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Crystal chemistry of REVO3 phases (RE = La-Lu, Y)
Mat. Res. Bull. Vol. 9, pp. 1279-1284, 1974. Pri nt e d in the United States.
P e r g a m o n P r e s s , Inc.
CRYSTAL CHEMISTRY OF REVO 3 PHASES (RE = L a - L u , Y) G r e g o r y J. McCarthy, Carol A. Sipe and Kenneth E. McIlvried Mater ials R e s e a r c h L a b o r a t o r y The Pennsylvania State U n i v e r s i t y U n i v e r s i t y P ark , P e n n s y l v a n i a 16802 (Received June 5, 1974; C o m m u n i c a t e d by R. C. D e V r i e s ) ABSTRACT All REVO 3 phases have been p r e p a r e d and c h a r a c t e r i z e d by x - r a y powder diffraction for s y m m e t r y and unit c e l l p a r a m e t e r s . LaVO 3 and CeVO 3 have t e t r a g o n a l v a r i a t i o n s of the perovskite s t r u c t u r e while all r e m a i n i n g REVO 3 phases a r e o r t h o r h o m b i c and i s o s t r u c t u r a l with GdFeO 3. The variation of cell p a r a m e t e r s with RE 3+ ionic r a d i u s shows a s t e a d y c o n t r a c t i o n with d e c r e a s i n g r a d i u s for ao, c o and V while bo goes through a maximum. Introduction This w o r k c ons t i t ut e s part of a s y s t e m a t i c study of the c r y s t a l c h e m isti-y of REMO 3 phases w h e r e RE = r a r e e a r t h cations and M = f i r s t row t r a n s i t i o n m e t a l cations. E a r l i e r p a p e r s in this study examined RETiO 3 (1) and REMnO 3 (2) phases. Many REMO 3 phases a r e of i n t e r e s t b e c a u s e of t h e i r e l e c t r i c a l , magnetic, m a gne t o- optic and catalytic p r o p e r t i e s . The f i r s t s y s t e m a t i c study of REVO 3 phases was c a r r i e d out by Wold and Ward (3). They r e p o r t e d that phases with RE = La, Ce, Pr, Nd and Sm had the cubic p e r o v s k i t e s t r u c t u r e with ao ~ 3.905. They used a s m a l l d i a m e t e r Debye S c h e r r e r c a m e r a which a p p a r e n t l y did not res o lv e the peak splittings indicative of lower s y m m e t r y . B e r t a u t and F o r r a t (4) p r e p a r e d phases with RE = La, Ce, P r , Nd, Sm and Gd. They found that LaVO 3 and CeVO 3 had t e t r a g o n a l s y m m e t r y and the r e m a i n d e r w e r e o r t h o r h o m b i c and i s o s t r u c t u r a l with the newly d e t e r m i n e d GdFeO3 type (5). Geller (6) listed cell p a r a m e t e r data for PrVO3, NdVO 3 and GdVO 3 in one of his c l a s s i c papers on c o m pounds with the GdFeO 3 s t r u c t u r e type. In the most r e c e n t and most c o m p l e t e study of REVO 3 phases R e u t e r (7) l i s t s r e s u l t s for nine of the fifteen possible RE+Y phases. He also found that LaVO 3 and CeVO 3 w e r e t e t r a g o n a l while the r e m a i n i n g RE+Y phases w e r e o r t h o r h o m b i c GdFeO 3 type. In this paper we r e p o r t the p r e p a r a t i o n and unit cell p a r a m e t e r s for all REVO 3 phases and give complete x - r a y powder data for two typical phases. 1279
1280
REVO 3 PHASES
Vol. 9, No. 10
