Structural characterization of the orthorhombic perovskites: [ARuO3 (A = Ca, Sr, La, Pr)]

Pergamon

Materials Rest,arch Bulletin, Voi. 29, No. 12, pp. 1271-1280, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/94 $6.00 + .00

0025-5408(94)00086-7

STRUCTURAL CHARACTERIZATION OF THE ORTHORHOMBIC P E R O V S K I T E S : [ARuO3 (A = Ca, Sr, La, Pr)]

H. Kobayashi, M. Nagata, R. Kanno * and Y. Kawamoto Department of Chemistry, Faculty of Science, Kobe University Nada, Kobe, Hyogo, 657 Japan (Received:

August 31, 1994; Communicated by M. Koizumi)

ABSTRACT The GdFeOs-type perovskites, ARuOa (A = Ca, Sr, La, Pr) and their solid solutions have been synthesized and their structures were refined by X-ray Rietveld analysis. In the Srl_,Ca~RuOa system, the twelve A - O bondlengths split in two groups of eight short and four long distances, and the difference between two groups becomes larger with increasing z. The bend in the zig-zag chains formed by the corner-sharing RuO6 octahedra increases with z, while no change in the Ru-O distances was observed. These structural changes correspond to the weakening of the ferromagnetic interactions from x = 0 to 0.6. For the ARuOa (A = Ca, Sr, La, Pr) series, the distortion from the ideal cubic perovskite increases in the order of Sr, La, Ca, Pr. The splitting of the two groups for the long and short A-O distances and the Ru-O-Ru angle decreases in that order, which corresponds to the metallic property for A = St, La, Ca and semiconducting for A = Pr. MATERIALS INDEX: ruthenium oxides, perovskite

Introduction The orthorhombic perovskites ARuO3 (A = Ca, Sr, La, Pr) with the GdFeO3-type structure give a wide variety in physical properties. SrRuO3, CaRuO3, and LaRuO3 exhibit metallic property with low resistivities of 10-3 fi cm at room temperature(l, 2), while PrRuOa showed semiconducting property(3). The striking difference in magnetic properties of SrRuO3 and CaRuO3 makes the solid solution Srx_~Ca~RuO3 quite interesting; SrRuOa is ferromagnetic below Tc = 160 K (4, 5, 6) accompanied by a slight change in resistivity(l) at the Curie temperature, and CaRuO3 follows a Curie Weiss law at high temperatures with *Author to w h o m correspondence should be addressed.

