High pressure synthesis of a new chromite, ScCrO3

Materials Research Bulletin, Vol. 32, No. 12, pp. 1617-1624, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0025.5408/97 $17.00 + .oO

Pergamon

PI1 SO0255408(97)00151-7

HIGH PRESSURE SYNTHESIS OF A NEW CHROMITE,

ScCrO,

J.-H. Park and J.B. Parise* Center for High Pressure Research, Department of Earth and Space Science and Department of Chemistry, State University of New York, Stony Brook, NY 11790-2100

(Received

(Refereed) July 23, 1997; Accepted July 30, 1997)

ABSTRACT A new compound, ScCrO, the cylindrical type press 5.0329(2) A, b = 5.3602(3) refined using the Rietveld fraction data. 0 1997 Else~~irr KEYWORDS: structure

has been synthesized at 45 kbar and 1200°C using (USCA-1000). It has Pbnm symmetry with a = A, and c = 7.3790(4) A, and its structure has been technique and the synchrotron powder X-ray difSC~CIW Lrd

A. oxides, C. high pressure,

C. X-ray diffraction.

D. crystal

INTRODUCTION The perovskite family of materials has been widely studied because of its unique electronic, ionic conductive, and catalytic properties. For example, the chromite perovskites are oxygen ionic conductors and LaCrO, is used as a common interconnector in ceramic fuel cells [ 11. Additionally, structural studies of the orthorhombic-to-rhombohedral phase transition in alkaline earth metal doped chromite systems have been undertaken in an attempt to improve ferroelectric behavior [2]. The ideal perovskite structure, ABX,, is of cubic (Pm3m) symmetry and can be described as a three-dimensional network of corner-sharing BX, octahedra. The A cation is in the center of a cube defined by eight of these octahedra. Comparatively few perovskites have the ideal cubic structure with B-X-B angles of 180”; most bear some extent of distortion [3]. The symmetry of perovskites depends upon the relative size and the electron configuration of cations occupying the A or B sites [4]. To predict the stability of these structures as a function

*To whom correspondence

should be addressed. 1617

I618

of ionic sizes. Goldschmidt

J.-H. PARK ANI) .I.H. PARISE

(5 1detined a tolerance tactor. t = (rjx + r\)/,

Vol. 32, No. 12

A2(r,, + rN) where

r,\. rB. and rx are ionic radii of the A- and B-site cations and the X anion. respectively. If the A cation is too small. ;I structure in which thts :2 and B cations are both six-coordinated becomes competitive. Application 01‘ pressure can compress the A-X bond distance to a greater degree than the B-X bond distance. decreasing the tolerance factor (dt/dP < 0) [6] and resulting in an cxtcnsion of the perovskitc stability field into that of the corundum or ilmenite structure type. A good example of this approach includes the geologically important MgSiO, composition. which ha\ the p!roxenc structure near the earth’s surface and transforms via the ilmenite structure to that ot’ perovskite at high pressure 171. Thus, a useful method to find new perovskitea is by high-pressure treatment of promising starting compositions. Under high pressure. the structure of SczO; transforms to the structure type common to small cation rare-earth oxides (Ndm-Lu) [Xl whereas Cr,O, maintains the corundum structure. The different compressibilitics of SczO; and Cr,O, suggest that a new compound in the Sc,Ol-Cr,O, system may cxicr under high pressure as in the case of the Sc@-AI,O, system 19,lO]. In order to investigate this possibility. the oxides of the two-component system. SclO-CrIO I. have heen studied under high-pressure and hightemperature conditions.

EXPERIMENTAL A polycrystalline specimen of ScCrO: \btis yynthesized in an l(IO@ton Uniaxial Split Cylinder Apparatus (USCA- 1000). which consists 01.a two-stage anvil system 1I 1I. The first stage is a steel cylinder split into six parts that encloses a cubic cavity. The second stage resides in this cavity and contains eight tungsten carbide cubes, each of which is truncated at one corner and separated by pyrophyllite gashet\. Teflon back-up gaskets. and balsa wood spacers. The truncations define an octahedral cavity that holds the sample assembly (Fig. 1). An equimolar mixture of Sc,O, and Cr,O, sealed in ;I ~
Vol. 32, No. 12

1619

SCANDIUM CHROMITE

14 mm

I+

b)

6.35 mm

Gold

capsule

m ..,.~.:~..~....~~..~ Tm ring

m

Brconia

sleeve

m

Alumina

m

Graphite

furnace

n

MgO

FIG.

plug

1

A schematic diagram of the cell assembly in MgO octahedral pressure medium. Electricity passes through the tungsten carbide cube, the TZM ring, the graphite disk, and the furnace from the top to the bottom. A hole 6.35 mm in diameter and 11.42 mm in length is drilled in the center of the MgO octahedron, which has a length of 14 mm. The thicknesses of the zirconia end-cap, the MgO disk, and the graphite disk are 2.44, 0.76, and 0.76 mm, respectively. The lengths of the TZM ring, the graphite furnace, and the sample capsule are 3.15, 6.35, and 3.50 mm, respectively.