Experimental REVO 3 phases (except CeVO 3) were prepared from equimolar amounts of flE203 and V203. The r a r e e a r t h oxides were obtained from R e s e a r c h Chemicals and were of minimum 99.9% purity. P r and Tb " a s received" oxides were converted to sesquLoxides by hydrogen reduction at 800°C. The remaining sesquioxides were fired at 1200°C to remove volatiles. Vanadium sesquioxide was prepared by hydrogen reduction of V20 5 (Baker Chemicals) at 850°C. Phase parity of all s t a r t i n g m a t e r i a l s was checked by x - r a y powder diffraction. The s t a r t i n g m a t e r i a l s were mixed and ground in an agate mortar, p r e s s e d into pellets at 28,000 psi, sealed into evacuated silica vials at 10 -3 t o r r and fired at 1200°C for 24-36 h r s with one intermediate regrinding. The s a m p l e s were allowed to cool slowly to room t e m p e r a t u r e . Since Ce203 is difficult to keep in the reduced form a f t e r preparation, CeVO3 was prepared by hydrogen reduction of a (2CEO2 + V205) mixture at 850°C. X - r a y examination indicated that the phase had formed but it was given the same 1200°C heat t r e a t m e n t as the other REVO 3 phases in o r d e r to maintain constant t h e r m a l h i s t o r y throughout. X - r a y powder diffraction a n a l y s i s was performed with a Siemens diff r a c t o m e t e r equipped with a diffracted b e a m m o n o c h r o m e t e r , scintillationdetector and solid state e l e c t r o n i c s . CuK~ radiation (;~= 1. 54178A) was used throughout. The d i f f r a c t o m e t e r was calibrated with high purity silicon (ao = 5.4301~,) and with Ag (ao = 4.0864A), W (ao = 3. 1652,~) and CdO (ao = 4. 6958~,) standards obtained from the National Bureau of Standards. Cell p a r a m e t e r s were obtained from a least s q u a r e s computer refinement of the i n t e r p l a n a r spacings of ten to twenty unambiguously indexed reflections. Results and D i s c u s s i o n As reported previously (4, 7) LaVO3 and CeVO3 have a tetragonal cell related to the basic perovskite cell p a r a m e t e r b y a o ~ v'~'ap, Co~ 2ap.
TABLE I Cell P a r a m e t e r s of REVO., P h a s e s .
REVO 3
ao(~)=
LaNO 3 CeV(~ 3 PrVO 3 NdVO3 SmVO 3 EuVO 3 GdVO 3 TbVO 3 DyVO3 HoVC~3 YVO3 ErVO 3 TmVO 3 YbVO3 LuVC3
5. 535 5. 519 5. 472 5.451 5. 394 5. 362 5. 342 5,325 5. 299 5. 276 5. 274 5. 256 5. 237 5. 223 5.214
bo(,~) ,
5. 529 5. 575 5.58.1 5. 599 5. 604 5. 606 5. 594 5. 592 5. 590 5. 581 5. 573 5. 564 5.561
Co(A),
j(}~3),,
7. 830 7. 809 7. 774 7. 740 7. 684 7. 651 7. 637 7. 614 7. 593 7. 576 7. 574 7. 559 7. 545 7. 534 7.530
239.9 237.9 235.2 235.2 231.3 229.7 228.6 227.3 225.1 223.5 223.3 221.8 220.2 219.0 218.3
* =0.0 03 o r b e t t e r for La-Sm, -t0.002 o r b e t t e r for E u - L u , Y. "*--0.1 or b e t t e r for La°Sm, : 0 . 0 5 or b e t t e r fot" E u -L u . Y.
The pseudocubLc cell p a r a m e t e r s , ap, d e : rived f r o m a o and c o d i f f e r by only 0. 001A for LaVO 3 and 0. 002A for CeVO 3 and this could account for the e a r l y a s s i g n m e n t of these phases to cubic s y m m e t r y (3, 8). The tetragonal (202) reflection is a c l e a r indicator of s y m m e t r y . In LaVO 3 and CeVO 3 it has normal peak shape but in PrVO 3 it has split into two c l e a r l y resolvable r e f l e c tions (022, 202) separated by 0.28°28 which is indicative of orthorhombic s y m m e t r y . REVO 3 with P r to Lu and Y a r e i s o s t r u c t u r a l with orthorhombic GdFeO 3 (5) with space group Pbnm and Z = 4 and have Cell p a r a m e t e r s related to the pseudocubtc perovskite p a r a m e t e r by a o ~ V~ap, b o ~- / 2 a p and c o ~ 2ap.
Vol. 9, No. I0
REVO3 PHASES
TABLE 2 X-ray
Powder
240
Data for EuVO3 and Y b V O 3.