1271

1272

H. KOBAYASHI et al.

Vol. 29, No. 12

a negative Weiss constant and no anomalies in resistivity down to 4.3 K(4, 5). The solid solution has therefore been investigated by magnetic measurements and 99Ru MSssbauer spectroscopy(7) in order to elucidate the possible origin of the ferromagnetism of SrRuO3, and the striking difference in magnetic properties although the two compounds are closely related both chemically and structurally. The magnetic susceptibility measurements indicated a decrease in the Curie temperature with the substitution of Ca ~+ for Sr ~+, indicative of a weakening of the ferromagnetic exchange interaction(8). The magnetic interaction was also studied by the ~ R u MSssbauer spectroscopy and the difference in magnetic properties on the Srl_,Ca, RuO3 system is understood by the role of counter cations, Sr 2+ and Ca2+; the greater electron-pair acceptor strength of Ca 2+ compared to Sr2+ results in a more effective competition with ruthenium for the oxygen anion orbitals involved in the superexchange interaction(7). ARuO3 (A=La, Pr) is one of the few oxide series in which ruthenium adopts the trivalent state (2, 3). LaRuO3 is metallic and the 99Ru MSssbaner spectroscopy indicated paramagnetic at 4.2 K(2, 9). PrRuO3 was synthesized by Greatrex et al. (3) under a pressure of 2 GPa. Electrical measurement using a sample compacted at 16 MPa indicated the semiconducting behavior with an activation energy of 0.17 eV between 373 and 673 K. Metallic-to-semiconducting property change with the A cation changing from La to Pr is similar to those reported for the ANiO3 and ATiO3 perovskites (10, 11). The GdFeOs-type structure consists of a framework of corner-sharing BOs octahedra linked into zig-zag chains. The structural studies on ANi03 and ATiO3 perovskites indicate that the B-O-B angle of the network particularly plays an important role of the superexchange interaction and also of the formation of electric conduction bands(10, 11). For the ruthenium perovskites, however, the relationship between the structural changes and the electrical and magnetic properties has not been reported. In the present study, the structures of the solid solutions, Srl_~Ca~RuO3, were refined by X-ray Rietveld analysis. We synthesized the perovskites, LaRuO3 and PrRuOs, containing trivalent ruthenium ion and determined their structures. Further, electrical properties were re-investigated for Srl_~Ca~RuO3, LaRuO3 and PrRuOs; their physical properties and the structural changes were discussed. Experimental The quaternary oxides Srl_,Ca~RuO3 were prepared by heating appropriate molar ratios of SrCO3, CaCO3, and RuO2 (SrCOs: Nakarai Chemicals Ltd.; CaCO3 > 99.9 % purity, Wako Chemical Industries Ltd.; RuO2: > 99.99% purity, Furuuchi Chemicals Ltd.). The samples were calcined at 1223 K for 24 h. After being reground, the samples were pressed into pellets again, and fired at 1473 K for 48 h. Nitrates, St(NO3)2 and Ca(NO3)2 • 4H20, were also used as the starting materials for synthesizing sample at x = 0.5 in Srl_,CaxRuO3 (Sr(NO3)2: > 98.0% purity, Ca(NO3)2" 4H20: > 98.5% purity, Nakarai Chemicals Ltd.). The ternary oxides ARuO3 (A = La, Pr) were synthesized at a high pressure condition using a piston-cylinder apparatus. The starting materials, La203, Pr6Oll, RuO2, and Ru were weighed, mixed, put into a platinum capsule in an argon atmosphere, and then reacted at 2 GPa and 1573 K for 30 min. The pressure was released after the samples were cooled to

Vol. 29, No. 12

PEROVSKITES

1273

room temperature. X-Ray diffraction (XRD) patterns of powdered samples were obtained with a X-ray powder diffractometer (Rigaku RAD-C). A curved graphite monochromator equipped after the sample position was used for lowering the background. Diffraction data were coLlected with CuKa radiation for 3 - 6 s at each 0.04 ° step-width over a 20 range from 18° to 150° at room temperature. Structural parameters were refined by Rietveld analysis with the computer program RIETAN (12). Reflection positions and intensities were calculated for both CuKcrl and CuKct2 with a factor of 0.5 applied to the calculated integrated intensities of CuKa2 peaks. A pseudo-Voigt profile-shape function was used; the mixing parameter 3' was included in the least-squares refinement. The electrical resistivity for Srx_~Ca~RuO3 was measured for the sintered materials with dimensions of approximately 2 × 2 × 5 mm. For LaRuO3 and PrRuO3, we used the pellets of 2 mm diameter and 2 mm length which were directly obtained from the high pressure synthesis. The data were obtained by dc four-probe method in the temperature range 15 < T < 300 K using Toyo-Sanso low-temperature electrical-conductivity measurement unit. The temperature of the samples was measured using a (Au + 0.07% Fe) versus chromel thermocouple. R e s u l t and discussion Srl_~CarRuOa: Monophasic properties of the perovskite structure were observed for the whole range of solid solutions, Sh_~Ca~RuO3, as reported previously. No superlattice reflections were observed for the XRD patterns obtained. Table 1 lists of the lattice parameters after Rietveld refinement. The lattice parameters decrease with the substitution of Ca for Sr, and Sr0.6Ca0.4RuO3 was indexed with a pseudo-cubic cell. These results are in good agreement with those reported by G i b b e t al. (7). Figure 1 shows the temperature dependence of the resistivity for Srl_~Ca~RuO3. Although the resistivity values are slightly higher than those from single crystals reported previously for SrRuO3 and CaRuO3(1), the temperature dependency is quite similar to the behavior of the single crystals. For example, the resistivity curve for SrRuO3 shows a slope change around 160 K, which corresponds to the ferromagnetic transition Tc. Metallic behavior was observed from x = 0.0 to 0.4 in Sh_xCa~RuO3 similar to the previous data, while the sample at x = 0.5 showed a slight semiconducting behavior. We measured densities of the samples by the pycnometric method and compared to the calculated densities; the relative densities of 92 ,,~ 95 % with no significant composition dependence were observed. Further, we synthesized the sample at x = 0.5 from Sr(NO3)2 and Ca(NO3)2 in order to clarify the starting materials dependence of the densities and resistivities. However, the sample showed semiconducting behavior with the same resistivity values as that synthesized from the carbonates, and also showed the relative density of 95 %. This indicates that the temperature dependency of the resistivity might not be affected by the grain boundary contribution. The crystal structures have been reported for SrRuO3 and CaRuO3 using neutron powder diffraction and single crystal X-ray diffraction measurements, respectively(13, 14). Bensch et al. reported the structures of SrRuO3 and CaRuO3 using single crystal X-ray

