported for the stoichiometric compositions. Fig. 1a shows the room temperature XRD patterns for the oxidized samples. The pattern of SFO could be indexed assuming a cubic cell of space group Pm3m with a= 3.850 A, , while CFO showed superlattice reflections suggesting an orthorhombic structure of Pnma. As can be seen in the enlarged patterns shown in Fig. 1b, the 221 reflection indicative of orthorhombic symmetry persisted up to x= 0.6, although the intensity decreased. However, the shift of the position of this peak to lower angles ceased at x: 0.5. The orthorhombic 220 reflection,

T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

which corresponds to the cubic 111 reflection, on the other hand, continued to shift to lower angles up to x= 1. It may thus be possible that the orthorhombic and cubic phases coexist at x :0.6. Fig. 2 shows the composition dependence of the lattice parameters

683

determined by the Rietveld refinement. It is evident that the orthorhombic distortion decreases with increasing x and the perovskite cell expands as Sr is substituted for the smaller Ca (ionic radii: Sr2 + (XII)= 1.44 A, , Ca2 + (XII)= 1.34 A, [16]).

Fig. 8. Temperature dependence of the lattice parameters, unit cell volume, and monoclinic beta angle for CaFeO3 (a) and Ca0.8Sr0.2FeO3 (b).

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T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

Fig. 9. Monoclinic structure of CaFeO3 illustrating the two different FeO octahedra at low temperatures.

listed in Table 1, and selected interatomic distances and bond angles calculated with ORFFE[17] are listed in Table 2. In Fig. 3, the Ca (Sr)O and FeO distances and the FeOFe and OFeO bond angles are plotted against composition. As can be seen in Table 1, the AO distances are clearly split into two sets, eight short and four long bonds, in the orthorhombic region. The averaged values for these sets are plotted in Fig. 3a. The difference between these two sets diminishes continuously as Sr content increases up to x:0.8. On the other hand, the FeO distances always fall within 0.5% of each other, and the OFeO bond angles deviate from 90° by at most 0.55% throughout this composition range. Therefore, the FeO6 octahedra are almost regular, even in the orthorhombic region, and its volume is almost constant. The tilting of the octahedra tends to be suppressed as the Sr content increases, with the averaged FeOFe bond angle increasing from 158 to 180°, as seen in Fig. 3b.

3.2. Resisti6ity, magnetic susceptibility, and Mo¨ssbauer spectroscopy Generally speaking, the results of the measurements of resistivity, magnetic susceptibility, and

Fig. 10. Temperature dependence of the average FeO bond lengths for CaFeO3.

The refinement was performed with space group Pnma for x 50.5 and with Pm3m for 0.85 x. The following sets of atomic coordinates were used: Ca (Sr) 4c (x, 1/4, z); Fe 4b (0, 0, 1/2); O1 8d (x, y, z); O2 4c (x, 1/4, z) for Pnma, and Ca (Sr) 1a (0, 0, 0); Fe 1b (1/2, 1/2, 1/2); O 3c (0, 0, 1/2) for Pm3m. These coordinates and the thermal parameters were allowed to vary after the scale, background, halfwidth, and unit-cell parameters almost converged to their optimum values. The final R factors, structural parameters, and their estimated deviations are

Fig. 11. Valence bond sums for the Fe ions in CaFeO3 plotted against temperature. The default valence parameter R0 for the Ca– O interaction (1.967) was used. For the Fe4 + – O interaction, the value (1.772) calculated from the interatomic distances at 305 K was used.

T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

Mo¨ssbauer spectroscopy were in very good agreement with the previous data. We add here some new unreported data mainly. Fig. 4 shows the resistivity data. The transition to the charge-disproportionated semiconducting phase is marked by a kink at 290 K for CFO (x = 0.0). The transition temperature decreased to : 200 K as Sr content increased to x =0.4. Such a well-defined anomaly was not found for x ] 0.5, and SFO showed metallic behavior down to the lowest temperature. It is reasonable to assume that the oxygen-hole band broadens as x increases because the structure approaches cubic symmetry. We previously reported the relationship between structural details and electrical conductivity for the GdFeO3-type and the pyrochlore-type oxides, the frameworks of which are both made of corner-sharing MO6 octahedra. The most important factor that suppresses metallicity is no doubt the decreasing MOM angle, from which the expansion and the distortion of the MO6 octahedra follow. The perovskites RRuO3 (R =Ca, Sr, La, Pr) [18], RNiO3 (R = La, Pr, Nd, Sm) [19], and RTiO3 (R= La, Ce, Nd, Sm, Gd, Y) [20], and the pyrochlores R 2M2O7 (R = Bi, Tl, Pb, Ln, M= Ru, Mn, Mo) [21 – 25] provide examples of such geometric effects. It is evident that the effect of the FeOFe angle also dominates in the present system. Magnetic susceptibility (M/H) was measured as a function of temperature at a fixed field of 1 kOe. All the compositions synthesized in this study exhibited antiferromagnetism with TN s of 116 – 134 K as reported previously. Shown in Fig. 5 are the temperature dependences of the susceptibility,  = M/H, and the derivative, d/dT, for x= 0.0 – 0.2. Note that for every composition, the susceptibility increases as temperature decreases from 400 K but the rate of increase slightly drops at the metal – semiconductor transition (290, 260, and 240 K for x = 0.0, 0.1, and 0.2, respectively), as clearly marked by the anomaly of d/dT, indicating that the CD suppresses not only the conductivity but also the magnetism. This is probably because the localization of holes weakens the n.n. FM interactions (see Introduction). The fact that the TN s of CSFO remain around 120 K suggests that the same type of screw spin structure persists throughout the whole composition range. Fig. 6 shows the difference in isomer shift (IS) between the two kinds of Fe ions, Fe(4 − l) + and Fe(4 + l) + , in Ca0.8Sr0.2FeO3 plotted against tempera-

ture. It is clear ductivity set in The difference saturated value

685

that both the CD and the semiconat the same temperature of 240 K. in IS increased continuously to a of 0.28 mms − 1 around 150 K.