EuVO3 hkl I01 Ii0 002 III 020 112 200 021 211 103 022 202 113 t22 212 220 0O4
023 221 .[23 213 131 114 310 311 132 024 204 312 223
235 230
YbVO 3
225
dcalc
dobs
I/I 1
dcalc
dobs
3.872 3.825 3.455 2. 799 2.721 2.681 2.629 2.306 2.303 * 2.259 2, 195 2. 130 2.082
3.868 3.822 3.454 2. 799 2.720 2.681 2.629
16 8 14 20 100 25 10
4.29 3. 808 3. 766 3. 398 2. 782 2. 678 2. 612 2,610 ~
4.29 3. 803 3. 768 3. 396 2. 780 2. 678
100
2.610
45
7.70~-
2.304 2.259 2.196 2. 130 2. 082
6 8 14 8 2
2. 256
2. 255
8
7.65 r
1.936 1.9127 1.8853 1.8771
1.937 1.9132 1.8849 1.8773
25 16 8 16
1.7176
1.7175
25
2. 2 3 7 2. 146 2. 096 2. 057 2. 002 1. 9039 1. 8831 1. 8638 1. 8459 1. 7554 1. 7212 1. 7023 1. 6 8 8 0 1. 6615 1. 6226 1, 5852 I, 5594 i, 5274 1. 5 2 0 2 1, 5171 ~
2. 237 2. 146 2. 096 2. 057 2.001 1. 9 0 3 9 1. 8831 I. 8639 1. 8462 1. 7553 1. 7214 1. 7019 1. 6885 1. 6 6 1 6 1. 6229 1. 5854 1. 5595 1. 5271
1. 7026 1.6619 1,6008 1.5793 1.5570 1.5556 ~
1.7029 1.6621 1.6007 1.5796
3 2 10
1.556
40
1.5421
1.5422
lO S
1281
I'I 1
220
2
25 25
4
1.519
7.80
14
zssr 18 14
5.60 -J
12
~
18
O
3 2
25 6
5.555,50
5.45
3 4
5.40
5
535
s
5.30
25
5.25 I ill
RE3+ is in 8-fold coordination while V3+ is in 6-fold. Cell parameters for all fifteen phases are listed in Table 1. Only LaVO3 has previously reported x-ray powder data (~). Powder data for two other typical phases, EuVO3 and YbVO3 are given in Table 2.
1.20 I Lo
Illl
1.15 I
It
I H
1.10 I I
Cepr
Nd
I I I II
1.05 I I
I I|'T[
1.00 0.95 I I II
Sm Eu I Tb ~HoI Et ,IYb L u Gd
Dy Y
Tm
RE 3+ RADIUS in vnl~ COORDINATION (4)
FIG. 1 V a r i a t i o n of the Cell P a r a m e t e r s of R E V O 3 P h a s e s with RE Ionic Radius. Open c i r c l e s a r e the data for CeVO 3.
