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AgSbO3: Chemical characterization and structural considerations
Mat. Res. Bull. Vol. 4, pp. 377-380, 1969. Pergamon Press, Inc. Printed in the United States.
AgSbO3: CHEMICAL CHARACTERIZATION AND STRUCTURAL CONSIDERATIONS A. W. Sleight Central R e s e a r c h Department* E. I. du Pont de N e m o u r s and Company Wilmington, Delaware 19898 (Received April 21, 1969; Communicated by J. B. Goodenough) ABSTRACT It is confirmed that AgSbO3 has a pyrochlore related structure without being hydrated. Furthermore, attempts to prepare Ag2Sb205(OH)2 or Ag2Sb205F2 were unsuccessful. It is concluded that the occurrence of the pyrochlore structure instead of the perovskite structure for certain ABO3 compounds is the result of very strong covalent bonding which tends to restrict oxygen to a maximum coordination number of four.
Introduction Compounds of the type ASbO 3 have been r e p o r t e d (1) to have the pyrochlore s t r u c t u r e w he r e A is Ag, Na, or K.
However, the usual fo rmu la for
the pyrochlore s t r u c t u r e is A2B207, not ABO3, and s e v e r a l w o r k e r s (2, 3) have recently shown that NaSbO 3 r e a l l y has the ilmenite s t r u c t u r e .
Only the
hydrated form of NaSbO 3 p o s s e s s e s the pyrochlore s t r u c t u r e , and its fo rmu la can be written as Na2Sb206 • H20 or Na2Sb205(OH)2.
The purpose of this in-
vestigation was to d e t e r m i n e whether AgSbO 3 must also be hydrated to have the p y r o c h l o r e s t r u c t u r e . Experimental Four different p r e p a r a t i v e methods w e r e used.
*Contribution No. 1560. 377
378 1.
AgSbO3
Vol. 4, No. 6
A 2AgO/Sb203 mixture was heated at 500°C under about 1 a t m o s p h e r e of oxygen for about 24 hours.
2.
A 2AgO/Sb205 mixture was sealed in a collapsible gold tube with water. This tube was heated at 700°C for about eight hours under 3000 atm of supporting p r e s s u r e .
3.
Same as 2 except 48% aqueous HF was used instead of water.
4.
The product of 2 was subjected to 65 kbars p r e s s u r e at 900°C for 2 hours. Results R e g a r d l e s s of the method of preparation, all the products analyzed
well for AgSbO3 (Table 1), and the cell dimension was always 10. 249 ~-. 002/~. The systematic absences and the qualitative intensities were those expected of the pyrochlore structure (4).
Analysis of h y d r o t h e r m a l products detected no
hydrogen; if present, it must be less than 0. 04% by weight. Analyses of the preparations in 48% HF showed that the fluorine content must be less than 0. 12% by weight.
The product of the 65 kbar experiment showed the usual
AgSbO3 pyrochlore x - r a y pattern. TABLE 1 Analytical Data Method
Ag
Sb
O
1
39.1%
43.4 %
17.2 %
2
38.9
43.6
17.5
3
38.7
43.5
17. 3
Theory
38.86
43.85
17.29
Discussion It has been shown that, unlike NaSbO3, AgSbO3 can have the "pyrochlore structure".
Substitution of e i t h e r hydroxyl or fluorine for oxygen
could bring the formula c l o s e r to the ideal pyrochlore formula, i . e . , Ag2Sb2X7, but attempts to do this failed (Methods 2 & 3).
Many compounds
with the pyrochlore structure have been reported with the ABO 3 stoichiometry,
Vol. 4, No. 6
AgSbO 3
379
and f o r t h e s e c o m p o u n d s A is a l w a y s Pb o r Bi (5, 6) with AgSbO 3 (and p o s s i b l y KSbO 3) being the only e x c e p t i o n . T h e r e a r e two c o m m o n w a y s of v i e w i n g the p y r o c h l o r e s t r u c t u r e : f i r s t , a s a BO 3 n e t w o r k with A and O i n t e r s t i t i a l s (4); s e c o n d l y , a s a d e r i v a tive of the f l u o r i d e s t r u c t u r e .
T h e c a t i o n s f o r m the s a m e l a t t i c e in f l u o r i t e a s
in the p y r o c h l o r e s t r u c t u r e , and in the f l u o r i t e s t r u c t u r e the c a t i o n s a r e in cubic c o o r d i n a t i o n .
In the ideal p y r o c h l o r e s t r u c t u r e the A and B c a t i o n s a r e
o r d e r e d , and the two o x y g e n s r e l a t e d by the cube body d i a g o n a l a r e m i s s i n g f r o m the cube a r o u n d the B cation. a r r a n g e m e n t a r o u n d the B cation.
