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Nonstoichiometry and phase transition in NdMnO3
Mat. R e s . B u l l . , V o l . 19, p p . 1201-1206, 1984. P r i n t e d in t h e U S A . 0 0 2 5 - 5 4 0 8 / 8 4 $3.00 + .00 C o p y r i g h t ( c ) 1984 Pergamon P r e s s Ltd.
NONSTOICHIOMETRY
AND PHASE
TRANSITION
IN NdMnO 3
Naoki K a m e g a s h i r a and Yuji M i y a z a k i Department of M a t e r i a l s Science, Toyohashi University Technology, Tempaku-cho, Toyohashi, 440, Japan.
of
(Received July 2, 1984; Communicated by M. Koizumi)
ABSTRACT The n o n s t o i c h i o m e t r y of n e o d y m i u m m a n g a n i t e N d M n O 3 phase has been studied by a g r a v i m e t r i c m e t h o d at 1273 K. The oxygen partial pressures were controlled by using flowing 0 2 / A r or H 2 / C O 2 m i x t u r e s . The existence of hyperstoichiometric phase and the h o m o g e n e i t y range of the • p h a s e N d M n O - J-~ X . f r o m x = 0 to 0 " 065 at 1 2 7 3 K u n d e r various o x y g e n partial p r e s s u r e s has b e e n revealed. And also the effect of o x y g e n n o n s t o i c h i o m e t r y on the phase transition at high temperature has b e e n s t u d i e d by electrical conductivity and DTA measurements. The transition temperature decreases with increasing oxygen content.
Introduction Among rare earth manganites lanthanum manganite has a considerably wide ra~e of n o n s t o i c h i o m e t r y (1-5) a n d m i x e d v a l e n c e of Mn 3+ and Mn ~ p l a y an i m p o r t a n t role in s o m e physical properties (6). L a M n O 3 a l s o h a s a p h a s e t r a n s i t i o n n e a r 800 K f r o m o r t h o r h o m b i c at low t e m p e r a t u r e to r h o m b o h e d r a l s y s t e m at high t e m p e r a t u r e , d u e to the l i b e r a t i o n f r o m J a h n - T e l l e r e f f e c t (1,7). The e x i s t e n c e of phase t r a n s i t i o n in N d M n O 3 phase at high t e m p e r a t u r e as in the case of L a M n O 3 was r e v e a l e d by the authors (8). T h i s k i n d of t r a n s i t i o n should inevitably be a f f e c t e d by the n o n s t o i c h i o m e t r y , w h i l e the a v a i l a b l e e x p e r i m e n t a l results on n o n s t o i c h i o m e t r i c behavior of N d M n O 3 h a v e been rather limited. The oxygen contents in N d M n O 3 phase have been determ i n e d by M c C a r t h y e t al. (9) f r o m m e a s u r i n g the weight loss during the r e a c t i o n Of oNcd~O~ w i t h MnO2, and N d M n O 2 97 at 1200 °C a n d N d M n O 2 96 at 1400 air have been obtained. Magnetic properties'of NdMnO 3 were investigated by several authors and their samples were synthesized in a r g o n (I0) or in a i r (ll) at high temperature. So far the r e l a t i o n s h i p a m o n g o x y g e n partial 1201