A comparison of cell parameters for those phases reported by R e u t e r (7) with t h o s e d e t e r m i n e d in this stud:~ i n d i c a t e s that although i n d i vidual c e l l e d g e s may v a r y by a s m u c h a s 0 . 0 1 5 A , c e l l v o l u m e s a g r e e very w e l l ( u s u a l l y w i t h i n O. 2 ~ ) . The v a r i a t i o n of c e l l p a r a m e t e r s a s a function of r a r e e a r t h ionic r a d i u s in 8 - f o l d c o o r d i n a t i o n (10) is plotted in Fig. 1. A s m o o t h d e c r e a s e in ao, Co and V is noted a s would be e x p e c t e d f r o m the lanthanide c o n t r a c t i o n . The b o p a r a m e t e r g o e s through a m a x i m u m n e a r TbVO3. T h i s b e h a v i o r h a s b e e n noted p r e v i o u s l y for RETiO3 (1), RECrO3 (11), REMnO3 (2) and RECoO3 (12). The p a r a m e t e r s for CeVO3 c l e a r l y do not fit the trend of the p a r a m e t e r s of the o t h e r p h a s e s . T h e r e a r e two p o s s i b l e e x p l a n a t i o n s for this. The c o o r d i n a t i o n of the r a r e earth cation in the t e t r a g o n a l LaVO 3 and CeVO3 p h a s e s
1282
REVO 3 PHASES
Vol. 9, No. 10
is not known because no s t r u c t u r e analysis has been p e r f o r m e d on them. It may be that the actual coordination is between the 12-fold in ideal cubic perovskite and 8-fold in the orthorhombic GdFeO:~type. In this case the Ce 3+ r a dius used should have been l a r g e r than 1.14A. Another explanation could be that not enough crystallographic data was available when Shannon and Prewitt (10) d e t e r m i n e d the "effective ionic radius" of Ce 3+ in 8-fold coordination (1.14.~) which is the same value assigned to P r 3+. If the coordination of Ce 3+ in CeVO 3 is 8-fold then the radius of Ce 3+ should not be equal to that of Pr 3+ because of the obviously s m a l l e r cell and different s y m m e t r y of PrVO 3 compared to CeVO3. In e i t h e r case, the value for the ionic radius of Ce3+ in CeVO3 should be ~- 1. 1552~ to fit the trend of REVO3 cell p a r a m e t e r s . Finally, it is of i n t e r e s t to note that EuVO3 has Eu3+-V 3+ valences w h e r e a s the titanium analog EuTLO 3 has 2+-4+ valences. The sizes of these cations place EuVO 3 in GdFeO 3 type s t r u c t u r e field and EuTLO 3 in the ideal cubic perovskite s t r u c t u r e field. T h e s e o b s e r v a t i o n s may be rationalized by r e f e r e n c e to the oxidation/reduction t h e r m o d y n a m i c s of each t r a n s i t i o n metal. Using data compiled by Coughlin (13) one d e r i v e s AG~ (1000° K) 2TiO2~ T[203+ 1/202
67
Kcal/mole
2VO2 -~ V203 + 1/202
26.5 K c a l / m o l e .
It takes m o r e than twice the e n e r g y to reduce Ti 4+ to TL3+ than to r e duce V4+ to V 3+. Apparently, this extra " r e s i s t a n c e " to reduction allows TL to r e m a i n tetravalent in the p r e s e n c e of Eu 2+ It is i n t e r e s t i n g to speculate on whether a cubic perovsktte "Eu2+V4+O3" phase could be stabilized at high p r e s s u r e s in spite of the apparently unfavorable t h e r m o d y n a m i c s . "EuZ+V4+O3 '', with its higher Eu coordLnatLon than Eu3+V3+O3 (12 vs 8) should be the more d e n s e phase and thus could be stab[llzed at high p r e s s u r e s . Such a phase transLtion, if it exists, should be a c companied by some abrupt changes Ln e l e c t r i c a l and magnetic properties. Acknowledgment This work was supported in part by the Joint Committee on Powder Diffraction Standards. References 1. G. J. McCarthy, W. B. White and R. Roy, Mat. Res. Bull. 4, 251 (1969). 2. G. J. McCarthy, P. V. Gallagher and C. A. Sipe, Mat. Res. Bull. 8, 1277 (1973). 3. A. Wold and R. Ward, J. Am. Chem. Soc. 76, 1029 (1954). 4. E. F. Bertaut and F. F o r r a t , J. Phys. Pad. 1/7, 129 (1956). 5. S. Geller and E. A. Wood, Acta Cryst. 9, 563 (1956). 6. S. Geller, Acta Cryst. 10, 243 (1957). 7. B. Reuter, Bull. Soc. Chim. Fr. 1965, 15 (1965). 8. W. Rtidorff and H. Becker, Z. Naturforschung 9B, 613 (1954).
Vol. 9, No. 10
REVO 3 PHASES
1283
9. M. Kestigian, J. C. Dickinson and R. Ward, J. Am. Chem. Soc. 7.._99, 5598 (1957). 10. R. D. Shannon and C. T. Prewitt, Acta Cryst. B25, 925 (1969). 11. S. Quezel-Ambunaz and M. Mareschal, Bull. Soc. Fr. Miner. Crist. 86, 204 (1963). 12. G. Demazeau, M. Pouchard and P. Hagenmuller, J. Solid State Chem. 9, 202 (1974). 13. J. P. Coughlin, Bull. 542.