I

I

1274

H. KOBAYASHI et al.

[

101

i

Vol. 29, No. 12

10-2

i

(a)

(b) / Sro,sCao.sRu~

loO

PrRuOs

•

---.. 10-1 Z' "F

Q.

LaRuO3

,F~'10-3

m

=_= lO -2

CaRuO3

10-3 10-4

,

I

100

~

I

200

TI K

,

L, 3

300

10-4 0

,

I

1O0

~

I

200

,

I

300

TI K

FIGURE 1 Temperature dependence of the resistivity for Sra_~CaxRu03, LaRu03 and PrRu03. diffraction measurements (13). They refined the structures using the space group Pm3 for SrRuO3. However, our sample clearly indicated the symmetry reduction from cubic to orthorhombic as reported by Jones et al.(14). In the ideal cubic perovskite, each A cation has 12 equidistant neighboring oxygen ions and RuOs octahedra are linked with each other by sharing corner along the cell axis. The orthorhombic perovskites have twelve A-O distances which separate into two groups of eight short and four long distances; RuOs octahedra are then tilted and rotated to fill the extra space around the A site. The orthorhombic perovskite structure was refined with space group Pnma with the structural model, A (Ca, Sr) at 4c (z, 1/4, z), Ru at 4b (0, 0, 1/2), O(1) at 8d (z, y, z), 0(2) at 4c (z, 1/4, z). The site occupation parameters, 9, were fixed at g = 1.0, except for those of the 4c site which were fixed at the value of the starting composition. Isotropic thermal parameters, B, were refined in the final refinement cycles. The B's for the 0(1) and 0(2) sites had to be constrained to the same value; otherwise the refinements were unstable. Table 1 lists final R factors, structural parameters, and their estimated standard deviations. Table 2 gives interatomic distances and bond angles calculated with ORFFE (15). The calculated interatomic distances and bond angles from the reported structures (13, 14) are also indicated in Table 2. Our refinement results on SrRuO3 and CaRuO3 are well consistent with the reported values. Figure 2 shows the composition dependence of the interatomic distances and bond angles in Srl_~Ca, RuO3. The twelve A-O bondlengths split in two groups of eight short and four long distances, and the difference between two groups becomes larger with increasing z. On the other hand, no significant change was found for the Ru-O distances. The Ru-O-Ru angle decreases from 164 to 150° with increasing x, which indicates that the bend in the zig-zag chains formed by the RuOs octahedra connected by corner sharing increases with x. No significant deviation from 90° for the O-Ru-O angles indicates no distortion of the

PEROVSKITES

Vol. 29, No. 12

1275

I

3'5 f

I

I

I

l

l

l

t

l

l

l

170

[

(b)