T h i s l e a d s to a f l a t t e n e d t r i g o n a l a n t i p r i s m H o w e v e r , t h e r e is a r e a r r a n g e m e n t of the
o x y g e n s so that the e n v i r o n m e n t of B is m o r e n e a r l y o c t a h e d r a l . t h i s p r o c e s s h a s b e e n c a r r i e d one s t e p f u r t h e r .
In AgSbO 3
H e r e the two o x y g e n s r e l a t e d
by t h e cube body d i a g o n a l a r e a l s o m i s s i n g a r o u n d the A cation. In s o m e r e s p e c t s the AgSbO 3 s t r u c t u r e is s i m i l a r to the C - r a r e e a r t h sesquioxide structure.
Both s t r u c t u r e s a r e r e l a t e d to the f l u o r i t e s t r u c t u r e ,
and both have the s a m e n u m b e r of " o x y g e n v a c a n c i e s " .
T h e r e a r e two t y p e s
of c a t i o n s in the C - r a r e e a r t h s e s q u i o x i d e s t r u c t u r e , but t h e s e a r e in a 3:1 r a t i o (AB306) i n s t e a d of the 1:1 r a t i o in AgSbO 3.
In the C - r a r e e a r t h s e s q u i -
oxide s t r u c t u r e two o x y g e n v a c a n c i e s a r e r e l a t e d by the cube body d i a g o n a l f o r the A c a t i o n but by the cube f a c e d i a g o n a l f o r the B cation. A and B a r e d i f f e r e n t c a t i o n s , e . g . ,
Cases where
+ 2 0 6 , a r e a p p a r e n t l y unknown. A + 6 -~3
T h u s , the s t r u c t u r e s of f l u o r i t e , p y r o c h l o r e , C - r a r e e a r t h s e s q u i o x i d e s , and AgSbO 3 m a y be r e g a r d e d a s c l o s e l y r e l a t e d .
C e r t a i n l y , the p y r o -
c h l o r e and AgSbO 3 s t r u c t u r e s a r e the m o s t c l o s e l y r e l a t e d , and t h e s e two have the s a m e s p a c e g r o u p .
If the one p o s i t i o n a l p a r a m e t e r is 0. 425 (Ag at the
origin), the A g - O and Sb-O d i s t a n c e s would be 2 . 4 9 ~ a n d 1.97/~, r e s p e c t i v e l y . T h i s is in good a g r e e m e n t with the v a l u e s p r e d i c t e d by the radii of Shannon and P r e w i t t (7) (Ag-O = 2 . 5 3 ~ and Sb-O = 1 . 9 9 ~ ) . It would a p p e a r t h a t t h e AgSbO 3 s t r u c t u r e o c c u r s only f o r v e r y c o v a l e n t ABO 3 c o m p o u n d s .
F o r e x a m p l e , if the A o r B c a t i o n is r e a s o n a b l y e l e c t r o -
p o s i t i v e [Sr(Ti, Zr, Hf)O3, P b ( T i , Zr, Hf)O3, Sr(Ru, Ir, Tc)O3, and Ag(Nb, Ta)O3] , the p e r o v s k i t e s t r u c t u r e is found.
H o w e v e r , w h e n both the A and B c a t i o n a r e
380
AgSbO3
Vol. 4, No. 6
not very electropositive (5, 8) [Pb(Ru, Ir, Tc)O 3 and AgSbO3] , the "pyrochloretype" structure occurs.
In the ideal perovskite structure, oxygen is coordi-
nated to six cations (2 X B and 4 X A) whereas in the pyrochlore structure the oxygen coordination is roughly t e t r a h e d r a l (2 X A and 2 X B).
Since oxygen
can only form four strongly covalent bonds, the "pyrochlore-type" structure is favored over the perovskite structure when the bonding is very covalent. An alternate explanation for the o c c u r r e n c e of the pyrochlore structure for c e r t a i n ABO 3 compounds has recently been given by Longo, Raccah and Goodenough (5). References I.
N. Schreweltus, Z. Anorg. Allgem. Chem. 238, 241 (1938).
2.
M. C. Montmory, A. Durif-Varambon and X. Pare, Bull. Soc. Franc. Mineral. Crist. 8__66,434 (1963).
3.
F. Brisse, Ph.D. Thesis, Dalhousie University, Halifax, Nova Scotia (1967).
4.
A.W. Sleight, Inorg. Chem. _7, 1704 (1968).
5.
J . M . Longo, P. M. Raccah and J. B. Goodenough, Mat. Res. Bull. 4, 191 (1969).
6.
L.G. Nikiforov, V. V. Ivanova, Yu. N. Venevtsev and G. S. Zhdanov, Neorg. Mat. 4, 381 (1968).
7.
R. D. Shannonand C. T. Prewitt, Acta Cryst (in press).
8.