U. S. Bureau of Mines (1954).
P e r g a m o n P r e s s , Inc.
CRYSTAL CHEMISTRY OF REVO 3 PHASES (RE = L a - L u , Y) G r e g o r y J. McCarthy, Carol A. Sipe and Kenneth E. McIlvried Mater ials R e s e a r c h L a b o r a t o r y The Pennsylvania State U n i v e r s i t y U n i v e r s i t y P ark , P e n n s y l v a n i a 16802 (Received June 5, 1974; C o m m u n i c a t e d by R. C. D e V r i e s ) ABSTRACT All REVO 3 phases have been p r e p a r e d and c h a r a c t e r i z e d by x - r a y powder diffraction for s y m m e t r y and unit c e l l p a r a m e t e r s . LaVO 3 and CeVO 3 have t e t r a g o n a l v a r i a t i o n s of the perovskite s t r u c t u r e while all r e m a i n i n g REVO 3 phases a r e o r t h o r h o m b i c and i s o s t r u c t u r a l with GdFeO 3. The variation of cell p a r a m e t e r s with RE 3+ ionic r a d i u s shows a s t e a d y c o n t r a c t i o n with d e c r e a s i n g r a d i u s for ao, c o and V while bo goes through a maximum. Introduction This w o r k c ons t i t ut e s part of a s y s t e m a t i c study of the c r y s t a l c h e m isti-y of REMO 3 phases w h e r e RE = r a r e e a r t h cations and M = f i r s t row t r a n s i t i o n m e t a l cations. E a r l i e r p a p e r s in this study examined RETiO 3 (1) and REMnO 3 (2) phases. Many REMO 3 phases a r e of i n t e r e s t b e c a u s e of t h e i r e l e c t r i c a l , magnetic, m a gne t o- optic and catalytic p r o p e r t i e s . The f i r s t s y s t e m a t i c study of REVO 3 phases was c a r r i e d out by Wold and Ward (3). They r e p o r t e d that phases with RE = La, Ce, Pr, Nd and Sm had the cubic p e r o v s k i t e s t r u c t u r e with ao ~ 3.905. They used a s m a l l d i a m e t e r Debye S c h e r r e r c a m e r a which a p p a r e n t l y did not res o lv e the peak splittings indicative of lower s y m m e t r y . B e r t a u t and F o r r a t (4) p r e p a r e d phases with RE = La, Ce, P r , Nd, Sm and Gd. They found that LaVO 3 and CeVO 3 had t e t r a g o n a l s y m m e t r y and the r e m a i n d e r w e r e o r t h o r h o m b i c and i s o s t r u c t u r a l with the newly d e t e r m i n e d GdFeO3 type (5). Geller (6) listed cell p a r a m e t e r data for PrVO3, NdVO 3 and GdVO 3 in one of his c l a s s i c papers on c o m pounds with the GdFeO 3 s t r u c t u r e type. In the most r e c e n t and most c o m p l e t e study of REVO 3 phases R e u t e r (7) l i s t s r e s u l t s for nine of the fifteen possible RE+Y phases. He also found that LaVO 3 and CeVO 3 w e r e t e t r a g o n a l while the r e m a i n i n g RE+Y phases w e r e o r t h o r h o m b i c GdFeO 3 type. In this paper we r e p o r t the p r e p a r a t i o n and unit cell p a r a m e t e r s for all REVO 3 phases and give complete x - r a y powder data for two typical phases. 1279
1280
REVO 3 PHASES
Vol. 9, No. 10