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pressure, composition and temperature in N d M n O 3 p h a s e h a s n o t b e e n k n o w n , a l t h o u g h l o w e r l i m i t s of o x y g e n p a r t i a l p r e s s u r e s for t h e s t a b l e e x i s t e n c e of N d M n O 3 p h a s e h a v e r e c e n t l y been determined at 1 1 7 3 - 1473 K with electrical conductivity measurement (12). T h e p r e s e n t investigation was undertaken f i r s t l y to d e t e r m i n e the relationship between oxygen partial p r e s s u r e and d e v i a t i o n of o x y g e n c o n t e n t f r o m stoic h i o m e t r y at 1 2 7 3 K, a n d s e c o n d l y to c l a r i f y the e f f e c t of o x y g e n n o n s t o i chiometry on the transition temperature from the electrical c o n d u c t i v i t y and D T A m e a s u r e m e n t s . Experimental Starting materials were 99.99 % Nd20 3 and Mn203 (Rare M e t a l l i c Co. Ltd.). The Nd203 was dried at 1273 K in v a c u u m and M n 2 0 3 w a s c a l c i n e d a t 1 0 7 3 K in air. An equimolar m i x t u r e of Nd203 and Mn203 w a s p r e s s e d into p e l l e t s and f i r e d at 1473 K in air ~or 3 daysf The f o r m a t i o n of o r t h o r h o m b i c N d M n O 3 phase was i d e n t i f i e d by X ray d i f f r a c t o m e t r y . Oxygen partial pressures were achieved by m e a n s of t w o methods. For~ressures f o r 105 to I0 Pa, m i x t u r e s of 02 a n d Ar gases a t i 0 ~ Pa t o t a l pressure were used in t h i s study. C o n t r o l l e d oxygen partial p r e s s u r e s in the low p r e s s u r e region w e r e ~ b t a i n e d by i n t r o d u c i n g k n o w n m i x t u r e s of CO 2 and H 2 gases at 10 ~ Pa t o t a l p r e s s u r e . For mixtures of H 2 a n d CO 2, t h e o x y g e n partial p r e s s u r e s w e r e c a l c u l a t e d f r o m the JANAF t h e r m o c h e m i c a l table (13). A f l o w rate of 1 ec/sec w a s m a i n t a i n e d for all gas m i x t u r e s l u s e d in this investigation. The f l u c t u a t i o n s of o x y g e n partial p e s s u r e s in s t r e a m i n g gas d u r i n g e q u i l i b r a t i o n were monitored by measuring electrical conductivity of s o m e m e t a l o x i d e , T i O 2 or C o O (14). After each sample was equilib r a t e d under a p p r o p r i a t e o x y g e n partial p r e s s u r e at 1273 K for 20-48 hours, it w a s q u e n c h e d into i c e - b a t h and w e i g h t w a s m e a sured by microbalance. Four probe e l e c t r i c a l c o n d u c t i v i t y m e a s u r e m e n t s w e r e c a r r i e d o u t o n p r e s s e d p e l l e t s as d e s c r i b e d in an e a r l i e r p u b l i c a t i o n
(8). Results (a) H o m o g e n e i t y
and d i s c u s s i o n
Range
The i s o t h e r m graph log (Po2/Pa) versus x in N d M n O 3 + x is s h o w n in Fig. I. N d M n O 3 d e c o m p o s e ~ into NdMnO 3 and MnO according to the f o l l o w i n g reaction, NdMnO 3 (s) = N d 2 0 3
(s) + M n O
(s) + 1/4 02
(i)
b e l o w 10 -7.79 Pa of o x y g e n partial p r e s s u r e at 1273 K. F r o m the p r e v i o u s results for the d e c o m p o s i t i o n p r e s s u r e (12) log (Po2 / Pa) = -7.68 h a s b e e n o b t a i n e d a n d in g o o d a g r e e m e n t with the present result. If it is a s s u m e d that Nd203 (15) a n d M n O (15,16) are n e a r l y s t o i c h i o m e t r i c at 1273 K and log (Po2/Pa) = 8.41, the o x y g e n c o n t e n t s in N d M n O 3 phase can be c a l c u r a t e d f r o m the r e s u l t s of w e i g h t c h a n g e u p o n t h e d e c o m p o s i t i o n in Fig. i, a n d N d M n O 3 0 0 1 3 w a s o b t a i n e d a t l o g ( P o 2 / P a ) = -7.58 at 1 2 7 3 K, which can 6e-cdnsidered to b e a l m o s t { £ o i c h i o m e t r i c . Then the