O. Muller, W. B. White and R. Roy, J. Inorg. Nucl. Chem. 2__66, 2075 (1964).
AgSbO3: CHEMICAL CHARACTERIZATION AND STRUCTURAL CONSIDERATIONS A. W. Sleight Central R e s e a r c h Department* E. I. du Pont de N e m o u r s and Company Wilmington, Delaware 19898 (Received April 21, 1969; Communicated by J. B. Goodenough) ABSTRACT It is confirmed that AgSbO3 has a pyrochlore related structure without being hydrated. Furthermore, attempts to prepare Ag2Sb205(OH)2 or Ag2Sb205F2 were unsuccessful. It is concluded that the occurrence of the pyrochlore structure instead of the perovskite structure for certain ABO3 compounds is the result of very strong covalent bonding which tends to restrict oxygen to a maximum coordination number of four.
Introduction Compounds of the type ASbO 3 have been r e p o r t e d (1) to have the pyrochlore s t r u c t u r e w he r e A is Ag, Na, or K.
However, the usual fo rmu la for
the pyrochlore s t r u c t u r e is A2B207, not ABO3, and s e v e r a l w o r k e r s (2, 3) have recently shown that NaSbO 3 r e a l l y has the ilmenite s t r u c t u r e .
Only the
hydrated form of NaSbO 3 p o s s e s s e s the pyrochlore s t r u c t u r e , and its fo rmu la can be written as Na2Sb206 • H20 or Na2Sb205(OH)2.
The purpose of this in-
vestigation was to d e t e r m i n e whether AgSbO 3 must also be hydrated to have the p y r o c h l o r e s t r u c t u r e . Experimental Four different p r e p a r a t i v e methods w e r e used.
*Contribution No. 1560. 377
378 1.
AgSbO3
Vol. 4, No. 6
A 2AgO/Sb203 mixture was heated at 500°C under about 1 a t m o s p h e r e of oxygen for about 24 hours.
2.
A 2AgO/Sb205 mixture was sealed in a collapsible gold tube with water. This tube was heated at 700°C for about eight hours under 3000 atm of supporting p r e s s u r e .
3.
Same as 2 except 48% aqueous HF was used instead of water.
4.
The product of 2 was subjected to 65 kbars p r e s s u r e at 900°C for 2 hours. Results R e g a r d l e s s of the method of preparation, all the products analyzed
well for AgSbO3 (Table 1), and the cell dimension was always 10. 249 ~-. 002/~. The systematic absences and the qualitative intensities were those expected of the pyrochlore structure (4).
Analysis of h y d r o t h e r m a l products detected no
hydrogen; if present, it must be less than 0. 04% by weight. Analyses of the preparations in 48% HF showed that the fluorine content must be less than 0. 12% by weight.
The product of the 65 kbar experiment showed the usual
AgSbO3 pyrochlore x - r a y pattern. TABLE 1 Analytical Data Method
Ag
Sb
O
1
39.1%
43.4 %
17.2 %
2
38.9
43.6
17.5
3
38.7
43.5
17. 3
Theory
38.86
43.85
17.29
Discussion It has been shown that, unlike NaSbO3, AgSbO3 can have the "pyrochlore structure".
Substitution of e i t h e r hydroxyl or fluorine for oxygen
could bring the formula c l o s e r to the ideal pyrochlore formula, i . e . , Ag2Sb2X7, but attempts to do this failed (Methods 2 & 3).
Many compounds
with the pyrochlore structure have been reported with the ABO 3 stoichiometry,
Vol. 4, No. 6
AgSbO 3
379
and f o r t h e s e c o m p o u n d s A is a l w a y s Pb o r Bi (5, 6) with AgSbO 3 (and p o s s i b l y KSbO 3) being the only e x c e p t i o n . T h e r e a r e two c o m m o n w a y s of v i e w i n g the p y r o c h l o r e s t r u c t u r e : f i r s t , a s a BO 3 n e t w o r k with A and O i n t e r s t i t i a l s (4); s e c o n d l y , a s a d e r i v a tive of the f l u o r i d e s t r u c t u r e .
T h e c a t i o n s f o r m the s a m e l a t t i c e in f l u o r i t e a s
in the p y r o c h l o r e s t r u c t u r e , and in the f l u o r i t e s t r u c t u r e the c a t i o n s a r e in cubic c o o r d i n a t i o n .
In the ideal p y r o c h l o r e s t r u c t u r e the A and B c a t i o n s a r e
o r d e r e d , and the two o x y g e n s r e l a t e d by the cube body d i a g o n a l a r e m i s s i n g f r o m the cube a r o u n d the B cation. a r r a n g e m e n t a r o u n d the B cation.