RESULTS

AND DISCUSSION

The tolerance factor [5] for ScCrO, calculated using Shannon’s ionic radii [ 181 is 0.796, indicating that it may be unstable as a perovskite under ambient pressure conditions. The experiment performed by Schneider et al. [19] was repeated to confirm that pressure is necessary to synthesize the perovskite compound. After heating a stoichiometric mixture of Sc,O, and Cr,O, in a sealed quartz tube at 1200°C for 1 day, X-ray powder diffraction analysis indicated the presence of Sc,O,-xCr,O, solid solution and excess Cr,O, (corundum structure) in agreement with the previous result.

.I -H. PARK AND .I.H. PARISE

Ih?O

0

0.002 I ( IO)

0.’

O.-KU(3

0

0.0000(?

O.XXI :

1

0.74.3

I I 1

0. I i-16(7, O.-M)2~7, 0.246(3

/ I’,,,,

1

IO0

1

0.6?(91

O(2) \ilt’\ 0.6X9( 1 J 03X(2 II,.,,

-

IO0

I

0.0733 I 1 0.25(9)

ot.31 \ite\

absences of certain rcllcctiw\ xc ~xm\~\tcnt M.ith the apace grouph 1512, and Phm (centric 1. Rietveld relinement u\in g hlarting models CdTiO, for /‘hn2, [ 201 and ScAIO, for P/v7777 [ IO] converged to similar models and discrepancy factors (Table 1). Furthermore. the ratio.
SCANDIUM

Vol. 32, No. 12

1621

CHROMITE

TABLE 2 Selected Bond Distances (A) and Angles (“) SC-O polyhedra 2.128(4) 2.072(4) 2. I l4(3) 2.372(3) 2.638(3) 3.163(4) 3.475(4) 3.539(3)

SC-O(l) (X I) SC-O(l) (XI) SC-O(2) (X2) SC-O(2) (X2) SC-O(2) (X2) SC-O( I )

SC-O(I ) SC-O(2) (X 2)

Cr-0

octahedra

Cr-O(l) Cr-O(2) Cr-O(2) (Cr-0)

(X2) (X2) (X 2)

I .950(3) I .965(3) 1.991(l) I .969(7) 87.6(l) (X2)

O(3)-Cr-O(4)

O(4)-Cr-O(4)

86.3(l)

(x2)

92.4(l)

(x2)

93.7(l)

(X2)

89.6(l)

(X2)

90.4(l)

(X2)

B-cation displacement is not likely in the space group Phnm and there was no indication of such behavior in the displacement parameter. The degree of octahedral distortion is represented by the bond-length distortion of CrO,, A,,, and bond-angle variance, u’. The former is defined as C {(ri - )/}* X lo’, and the latter as x [(@ -9O)‘/(n - 1)] where ri and I

I

I

50

20

30

40

50

I

60

60

I

70

70

61

80

2@(o)

FIG.

2

Observed (+) and calculated (-) X-ray powder diffraction profiles of ScCQ. Tick marks indicate the Bragg positions of ScCrO, (lower) and the gold trace from the capsule (upper). A difference curve (I,,,I,,,) is shown at the bottom. The high angle area (50” 5 28 5 SO’) is magnified.

J.-H. PARK

1622

AND

Vol. 32. No. 12

J.B. PARISE

08

0.9 ionic

(4

Cc)

of

1.2

1 .l

1

radius

A(Vlll)

cation

(b)

(4

(e)

(a), The structure of the ScCrO, perovskite pro.jected on (001) illustrates the tilting angles, O(Z)‘-O(2)““‘-O(2)” and O(2)“-O(2)“‘-O(2)“‘. . SC and Cr sites are denoted by large open and double circles. O(I) and O(2) are represented by small heavy and small open circles, respectively. The O(2)‘. O(2)““. O(2)“. O(2)“‘. and O(3)“” are indicated by lower case Roman numerals, and the I. values of O(2)‘,“. O(2)“.“‘. O(2)““, and O(2)““’ are 0.0669, 0.433 I. 0.5669, and -0.0669, respectively. Most atoms for which 0.5 < 7 < I .O have been omitted for clarity. (b), Plot of titling angles, O(2)‘-O(2)““-0(2)‘~ (diamond) and O(2)“0(2)-O(2)“” (square). (c). The bond-length distortion lrI‘. (d), the bond angle variance a2 of CrOb, and (e), the bond-length distortion A, of A0 II vs. ionic radius of A(VIII) cation in orthorhombic ACrO, perovskites (A = Nd. Gd. Er, In. and SC).