(a)

v vv'VV

o q~

0

v

<~

3.ot-'- • I- (A-O)I~Q

Ru-O-Ru

160

o~

(A-O)s~rt

o-

10G

150

o

2.5 e e ~I' 41, 41,

Ru -O 2.0 -.,,-o • I 0

f O-Ru-O

¢-

•••o•

i t r f 0.2 0.4 0.6 0.8

x in Sr l_xCaxRuO3

• t 1

•

8~

I J t r i 0

t I

I J i I 1

x in Srl_xCaxFluO3

FIGURE 2 Composition dependence of the (a)interatomic distances and (b)bond angles for Srl_,Ca~RuO3. RuO6 octahedra. The substitution of smaller Ca 2+ ions for Sr 2+ ions leads to the decrease in Ru-O-Ru angles, and then makes the ferromagnetic interactions weak from z = 0 to 0.6. ARuO3 (A -----La, P r ) : Reaction products of the high-pressure synthesis showed the Xray diffraction patterns of the orthorhombic perovskite structure under a condition of 2 GPa and 1573 K. However, a very small amount of the second phase, RuO2, was always present. The electrical resistivity was measured for the pellets of the samples directly obtained by the high pressure synthesis. Figure 1 shows the temperature dependence of the resistivity for LaRuO3 and PrRuOs. LaRuO3 is metallic with low resistivities of 10 -2 f~ cm at room temperature, while PrRuO3 is semiconducting. This behavior is consistent with the previous data obtained above room temperature (2, 3).

The structures of LaRuO3 and PrRuOs were refined with space group Pnma using the same structural model as those of Srl_~CaxRuOs. The site occupation parameters, g, were also fixed at g = 1.0. The B's for the O(1) and 0(2) sites were constrained to the same value. Table I lists final R factors, structural parameters, and their estimated standard deviations. Table 2 gives interatomic distances and bond angles. R e l a t i o n s h i p b e t w e e n crystal structure and physical p r o p e r t i e s : The GdFeO3-type perovskite structure consists of a framework of corner-sharing Ru06 octahedra linked into zig-zag chains. In the ideal perovskite structure, the B06 octahedra sit at the corners of a cubic unit cell, with their axes oriented along the cell edges. In the center, there is space for A ion which will fit perfectly if dA-o = v ~ dB-o (16). Since the A are too small to satisfy this criterion, the BOs octahedra are tilted and rotated to fill the extra space around the A ion. These rotations cause the unit cell to be smaller and distorted from the ideal cubic cell.

H. KOBAYASHI

1276

et al.

Vol. 29, No. 12

TABLE 1 Rietvelt refinement results for Srl_xCaxRuO3, LaRuO 3 and PrRuO 3 Sr/Ca

x=O.O

x=0.2

Sq ~Ca~guO, x=0.4 x=~.5

x=0.6

x=0.7

~/

5.53283(11) 5.5290(3)57.~9~3 / 5.5136(4) 5.5115(2) 5.51275(19) 7.84712(16) 7.8231(4) 7.7921(6) 7.7632(4) 7.7392(3) 5.56926(9) 5.5453(3) 5~5061(3) 5.4932(4) 5.4690(2) 5.44436(19)

x y ~(/~2)

R~

0,0170(5) 1/4 -0.0019(15) 0.45(4) 0 0 1/2 0.16(3) 0.276(4) 0.029(3) 0.72.3.(4) 0.6(2) 0.498(3) 1/4 0.046(5) 0.6 12.00 8.22 1.98

SrlCa

St, .zCaxRuO3 x=0.ffx=l.0

A

Ru x y 1~ O((A2) x y 2~B O((A2) x y ~(/~z) b

0.0227(4) 1/4 -0.002(2) 0.41(4) 0 0 1/2 0.13(3) 0.287(4) 0.031(1) 0.717(4) 0,4(2) 0,490(3) 1/4 014060(7) 11.53 8.49 1.87

0.0296(5) 1/4 -0.003(2) 0.49(4) 0 0 1/2 0.19(3) 0.291(4) 0.030(3) 0.71.3.(4) 0.4(2) 0.487(3) 1/4 0.080(7) 0.4 11.26 8.66 1.62