Experimental REVO 3 phases (except CeVO 3) were prepared from equimolar amounts of flE203 and V203. The r a r e e a r t h oxides were obtained from R e s e a r c h Chemicals and were of minimum 99.9% purity. P r and Tb " a s received" oxides were converted to sesquLoxides by hydrogen reduction at 800°C. The remaining sesquioxides were fired at 1200°C to remove volatiles. Vanadium sesquioxide was prepared by hydrogen reduction of V20 5 (Baker Chemicals) at 850°C. Phase parity of all s t a r t i n g m a t e r i a l s was checked by x - r a y powder diffraction. The s t a r t i n g m a t e r i a l s were mixed and ground in an agate mortar, p r e s s e d into pellets at 28,000 psi, sealed into evacuated silica vials at 10 -3 t o r r and fired at 1200°C for 24-36 h r s with one intermediate regrinding. The s a m p l e s were allowed to cool slowly to room t e m p e r a t u r e . Since Ce203 is difficult to keep in the reduced form a f t e r preparation, CeVO3 was prepared by hydrogen reduction of a (2CEO2 + V205) mixture at 850°C. X - r a y examination indicated that the phase had formed but it was given the same 1200°C heat t r e a t m e n t as the other REVO 3 phases in o r d e r to maintain constant t h e r m a l h i s t o r y throughout. X - r a y powder diffraction a n a l y s i s was performed with a Siemens diff r a c t o m e t e r equipped with a diffracted b e a m m o n o c h r o m e t e r , scintillationdetector and solid state e l e c t r o n i c s . CuK~ radiation (;~= 1. 54178A) was used throughout. The d i f f r a c t o m e t e r was calibrated with high purity silicon (ao = 5.4301~,) and with Ag (ao = 4.0864A), W (ao = 3. 1652,~) and CdO (ao = 4. 6958~,) standards obtained from the National Bureau of Standards. Cell p a r a m e t e r s were obtained from a least s q u a r e s computer refinement of the i n t e r p l a n a r spacings of ten to twenty unambiguously indexed reflections. Results and D i s c u s s i o n As reported previously (4, 7) LaVO3 and CeVO3 have a tetragonal cell related to the basic perovskite cell p a r a m e t e r b y a o ~ v'~'ap, Co~ 2ap.
TABLE I Cell P a r a m e t e r s of REVO., P h a s e s .
REVO 3
ao(~)=
LaNO 3 CeV(~ 3 PrVO 3 NdVO3 SmVO 3 EuVO 3 GdVO 3 TbVO 3 DyVO3 HoVC~3 YVO3 ErVO 3 TmVO 3 YbVO3 LuVC3
5. 535 5. 519 5. 472 5.451 5. 394 5. 362 5. 342 5,325 5. 299 5. 276 5. 274 5. 256 5. 237 5. 223 5.214
bo(,~) ,
5. 529 5. 575 5.58.1 5. 599 5. 604 5. 606 5. 594 5. 592 5. 590 5. 581 5. 573 5. 564 5.561
Co(A),
j(}~3),,
7. 830 7. 809 7. 774 7. 740 7. 684 7. 651 7. 637 7. 614 7. 593 7. 576 7. 574 7. 559 7. 545 7. 534 7.530
239.9 237.9 235.2 235.2 231.3 229.7 228.6 227.3 225.1 223.5 223.3 221.8 220.2 219.0 218.3
* =0.0 03 o r b e t t e r for La-Sm, -t0.002 o r b e t t e r for E u - L u , Y. "*--0.1 or b e t t e r for La°Sm, : 0 . 0 5 or b e t t e r fot" E u -L u . Y.
The pseudocubLc cell p a r a m e t e r s , ap, d e : rived f r o m a o and c o d i f f e r by only 0. 001A for LaVO 3 and 0. 002A for CeVO 3 and this could account for the e a r l y a s s i g n m e n t of these phases to cubic s y m m e t r y (3, 8). The tetragonal (202) reflection is a c l e a r indicator of s y m m e t r y . In LaVO 3 and CeVO 3 it has normal peak shape but in PrVO 3 it has split into two c l e a r l y resolvable r e f l e c tions (022, 202) separated by 0.28°28 which is indicative of orthorhombic s y m m e t r y . REVO 3 with P r to Lu and Y a r e i s o s t r u c t u r a l with orthorhombic GdFeO 3 (5) with space group Pbnm and Z = 4 and have Cell p a r a m e t e r s related to the pseudocubtc perovskite p a r a m e t e r by a o ~ V~ap, b o ~- / 2 a p and c o ~ 2ap.
Vol. 9, No. I0
REVO3 PHASES
TABLE 2 X-ray
Powder
240
Data for EuVO3 and Y b V O 3.