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V o l . 19, No. 9
free e n e r g y change for the rea3.25 ction (I) w a s calculated from z o o the decomposiL~ 3.00 I--tion pressure z and determined o 2.75 to be ~ % 1 ~ 3 o ° 77.9 i~ z Under the o x y g e n ILl partial pres2.50 ÷ sures above that of t h e d e c o m p o MnO s i t i o n the h y p e r stoichiometry was J , a ~) revealed in NdMnO 3 phase, while hypostoic h i o m e t r i c phase FIG. 1 was shown by M c C a r t h y et al O x y g e n c o n t e n t in N d M n O 3 + x as a f u n c t i o n of o x y g e n p a r t i a l p r e s s u r e at 1273 K. (9). However, as t h e e x i s t e n ces of the h y p e r s t o i c h i o m e t r y in some rare earth manganites L n M n O 3 (Ln = La, Sm, Dy, Y a n d Er) p h a s e a t 1 4 7 3 K h a v e b e e n c o n f i r m e d b y K a m a t a et al. (4,17) a n d N d M n O 3 s h o w s p - t y p e s e m i c o n d u c t i v i t y as d e s c r i b e d below, it can s u r e l y be c o n s i d e r e d t h a t N d M n O 3 a l s o has o x y g e n - e x c e s s n o n s t o i chiometry. The maximum a m o u n t of t h e o x y g e n d e v i a t i o n from s t o i c h i o m e t r y in NdMnO3+. w a s d e t e r m i n e d to be x= +0.065 at 1273 K and log(Po2/Pa) = 5, a n d it is a l i t t l e l e s s t h a n t h a t in L a M n 0 3 + x (5).
NdMnO3. x
Nd203~ -10
(b)
Lattice
-5 0 LOG(Po2/Pa)
constant
The n o n s t o i c h i o m e t r i c p h a s e c o u l d be i n d e x e d w i t h the o r t h o rhombic symmetry. The c h a n g e of l a t t i c e c o n s t a n t s in n o n s t o i chiometric NdMn03+ x is p l o t t e d a g a i n s t the e x c e s s o x y g e n cont e n t x i n Fig. 2, f r o m w h i c h it is s e e n t h a t t h e a - a x i s is a l m o s t c o n s t a n t and the c-axis g r a d u a l l y increases, w h i l e the baxis d e c r e a s e s w i t h i n c r e a s i n g x, w h i c h leads to the c o n t r a c t i o n of v o l u m e of the unit cell as the d e v i a t i o n f r o m s t o i c h i o m e t r y b e c o m e s large. (c)
Electrical
conductivity
T h e v a l u e s of l o g ( ~ T ) versus reciprocal temperatures of nonstoichiometric NdMnO 3 specimens a r e p l o t t e d in Fig. 3, in w h i c h the c o n d u c t i v i t y i n c r e a s e s as i n c r e a s i n g o x y g e n c o n t e n t at lower temperature region. This characteristic shows that N d M n O 3 is a p - t y p e s e m i c o n d u c t o r and that h o l e h o p p i n g m e c h a n i s m is d o m i n a n t in N d M n O 3 as w e l l as L a M n O 3 (7)'. A t h i g h t e m p e r a ture the e l e c t r i c a l c o n d u c t i v i t y b e c o m e s a l m o s t i n d e p e n d e n t of o x y g e n content, w h i c h w a s c o n f i r m e d by m e a s u r i n g the v a r i a t i o n of the e l e c t r i c a l c o n d u c t i v i t y w i t h p a r t i a l p r e s s u r e of o x y g e n
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5.90
...7f,k
"~<_~ ~8°t
|~..~
5.7o~
A : 0.064 B:o.o47
l\~.
-~ 5
)
s3ot
'
C : 0.029
% \ ~
%%, % ~ . ~
D : 0.017
~..
°%
%, ' , °
e....... D'"-...
"%
%
103KIT .
.
.
1100 0.02
.
0.04
0.06
X in NdMn03. x
FIG. 3 E l e c t r i c a l c o n d u c t i v i t y as a f u n c t i o n of r e c i p r o c a l t e m p e r a t u r e in N d M n 0 3 + x.