Bi are the i’h (Cr-0) bond distance and (0-Cr-0) bond angle; for a perfect octahedron, both of the values, AC, and c2. are zero 122,231. The tilting angles for irregular octahedra cannot be deduced from the cell parameters, therefore. the tilting angles suggested by Sasaki et al. [22J are used. The departure from cubic symmetry in the GdFeO,-type perovskite is

Vol. 32, No. 12

Distortion

SCANDIUM CHROMITE

I623

TABLE 3 of Polyhedral Sites and Tilting Angles (“) for ACrO, (A = SC, In, Er, Gd, and Nd)

4w, a2 CrWI1 A *(XII) O(2)‘-O(2)““‘-O(2)” 0(2)“-0(2)“‘-0(2)“” Bond-valence sum [27]

InCrO,

ErCrO,

GdCrO,

NdCrO,

ScCrO,

1261

1241

~241

1251

0.075 6.99 44.62 129.9(2) I 10.8(2) 2.78

0.096 7.31 38.40 131.96 110.39 2.83

0.041 3.66 30.4 I 141.2(l) 105X(2) 3.02

0.014 0.61 22.5 1 144.95 103.89 3.06

0.0054 I .48 16.98 145.29 104.85 3.24

The program VOLCAL was used for this calculation [28]. The bond-length distortion, A, [22] is defined as l/n ): {ri - (r))/(r)]* X IO’. The bond angle variance, u2, [23] is defined as 2 [Oi - 90)‘/(n - I)].

manifested by the deviation of the O(2)‘-O(2)‘“‘-O(2)” angle (Fig. 3a) from 180”, which corresponds to an octahedral tilt on the xy plane. Similarly, the variation of the O(2)“-O(2)“‘O(2)“” angle (Fig. 3a) from 90” constitutes a polyhedral tilt on the xz plane. The octahedral tilting and distortion, in turn, cause deformation of AO,, polyhedra from the ideal cuboctahedron and, consequently, an effective coordination number of 8 for the A cation. The octahedral tilting angles (O(2)‘-O(2)““‘-O(2)” and O(2)“-O(2)“‘-O(2)““), the degree of the octahedral distortion (A,-.,. a2), and the (A-O) bond-length distortion of the AO,, polyhedra are listed in Table 3 and are plotted against the ionic radius of the A ion (VIII) (A = Nd, Gd, Er, In, and SC) in orthorhombic ACrO, perovskites in Figure 3 [22-281. When Gd occupies the A site instead of Nd, little variation in either tilting angle is observed. As the A cation is changed from Gd, Er to In, there is a gradual decrease in the O(2)‘-O(2)““-O(2)” angle and a moderate increase in the O(2)“-O(2)“‘-O(2)‘” angle. The substitution of SC in the A site leads to a saturation in both tilting angles (Fig. 3b). As the size of A cation decreases from Nd to In, the degree of octahedral distortion is enhanced; however, further decreasing the size from In to SC leads to a decrease in octahedral distortion (Fig. 3c and d). The degree of bond-length distortion of AO,, clearly increases as the ionic size of the A cation decreases (Fig. 3e). The bond-valence sum calculation for the twelve A-O bonds, as shown in Table 3, indicates an evident diminution in the value across the series from Nd to SC. To maximize the effective coordination of the A cation by oxygens, the contribution of both octahedral tilting and octahedral distortion increases as the size of the A cation decreases; however, it reaches a maximum for In, and a further reduction in the A cation size to SC does not result in further tilting or distortion. Thus, the incorporation of SC into the perovskite structure must be accommodated by a larger translation of the A cation off center than was found for the other cations.

ACKNOWLEDGMENTS This work was supported by a grant from the DMR-9413003 to JBP. The powder diffractometry was carried out at the X-7A beamline at the NSLS, a facility operated by the U.S. Department of Energy, Divisions of Materials Science and Chemical Sciences.

162-l

J.-H. PARK

AND

Vol. 32, No. 12

J.B. PARISE

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