0.0337(7) 0.0362(5) 0.0400(5) 1/4 1/4 1/4 0-..3.O0~(2) -0.0071(14) -0.0089(13) 0.50(5) 0.46(6) 0 0 0 0 0 0 1/2 1/2 1/2 0.23(4) 0.21(3) 0.19(3) 0.294(5) 0.294(2) 0.296(2) 0.031(4) 0.037(2) 0.042709) 0.711(5) 0.708(3) 0.705(2) 0.4(3) 0.51(I9) 0.42(19) 0.488(4) 0.481(3) 0.480(3) 1/41/4 1/4 0.084(9) 00:501~5 ) 0.079(4) 0.4 0.42 13.21 9.69 10.13 7.67 6.87 6.95 2.46 1.58 1.42

LaguO3

PtRuO3

SrRu030

CaRuOab)

5.5141(2) 5.52238(16) 5.6847(2) 7.7113(3) 7.6626(2) 7.8897(3) 5.41531(19) 5.35991(15) 5.5462(2)

5.8344301) 5.5304(1) 7.74767(15) 7.8446(2) 5.37940(11) 5.5670(1)

x y z o_ B(A2) Ru x y 1~ O((JL2)

0.0437(6) 1/4 -0.0118(15) 0.50(8)

0.0538(7) 1/4 -0.0153(15) 0.34(10)

0.0691(6) 1/4 -0.0218(8) 0.38(6)

0.0157(4) 0.0552(4) 1/4 1/4 -0.0027(3) -0.0139(2) .......

0 0 1/2 0.16(3)

0 0 1/2 0.15(4)

0 0 1/2 0.38

0 0 1/2

0 0 1/2

x y z o_ O(,,xB(AZ),,, x y z o_ b R/

0.297(2) 0.040(2) 0.707(2) 0.3(2) 0.474(3) 1/4 0.089(4) 0.3 9.88 6.39 1.45

0.299(5) 0.042(4) 0.685(5) 0.38 0.468(7) 1/4 0.102(7) 0.38 15.07 6.90 8.42

0.2764(2) 0.0278(2) 0.7248(2) ............... 0.4966(5) 1/4 0.0532(4) 3.23 2.06 8.4

0.2979(5) 0.0482(3) 0.6973(4)

~t A

0.0491(7) 1/4 -0.0118(15) 0.71(12)

0 0 1/2 0.45(14) 0.28 0.2952(19) 0.040(7}5(6) 0.04 16) 0.70~92~21 0.723(8) 0.12(I8) 0.4(7) 0.473(3) 0.493(7) 1/4 1/4 0.093(3) 0.088(10) 0.12 0.4 11.40 16.44 6.52 6.87 1.53 6.47

5.524(1) 7.649(2) 5.354(1)

0.4742(6) 1/4 0.0920(6) ....... ....... .......

a)From Ref.(14) ; b)From Ref.(13) The distortion is discussed in terms of a tolerance factor defined as t = dA-o / V/2 dB-o. We calculated the tolerance factors for ARuOs with the observed A-O and B-O distances. The t-factor decreases in the order of Sr (0.95), La (0.90), Ca (0.88), Pr (0.87), indicating that the distortion from the ideM cubic perovskite structure increases in that order.

Vol. 29, No. 12

PEROVSKITES

1277

TABLE 2 Interatomic distances(~) and angles(°) for Sr]_~CaxRuO3, LaRuO3 and PrRuO 3 St/Ca Ru-~ Ru--C

Ru-O(2 -Ru A-O(1) x2l) A-O(1) x2 A--O(1) A--O(2~)

x=0.0 1.978(4) 91.6(I2) 89.0(13) 91.10(1~ 162.1(11 165.1(17 2.510(18 3.136(1~ 2.54(3) 2.6720~

x=0.2 1.98(3) 2.01(3)

158:~t~1)

~64~2~ 2 :7~1 2 .