FIG. 2 V a r i a t i o n of l a t t i c e p a r a m eters w i t h o x y g e n c o n t e n t in N d M n O 3 + x.
at 1 2 7 3 K a s w a s s h o w n in p r e v i o u s s t u d y (12). F o r all o x y g e n c o m p o s i t i o n s t h e r e is seen a sharp j u m p and a f t e r w a r d s a gradual increase in conductivity c u r v e as t e m p e r a t u r e increases, socalled Z-type jump, which would reflect the existence of some kind of s t r u c t u r a l c h a n g e and in this case the p h a s e t r a n s i t i o n f r o m o r t h o r h o m b i c s y s t e m at l o w t e m p e r a t u r e to r h o m b o h e d r a l f o r m at high temperature (8), as is t h e c a s e in L a M n O 3 (7). T h e t r a n s i t i o n t e m p e r a t u r e w a s d e t e r m i n e d f r o m the t e m p e r a t u r e corr e s p o n d i n g to the b r e a k p o i n t in c o n d u c t i v i t y curve. (d)
D T A study
DTA studies also clearly show thermal anomalies, although peaks are c o n s i d e r a b l y small. W h e n the h e a t of t r a n s i t i o n of Ni at 626 K w a s c h o s e n as t h e r e f e r e n c e , t h e n t h e e n t h a l p y c h a n g e of the p h a s e t r a n s i t i o n in N d M n O 3 near 1100 K w e r e a p p r o x i m a t e l y e v a l u a t e d to be 1 K J / m o l for stoichiometric composition. DTA studies a l s o s h o w e d a d e c r e a s e in the t r a n s i t i o n t e m p e r a t u r e and peak a r e a w i t h an i n c r e a s e in x. (e) T r a n s i t i o n
temperature
The t r a n s i t i o n t e m p e r a t u r e of N d M n O 3 w i t h o x y g e n n o n s t o i c h i o m e t r y is s h o w n i n Fig. 4, w h e r e t h e v a l u e s T t w e r e d e t e r m i n e d u s i n g t w o k i n d s of m e t h o d s , DTA and electrical conductivity
NdMnO 3
Vol. 19, No. 9
m e s u r e m e n t , respectively. DTA peak b e c o m e s s m a l l e r as x i n c r e a s e s , a n d so the t r a n s i t i o n temperature could not be d e t e c t e d in the r a n g e of x > 0.04, while that c o u l d be m o r e e a s i l y detected with electrical conductivity method, alt h o u g h t h e j u m p of e l e c trical conductivity at the t r a n s i t i o n temperature g r a d u a l l y d e c r e a s e s with increasing x and a larger uncertainty is included for the d e t e r m i n a t i o n of t h e t r a n s i t i o n temperature. Thus the phase t r a n s i t i o n o c c u r s in N d M n O 3 at a l i t t l e higher t e m p e r a t u r e t h a n t h a t in L a M n O 3, a n d the larger the d e v i a t i o n from stoichiometry, the l o w e r the t r a n s i t i o n temperature.
1150
1205
o Electrical conductivity' • DTA
% o
1100
o 0
,,t"
,r.
0
lO5O 0.02 ' 0.04 ' 0 .'o 6 X in NdMn03.x
0.00
FIG. 4 Transition temperature oxygen nonstoichiometry NdMnO3+ x •
0.08
vs in
Acknowledgement The author would express for his helpful discussion.
their
thanks
for
Prof.
T. T a k a i s h i
References i.