EuVO3 hkl I01 Ii0 002 III 020 112 200 021 211 103 022 202 113 t22 212 220 0O4
023 221 .[23 213 131 114 310 311 132 024 204 312 223
235 230
YbVO 3
225
dcalc
dobs
I/I 1
dcalc
dobs
3.872 3.825 3.455 2. 799 2.721 2.681 2.629 2.306 2.303 * 2.259 2, 195 2. 130 2.082
3.868 3.822 3.454 2. 799 2.720 2.681 2.629
16 8 14 20 100 25 10
4.29 3. 808 3. 766 3. 398 2. 782 2. 678 2. 612 2,610 ~
4.29 3. 803 3. 768 3. 396 2. 780 2. 678
100
2.610
45
7.70~-
2.304 2.259 2.196 2. 130 2. 082
6 8 14 8 2
2. 256
2. 255
8
7.65 r
1.936 1.9127 1.8853 1.8771
1.937 1.9132 1.8849 1.8773
25 16 8 16
1.7176
1.7175
25
2. 2 3 7 2. 146 2. 096 2. 057 2. 002 1. 9039 1. 8831 1. 8638 1. 8459 1. 7554 1. 7212 1. 7023 1. 6 8 8 0 1. 6615 1. 6226 1, 5852 I, 5594 i, 5274 1. 5 2 0 2 1, 5171 ~
2. 237 2. 146 2. 096 2. 057 2.001 1. 9 0 3 9 1. 8831 I. 8639 1. 8462 1. 7553 1. 7214 1. 7019 1. 6885 1. 6 6 1 6 1. 6229 1. 5854 1. 5595 1. 5271
1. 7026 1.6619 1,6008 1.5793 1.5570 1.5556 ~
1.7029 1.6621 1.6007 1.5796
3 2 10
1.556
40
1.5421
1.5422
lO S
1281
I'I 1
220
2
25 25
4
1.519
7.80
14
zssr 18 14
5.60 -J
12
~
18
O
3 2
25 6
5.555,50
5.45
3 4
5.40
5
535
s
5.30
25
5.25 I ill
RE3+ is in 8-fold coordination while V3+ is in 6-fold. Cell parameters for all fifteen phases are listed in Table 1. Only LaVO3 has previously reported x-ray powder data (~). Powder data for two other typical phases, EuVO3 and YbVO3 are given in Table 2.
1.20 I Lo
Illl
1.15 I
It
I H
1.10 I I
Cepr
Nd
I I I II
1.05 I I
I I|'T[
1.00 0.95 I I II
Sm Eu I Tb ~HoI Et ,IYb L u Gd
Dy Y
Tm
RE 3+ RADIUS in vnl~ COORDINATION (4)
FIG. 1 V a r i a t i o n of the Cell P a r a m e t e r s of R E V O 3 P h a s e s with RE Ionic Radius. Open c i r c l e s a r e the data for CeVO 3.
A comparison of cell parameters for those phases reported by R e u t e r (7) with t h o s e d e t e r m i n e d in this stud:~ i n d i c a t e s that although i n d i vidual c e l l e d g e s may v a r y by a s m u c h a s 0 . 0 1 5 A , c e l l v o l u m e s a g r e e very w e l l ( u s u a l l y w i t h i n O. 2 ~ ) . The v a r i a t i o n of c e l l p a r a m e t e r s a s a function of r a r e e a r t h ionic r a d i u s in 8 - f o l d c o o r d i n a t i o n (10) is plotted in Fig. 1. A s m o o t h d e c r e a s e in ao, Co and V is noted a s would be e x p e c t e d f r o m the lanthanide c o n t r a c t i o n . The b o p a r a m e t e r g o e s through a m a x i m u m n e a r TbVO3. T h i s b e h a v i o r h a s b e e n noted p r e v i o u s l y for RETiO3 (1), RECrO3 (11), REMnO3 (2) and RECoO3 (12). The p a r a m e t e r s for CeVO3 c l e a r l y do not fit the trend of the p a r a m e t e r s of the o t h e r p h a s e s . T h e r e a r e two p o s s i b l e e x p l a n a t i o n s for this. The c o o r d i n a t i o n of the r a r e earth cation in the t e t r a g o n a l LaVO 3 and CeVO3 p h a s e s
1282
REVO 3 PHASES
Vol. 9, No. 10