3.3. ND measurement Although CD has long been assumed to be coupled to the breathing mode, there has been no systematic structural analysis to test this idea. Morimoto et al. [26] found a structural change in CFO by synchrotron XRD, but unfortunately detailed information was not reported. In the present work we carried out a careful powder ND study on CFO and Ca0.8Sr0.2FeO3. Intensity data for a range of interplanar spacings between 0.5 and 4.97 A, were used for the Rietveld analysis, except those in TOF regions of 21000 –21500, 15100 –15150, 13740 – 13830, and 13960 –14060 where the sample holder showed additional peaks. In the initial refinement, structure parameters were refined with space group Pnma for all the diffraction patterns taken at temperatures of 130 –340 K. The site occupancy parameters, g, refined at 305 K for CFO indicated a stoichiometric composition of g (Ca)= 0.998 (4), g (Fe)=1.0001 (18), g (O1)= 1.004 (4), and g (O2)= 0.998 (7). The temperature dependence of the lattice parameters showed an anomaly at 290 K where the CD sets in, and then the refinement below 290 K was done with monoclinic space group P21/n as suggested by Morimoto et al. [26]. Fig. 7 illustrates the fitted profiles and difference patterns for the 305 K (Pnma) and 130 K (P21/n) data. Table 3 lists the final R factors and structure parameters and Table 4 lists the FeO distances. Plotted in Fig. 8a are the temperature dependences of the lattice parameters and the cell volume, all showing an anomaly at 290 K. The low temperature structure of CFO is illustrated in Fig. 9, where Fe1O6 octahedra (light) and Fe2O6 octahedra (dark), both being almost regular (see Table 4), are ordered in a rock-salt manner. Of special interest here is that these octahedra change their volumes in an inverse fashion as shown in Fig. 10. It is quite reasonable to assume that the concentration of electrons occupying the antibonding orbitals is higher in the larger Fe1O6 octahedra than in the smaller Fe2O6 octahedra. Or it may be said that Fe1O6 and Fe2O6 contain Fe(4 − l) + and Fe(4 + l) + , respectively. In other words, the oxygen hole concentration must be larger in the latter.

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T. Takeda et al. / Solid State Sciences 2 (2000) 673–687

Consistent with this, the valence bond sum calculated using the program EUTAX [27] for the Fe ions is 3.994 at 305 K but is split into 3.673 and 4.095 at 130 K, as plotted in Fig. 11. We emphasize here that the expansion and the contraction of the FeO octahedra (Fig. 10) begin at 290 K and become saturated around 200150 K as the difference in IS does [7]. This indicates clearly that the electronic transition is intimately coupled to the breathing mode in such a way that the CD proceeds (l: 0 “1), or the concentration of oxygen holes in the Fe2O6 octahedra increases, as the oxygen displacement increases below 290 K. The temperature dependences of the lattice constants and the cell volume of Ca0.8Sr0.2FeO3 are shown in Fig. 8b. In parallel to the case of CFO, all these parameters show an anomaly at : 240 K where the resistivity begins to increase rapidly (Fig. 4) and the Mo¨ssbauer spectrum splits into two components (Fig. 6). However, these changes are all smaller than those for CFO: for example, the saturated difference in IS is :0.28 mm s − 1 in comparison to 0.33 mm s − 1 for CFO. It is evident that the present oxygen hole-lattice coupled system changes the nature of the ground state from the holeconfined semiconductive state of monoclinic CFO to the hole-delocalized metallic state of cubic SFO quite smoothly depending on the Ca/Sr ratio, i.e. the FeOFe bond angle: it increases only by ca. 5° from CFO to Ca0.8Sr0.2FeO3 (Table 2 and Fig. 3b).

4. Concluding remarks Although the electronic phase transition in the CSFO system has long been assumed to result from the coupling to the breathing mode, there had previously been no systematic structural analysis to test this idea. The present study, done at various temperatures, succeeded in providing direct evidence for the intimate coupling. The structure changes from orthorhombic (Pnma) to monoclinic (P21/n) at the transition temperature, and the unique iron site (4b) in the former structure is divided into two (2c and 2b) in the latter, with slightly different average FeO bond lengths. Comparison of the structural, transport, and Mo¨ssbauer data indicates that the holeband width is very sensitive to the FeOFe bond angle and that a gap opens in the band when the breathing mode is frozen. The gap width increases

gradually as the size difference between the two kinds of FeO octahedra increases.

5. Note In the course of the preparation of this manuscript, we were informed that Woodward et al. also studied the low temperature structure of CFO using synchrotron XRD and ND at 15 K [28]. The lattice distortion and the structural parameters measured by them are consistent with the present ND analysis.

Acknowledgements This work wass supported partly by Grant-in-Aid for Scientific Research from The Ministry of Education, Science and Culture of Japan.

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