A-O(2~)

Sr1 ~Ca~uO. St/Ca x=0.8 x=~l.0 Re-O(1) ix2) 1.969(12) 1.992(11 Ru-O(1) (x2) 2.006(1 12) 1.994(11

R.-O¢2) (~1_ 1.992~5) O(1)-gu-:Of2)

1.9S50)

Sr~.~Ca~Ru03 x=0.4 x=0.5 1.97 ." } 1.97(: 2.01( ~.~ 2.003 )

89:: 15

154(2

~

0)

15~i~ !3)

2.67(, 2.45(~ 2.731~ 3.027 3.19(,

2.33( 2.55( 3.05( 3.19(

LaRuOs

PrRuO3

2.01(4) 2.05(4) 2.03~14)

O(1)7Ru-O(1) 89:4309) ~ : ~ * Ru-.~O(1)-Ru 152.9(6) 149.8(5) Ru- 2):-Ru 150.8(1]) 149.6(9) A~3~) (x2) 2.365(11) 2.326(1( 2.5 A-O(1) (x2) 2.624(15) 2.537(1~ 2.5~i A_-_-_~I 2.680(16) A 1t/~x~) x2 3.355(12) 2.704(1~ 3.433(11 A--O(2) 2.32(2) 2.306(1~ 2.37~i A-O(2) 2.437(17) 2.386(1~ 2"5~i A-O(2'F) 3.15(2) 3.129(17 3.2 A--O(2y) 3.18707 ) 3.260(1{ 3.21( a)From Ref.(14) ; b)From Ref.(13) ; c)Long A--O distances

21022(111 86.6(14) 91.2(14) ..6~571 6)2.~(~ ) 2.63(3) 2.63(3~ 3.61(3) 2.33(4) 2.42(4) 3.18(4) 3.57(4)

x=0.6

x=0.7

l:g~0~] ~

1.988 2.003

19 1(6) 91.4(9) 89.1(2) 154.3(7) 154.1(14) 2.391(14) 2.679(16) 2.689(17) 3.312(14) 2.3 ~i31~116~

1.985 ~)

89:b~; )) 152.1 154.2 2.353 2.642 2.704 3.359 2.37(: 2.471 3.125 3.125

SrRuOs0 CaRuOsb)

1.981,2) 1.987(2)

21. 1/1

1.98.3(2) 90.04 91.09 162.8 162.8

1.980(1) 89.28 89.05 90.16 148.9 150.0 2.31 3) 2.564(3) 2~17723~4) 2.67"/_.(_3) 3.447 2.505(4) 2.303(4) 2.67_8(4) 2.38_3(4) 2.888 3.127 3.067 3.259 90.41

Figure 3 shows the interatomic distances and bond angles versus tolerance factors for ARuOs. The Ru-O distances of LaRuO3 and PrRuO3 are slightly longer than those of CaRuO3 and SrRuO3, which indicates the existence of trivalent ruthenium ions in the former compounds (ionic radii, r = 0.68/~ for Ru 3+ and r = 0.62 A for Ru 4+) (17). The twelve A-O bondlengths split into two groups of eight short and four long distances, and the difference between two groups becomes larger with decreasing t-factor, or increasing distortion from the ideal cubic perovskite. The average Ru-O-Ru angle decreases from 164 to 150 ° with decreasing t-factor, which indicates that the bend in the zig-zag chains formed by the corner-sharing RuO6 octahedra increases. No significant deviation from 90° for the O-Ru-O angles indicates no distortion of the RuO6 octahedra. The structural changes indicated above are well consistent with the physical property changes. For magnetic property, the ferromagnetic interaction decreases with x in Srl_xCa~RuOa as described previously. For electrical property, the metallic conductivity was observed for ARuOa, except for PrRuOa, being consistent with the structural changes that the Ru-O-Ru angle of PrRuOa is smaller than those of other compounds. The bridging

I

I

1278

H. KOBAYASHI et al. I

I

I

I

I

]

I

I

I

I

I

]

I

Vol. 29, No. 12 I

I

I

I

170

1 1 1 1 1 1 1

(a)