A. W o l d a n d R.J. A r n o t t , J. P h y s . C h e m . S o l i d s ~ , 176 (1959). 2. B.C. T o f i e l d a n d W.R. S c o t t , J. S o l i d S t a t e Chem. i0, 183 (1974). 3. N.N. S i r o t a , A.P. K a r a v a y a n d V.I. P a v l o v , Kristall u. T e c h n i k ii, 861 (1976). 4. K. K a m a t a , T. N a k a j i m a , T. H a y a s h i a n d T. N a k a m u r a , Mat. Res. Bull. i~, 49 (1978). 5. T. N a k a m u r a , G. P e t z o w a n d L.J. G a u c k l e r , Mat. Res. Bull. 14, 649 (1979). 6. J.B. G o o d e n o u g h , Phys. Rev. 100, 564 (1955). 7. J.B. G o o d e n o u g h , P r o g r e s s in S o l i d S t a t e C h e m i s t r y , vol. 5, (H. R e i s s , Ed.) p.145. P e r g a m o n P r e s s , O x f o r d (1971). 8. N. K a m e g a s h i r a a n d Y. M i y a z a k i , Phys. Stat. Sol., (a) 76 K39 (1983). 9. G.J. M c C a r t h y , P.V. G a l l a g h e r a n d C. Sipe, Mat. Res. Bull. 8 , 1 2 7 7 (1973). i0. S. Q u e z e l - A m b r u n a z , Bull. Soc. fr. M i n e r a l . C r i s t a l l o g r . 91, 339 (1968). Ii. T. A r a k a w a , A. Y o s h i d a a n d J. S h i o k a w a , Mat. Res. Bull. 15, 269 (1980). 12. N. K a m e g a s h i r a , Y. M i y a z a k i a n d Y. H i y o s h i , M a t e r . Chem.
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Vol. 19, No. 9
Phys. 10, 299 (1984). 13. " J A N A F T h e r m o c h e m i c a l Tables" (D.R. S t a l l Ed.). D o w Chemical Co., Midland, Mich. (1965). 14. K. Naito, N. K a m e g a s h i r a and N. Sasaki, J. S o l i d State Chem. 35, 305 (1980). 15. P. Kofstad, N o n s t o i c h i o m e t r y , D i f f u s i o n , and E l e c t r i c a l C o n d u c t i v i t y in B i n a r y Oxides, p.213 ans p.265 Wiley, I n t e r s c i e n c e , (1972). 16. C. P i c a r d and P. G e r d a n i a n , J. S o l i d S t a t e Chem., ii, 190 (1974). 17. K. K a m a t a , T. N a k a j i m a and T. N a k a m u r a , Mat. Res. Bull. 14, 1007 (1979).
NONSTOICHIOMETRY
AND PHASE
TRANSITION
IN NdMnO 3
Naoki K a m e g a s h i r a and Yuji M i y a z a k i Department of M a t e r i a l s Science, Toyohashi University Technology, Tempaku-cho, Toyohashi, 440, Japan.
of
(Received July 2, 1984; Communicated by M. Koizumi)
ABSTRACT The n o n s t o i c h i o m e t r y of n e o d y m i u m m a n g a n i t e N d M n O 3 phase has been studied by a g r a v i m e t r i c m e t h o d at 1273 K. The oxygen partial pressures were controlled by using flowing 0 2 / A r or H 2 / C O 2 m i x t u r e s . The existence of hyperstoichiometric phase and the h o m o g e n e i t y range of the • p h a s e N d M n O - J-~ X . f r o m x = 0 to 0 " 065 at 1 2 7 3 K u n d e r various o x y g e n partial p r e s s u r e s has b e e n revealed. And also the effect of o x y g e n n o n s t o i c h i o m e t r y on the phase transition at high temperature has b e e n s t u d i e d by electrical conductivity and DTA measurements. The transition temperature decreases with increasing oxygen content.