is not known because no s t r u c t u r e analysis has been p e r f o r m e d on them. It may be that the actual coordination is between the 12-fold in ideal cubic perovskite and 8-fold in the orthorhombic GdFeO:~type. In this case the Ce 3+ r a dius used should have been l a r g e r than 1.14A. Another explanation could be that not enough crystallographic data was available when Shannon and Prewitt (10) d e t e r m i n e d the "effective ionic radius" of Ce 3+ in 8-fold coordination (1.14.~) which is the same value assigned to P r 3+. If the coordination of Ce 3+ in CeVO 3 is 8-fold then the radius of Ce 3+ should not be equal to that of Pr 3+ because of the obviously s m a l l e r cell and different s y m m e t r y of PrVO 3 compared to CeVO3. In e i t h e r case, the value for the ionic radius of Ce3+ in CeVO3 should be ~- 1. 1552~ to fit the trend of REVO3 cell p a r a m e t e r s . Finally, it is of i n t e r e s t to note that EuVO3 has Eu3+-V 3+ valences w h e r e a s the titanium analog EuTLO 3 has 2+-4+ valences. The sizes of these cations place EuVO 3 in GdFeO 3 type s t r u c t u r e field and EuTLO 3 in the ideal cubic perovskite s t r u c t u r e field. T h e s e o b s e r v a t i o n s may be rationalized by r e f e r e n c e to the oxidation/reduction t h e r m o d y n a m i c s of each t r a n s i t i o n metal. Using data compiled by Coughlin (13) one d e r i v e s AG~ (1000° K) 2TiO2~ T[203+ 1/202
67
Kcal/mole
2VO2 -~ V203 + 1/202
26.5 K c a l / m o l e .
It takes m o r e than twice the e n e r g y to reduce Ti 4+ to TL3+ than to r e duce V4+ to V 3+. Apparently, this extra " r e s i s t a n c e " to reduction allows TL to r e m a i n tetravalent in the p r e s e n c e of Eu 2+ It is i n t e r e s t i n g to speculate on whether a cubic perovsktte "Eu2+V4+O3" phase could be stabilized at high p r e s s u r e s in spite of the apparently unfavorable t h e r m o d y n a m i c s . "EuZ+V4+O3 '', with its higher Eu coordLnatLon than Eu3+V3+O3 (12 vs 8) should be the more d e n s e phase and thus could be stab[llzed at high p r e s s u r e s . Such a phase transLtion, if it exists, should be a c companied by some abrupt changes Ln e l e c t r i c a l and magnetic properties. Acknowledgment This work was supported in part by the Joint Committee on Powder Diffraction Standards. References 1. G. J. McCarthy, W. B. White and R. Roy, Mat. Res. Bull. 4, 251 (1969). 2. G. J. McCarthy, P. V. Gallagher and C. A. Sipe, Mat. Res. Bull. 8, 1277 (1973). 3. A. Wold and R. Ward, J. Am. Chem. Soc. 76, 1029 (1954). 4. E. F. Bertaut and F. F o r r a t , J. Phys. Pad. 1/7, 129 (1956). 5. S. Geller and E. A. Wood, Acta Cryst. 9, 563 (1956). 6. S. Geller, Acta Cryst. 10, 243 (1957). 7. B. Reuter, Bull. Soc. Chim. Fr. 1965, 15 (1965). 8. W. Rtidorff and H. Becker, Z. Naturforschung 9B, 613 (1954).
Vol. 9, No. 10
REVO 3 PHASES
1283
9. M. Kestigian, J. C. Dickinson and R. Ward, J. Am. Chem. Soc. 7.._99, 5598 (1957). 10. R. D. Shannon and C. T. Prewitt, Acta Cryst. B25, 925 (1969). 11. S. Quezel-Ambunaz and M. Mareschal, Bull. Soc. Fr. Miner. Crist. 86, 204 (1963). 12. G. Demazeau, M. Pouchard and P. Hagenmuller, J. Solid State Chem. 9, 202 (1974). 13. J. P. Coughlin, Bull. 542.
U. S. Bureau of Mines (1954).