3.5

(b)

o

o~

/

160

(A-Ohong

3.C

i

Ru-O-Ru o o Ca Pr

(A-O) short

"O

E 2.5

\

o

o

o o Pr Ca

La

Sr

o La

o 10(3

150

O-Ru-O Ru-O

2.0

¢-

I

0.85

t

I

j

L

t

I

I

I

I

I

0.90 0.95 tolerance factor t

I

800.~

5

I

I

I

t

I

I

,

~

~

,

0.90 0.95 tolerance factor t

j

FIGURE 3 (a)Interatomic distances and (b)bond angles versus tolerance factor for ARuO3. angles for the corner-sharing BOs octahedra linked into zig-zag chains plays an important role of the superexchange interaction and the formation of electrical conduction bands for the GdFeOz-type perovskite compounds. Table 3 summarizes the relationship between the structures and physical properties for the GdFeO3-type and the pyrochlore structures. In RB03 (R = rare earth, B = Ni, Ti), the variation of the B-O-B angles corresponds to the electrical and magnetic properties (10, 11). In RNiO3, the metallic-to-semiconducting transition was observed for R = Pr, Nd, Sm and the transition temperature shifted to higher temperatures for the smaller R cations, while LaNiO3 showed metallic property. The Ni-O-Ni angle decreases from the ideal cubic perovskite value of 180° with decreasing size of the R ions. The electron delocalization is then controlled by the Ni-O-Ni angle between neighboring, corner-sharing octahedra(10). Discontinuous changes in electrical and magnetic properties occur as the radius of R decreases in RTiO3 (11). Room temperature metallic conductivity with metallic-to-semiconducting transition at low temperatures was observed for R = La, Ce. However, all other RTiO3 are semiconducting with activation energies increases from Pr, Nd to Gd, Y. The above changes are also correlated to the variation of the Ti-O-Ti angles. We also reported the relationship between the structural changes and electrical properties for the pyrochlore compounds, A2Ru20~ (A = Pb, Bi, R) (18, 19, 20). The pyrochlore structure is similar to the GdFeO~-type perovskite structure in view of a framework of corner-sharing RuO6 octahedra. Their electrical property changes were well understood by the Ru-O distance and Ru-O-Ru angle variations. The metallic-to-semiconducting change is related to their structural changes as follows; (i) the increase in the Ru-O bondlength in the RuO6 octahedra, (ii) the increase in the distortion of the RuO6 octahedra, and (iii) the increase in the bend of the RuO6 zig-zag chains. These variations affect the Ru-O overlap integrals, leading to the transition from metallic to semiconducting.

i

i

1279

PEROVSKITES

Vol. 29, No. 12

TABLE 3 Relationship bctw~n physical properties and structure in the GdFcO3-tYpe perovskite and the pyrochlore compounds perovskite

La Ce Pr Nd I'm Sm Eu Gd Tb Dy Ho Er Tin Yb Lu Y

RNiO31°)

M

B-O-8 angle 1~rFiO311) B - O - B angle

pyrochlore R2Mo20721'22)

MS

MS

130 200

MS

400K.(transitiontemperature) > mct~ase

159 157 153" < increase MS MS S S 157 151 147 <

S 145

S 142"

increase

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho F.x Tm Yb Lu Y M

M M [S ferromagnetism [

R_MIL ~ 0723)

S

S S S S paramagnetism

S

S

S

S E

S

S S -,>S

increase

M: metallic ; MS: metallic-semiconductingtransition ; S: semiconducting; E: activationenergy

Similar property changes have also been reported for the pyrochlore compounds, R:Mo207 and RzMn~OT. Metallic behavior for R = Nd, Sin, Gd and semiconducting for R = Tb, Y were observed for R2MozO¢ (21, 22). The increase in activation energy was also reported for the semiconducting R2Mn20¢ with decreasing size of R ions(23). Both examples were previously described by the inductive effect of R cations; more acidic character of R cations leads to the semiconducting behavior. However, the physical property changes in the GdFeOa-type perovskite and the ruthenium pyrochlore structures are well understood by the structural changes, particularly the B-O-B angle changes. This indicates that these property changes in these pyrochlores, R2Mn207 and R2Mo2OT, might also be understood by the geometric effect. Conclusion The relationship between the crystal structures and physical properties was clarified in the Srl_xCa~RuO3 and ARuOa (A = Ca, Sr, La, Pr) systems. In both systems, the Ru-O-Ru angle variation highly affects their magnetic and electrical properties. Similar relationship between the structural changes and physical properties is widely observed for the perovskites with the GdFeOa-type structure, RTiOs and RNiOs, and the pyrochlores containing ruthenium and molybdenum. The geometric effect on the physical property changes is important for the compounds formed by the framework containing zig-zag BOo octahedra. Further, the structural characterization using X-ray Rietveld analysis is convenient and powerful tool to understand their electrical and magnetic properties.