T h i s l e a d s to a f l a t t e n e d t r i g o n a l a n t i p r i s m H o w e v e r , t h e r e is a r e a r r a n g e m e n t of the
o x y g e n s so that the e n v i r o n m e n t of B is m o r e n e a r l y o c t a h e d r a l . t h i s p r o c e s s h a s b e e n c a r r i e d one s t e p f u r t h e r .
In AgSbO 3
H e r e the two o x y g e n s r e l a t e d
by t h e cube body d i a g o n a l a r e a l s o m i s s i n g a r o u n d the A cation. In s o m e r e s p e c t s the AgSbO 3 s t r u c t u r e is s i m i l a r to the C - r a r e e a r t h sesquioxide structure.
Both s t r u c t u r e s a r e r e l a t e d to the f l u o r i t e s t r u c t u r e ,
and both have the s a m e n u m b e r of " o x y g e n v a c a n c i e s " .
T h e r e a r e two t y p e s
of c a t i o n s in the C - r a r e e a r t h s e s q u i o x i d e s t r u c t u r e , but t h e s e a r e in a 3:1 r a t i o (AB306) i n s t e a d of the 1:1 r a t i o in AgSbO 3.
In the C - r a r e e a r t h s e s q u i -
oxide s t r u c t u r e two o x y g e n v a c a n c i e s a r e r e l a t e d by the cube body d i a g o n a l f o r the A c a t i o n but by the cube f a c e d i a g o n a l f o r the B cation. A and B a r e d i f f e r e n t c a t i o n s , e . g . ,
Cases where
+ 2 0 6 , a r e a p p a r e n t l y unknown. A + 6 -~3
T h u s , the s t r u c t u r e s of f l u o r i t e , p y r o c h l o r e , C - r a r e e a r t h s e s q u i o x i d e s , and AgSbO 3 m a y be r e g a r d e d a s c l o s e l y r e l a t e d .
C e r t a i n l y , the p y r o -
c h l o r e and AgSbO 3 s t r u c t u r e s a r e the m o s t c l o s e l y r e l a t e d , and t h e s e two have the s a m e s p a c e g r o u p .
If the one p o s i t i o n a l p a r a m e t e r is 0. 425 (Ag at the
origin), the A g - O and Sb-O d i s t a n c e s would be 2 . 4 9 ~ a n d 1.97/~, r e s p e c t i v e l y . T h i s is in good a g r e e m e n t with the v a l u e s p r e d i c t e d by the radii of Shannon and P r e w i t t (7) (Ag-O = 2 . 5 3 ~ and Sb-O = 1 . 9 9 ~ ) . It would a p p e a r t h a t t h e AgSbO 3 s t r u c t u r e o c c u r s only f o r v e r y c o v a l e n t ABO 3 c o m p o u n d s .
F o r e x a m p l e , if the A o r B c a t i o n is r e a s o n a b l y e l e c t r o -
p o s i t i v e [Sr(Ti, Zr, Hf)O3, P b ( T i , Zr, Hf)O3, Sr(Ru, Ir, Tc)O3, and Ag(Nb, Ta)O3] , the p e r o v s k i t e s t r u c t u r e is found.
H o w e v e r , w h e n both the A and B c a t i o n a r e
380
AgSbO3
Vol. 4, No. 6
not very electropositive (5, 8) [Pb(Ru, Ir, Tc)O 3 and AgSbO3] , the "pyrochloretype" structure occurs.
In the ideal perovskite structure, oxygen is coordi-
nated to six cations (2 X B and 4 X A) whereas in the pyrochlore structure the oxygen coordination is roughly t e t r a h e d r a l (2 X A and 2 X B).
Since oxygen
can only form four strongly covalent bonds, the "pyrochlore-type" structure is favored over the perovskite structure when the bonding is very covalent. An alternate explanation for the o c c u r r e n c e of the pyrochlore structure for c e r t a i n ABO 3 compounds has recently been given by Longo, Raccah and Goodenough (5). References I.
N. Schreweltus, Z. Anorg. Allgem. Chem. 238, 241 (1938).
2.
M. C. Montmory, A. Durif-Varambon and X. Pare, Bull. Soc. Franc. Mineral. Crist. 8__66,434 (1963).
3.
F. Brisse, Ph.D. Thesis, Dalhousie University, Halifax, Nova Scotia (1967).
4.
A.W. Sleight, Inorg. Chem. _7, 1704 (1968).
5.
J . M . Longo, P. M. Raccah and J. B. Goodenough, Mat. Res. Bull. 4, 191 (1969).
6.
L.G. Nikiforov, V. V. Ivanova, Yu. N. Venevtsev and G. S. Zhdanov, Neorg. Mat. 4, 381 (1968).
7.
R. D. Shannonand C. T. Prewitt, Acta Cryst (in press).
8.
O. Muller, W. B. White and R. Roy, J. Inorg. Nucl. Chem. 2__66, 2075 (1964).