1280

H. KOBAYASHI et al.

Vol. 29, No. 12

Acknowledgments We thank Dr. F. Izumi of NIRIM for providing the computer program RIETAN. All computations for structure determination were carried out at the Kobe University Information Processing center.

References 1. R. J. Bouchaxd, and J. L. GiUson, Mater. Res. Bull., 7, 873 (1972). 2. R. J. Bouehard, and J. F. Weiher, J. Solid State Chem., 4, 80 (1972). 3. R. Greatrex, G. Hu, and D. C. Munro, Mater. Res. Bull., 21, 797 (1986). 4. J. M. Longo, P. M. Racca.h, and J. B. Goodenough, J. Appl. Phys., 39, 1327 (1968). 5. A. Callaghan, C. W. Moeller, and R. Ward, Inorg. Chem., 5, 1572 (1966). 0. A. Kanbaya~hi, J. Phys. Soc. Japan., 41, 1870 (1976). 7. T. C. Gibb, R. Greatrex, N. N. Greenwood, D. C. Puxley, and K. G. Snowdon, J. Solid State Chem., 11, 17 (1974). 8. A. Kanbayashi, J. Phys. Soc. Japan., 44, 108 (1978). 9. F. M. Da Costa, R. Gretarex, and N. N. Greenwood., J. Solid State Chem., $0, 381 (1977). 10. P. Lacorre, J. B. Torrance, J. Pannetier, A. I. Nazzal, P. W. Wang, and T. C. Huang, J. Solid State Chem., 91,225 (1991). 11. J. E. Greedan, J. Less-Common Met., 111,335 (1985). 12. F. Izumi, "The Rietveld Method", ed. by R. A. Young, Oxford Univ. Press, Oxford (1993), Chap. 13. 13. W. Bensch, H. W. Schmalle, and A. Relier, Solid State Ionics, 43, 171 (1990). 14. C. W. Jones, P. D. Battel, P. Lightfoot, and W. T. A. Harrison, Aeta CrystaUogr., C45, 305 (1989). 15. W. R. Busing, K. O. Martin, and H. A. Levy, Report ORNL-TM-306, Oak Ridge National Laboratory, Ork Ridge, TN (1964). 16. V. M. Goldschmidt, Geochemisehe Verteilungsgesetze der Elemente VII, VIII (1927[28). 17. R. D. Shannon, and C. T. Prewitt, Acta Crystallogr., B25, 925 (1969). 18. R. Kanno, Y. Takeda, T. Yamamoto, Y. Kawamoto, and O. Ya~namoto, J. Solid State Chem., 102, 106 (1993). 19. T. Yamamoto, R. Kanno, Y. Takeda, O. Yamamoto, Y. Kawamoto, and M. Takano, J. Solid State Chem., 109, 372 (1994). 20. It. Kobayashi, R. Kanno, Y. Kawamoto, T. Kamiyama, F. Izumi, and A. W. Sleight, J. Solid State Chem., in press. 21. J. E. Greedan, M. Sato, N. Ali, and W. R. Datars, J. Solid State Chem., 68, 300 (1987). 22. M. A. Subramanian, G. Aravamudan, and G. V. S. Rao, Mater. Res. Bull., 15, 1401 (1980). 23. M. A. Subramanian, C. C. Torardi, D. C. Johnson, J. Pannetier, and A. W. Sleight, J. Solid State Chem., 72, 24 (1988).