Introduction Among rare earth manganites lanthanum manganite has a considerably wide ra~e of n o n s t o i c h i o m e t r y (1-5) a n d m i x e d v a l e n c e of Mn 3+ and Mn ~ p l a y an i m p o r t a n t role in s o m e physical properties (6). L a M n O 3 a l s o h a s a p h a s e t r a n s i t i o n n e a r 800 K f r o m o r t h o r h o m b i c at low t e m p e r a t u r e to r h o m b o h e d r a l s y s t e m at high t e m p e r a t u r e , d u e to the l i b e r a t i o n f r o m J a h n - T e l l e r e f f e c t (1,7). The e x i s t e n c e of phase t r a n s i t i o n in N d M n O 3 phase at high t e m p e r a t u r e as in the case of L a M n O 3 was r e v e a l e d by the authors (8). T h i s k i n d of t r a n s i t i o n should inevitably be a f f e c t e d by the n o n s t o i c h i o m e t r y , w h i l e the a v a i l a b l e e x p e r i m e n t a l results on n o n s t o i c h i o m e t r i c behavior of N d M n O 3 h a v e been rather limited. The oxygen contents in N d M n O 3 phase have been determ i n e d by M c C a r t h y e t al. (9) f r o m m e a s u r i n g the weight loss during the r e a c t i o n Of oNcd~O~ w i t h MnO2, and N d M n O 2 97 at 1200 °C a n d N d M n O 2 96 at 1400 air have been obtained. Magnetic properties'of NdMnO 3 were investigated by several authors and their samples were synthesized in a r g o n (I0) or in a i r (ll) at high temperature. So far the r e l a t i o n s h i p a m o n g o x y g e n partial 1201
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pressure, composition and temperature in N d M n O 3 p h a s e h a s n o t b e e n k n o w n , a l t h o u g h l o w e r l i m i t s of o x y g e n p a r t i a l p r e s s u r e s for t h e s t a b l e e x i s t e n c e of N d M n O 3 p h a s e h a v e r e c e n t l y been determined at 1 1 7 3 - 1473 K with electrical conductivity measurement (12). T h e p r e s e n t investigation was undertaken f i r s t l y to d e t e r m i n e the relationship between oxygen partial p r e s s u r e and d e v i a t i o n of o x y g e n c o n t e n t f r o m stoic h i o m e t r y at 1 2 7 3 K, a n d s e c o n d l y to c l a r i f y the e f f e c t of o x y g e n n o n s t o i chiometry on the transition temperature from the electrical c o n d u c t i v i t y and D T A m e a s u r e m e n t s . Experimental Starting materials were 99.99 % Nd20 3 and Mn203 (Rare M e t a l l i c Co. Ltd.). The Nd203 was dried at 1273 K in v a c u u m and M n 2 0 3 w a s c a l c i n e d a t 1 0 7 3 K in air. An equimolar m i x t u r e of Nd203 and Mn203 w a s p r e s s e d into p e l l e t s and f i r e d at 1473 K in air ~or 3 daysf The f o r m a t i o n of o r t h o r h o m b i c N d M n O 3 phase was i d e n t i f i e d by X ray d i f f r a c t o m e t r y . Oxygen partial pressures were achieved by m e a n s of t w o methods. For~ressures f o r 105 to I0 Pa, m i x t u r e s of 02 a n d Ar gases a t i 0 ~ Pa t o t a l pressure were used in t h i s study. C o n t r o l l e d oxygen partial p r e s s u r e s in the low p r e s s u r e region w e r e ~ b t a i n e d by i n t r o d u c i n g k n o w n m i x t u r e s of CO 2 and H 2 gases at 10 ~ Pa t o t a l p r e s s u r e . For mixtures of H 2 a n d CO 2, t h e o x y g e n partial p r e s s u r e s w e r e c a l c u l a t e d f r o m the JANAF t h e r m o c h e m i c a l table (13). A f l o w rate of 1 ec/sec w a s m a i n t a i n e d for all gas m i x t u r e s l u s e d in this investigation. The f l u c t u a t i o n s of o x y g e n partial p e s s u r e s in s t r e a m i n g gas d u r i n g e q u i l i b r a t i o n were monitored by measuring electrical conductivity of s o m e m e t a l o x i d e , T i O 2 or C o O (14). After each sample was equilib r a t e d under a p p r o p r i a t e o x y g e n partial p r e s s u r e at 1273 K for 20-48 hours, it w a s q u e n c h e d into i c e - b a t h and w e i g h t w a s m e a sured by microbalance. Four probe e l e c t r i c a l c o n d u c t i v i t y m e a s u r e m e n t s w e r e c a r r i e d o u t o n p r e s s e d p e l l e t s as d e s c r i b e d in an e a r l i e r p u b l i c a t i o n
(8). Results (a) H o m o g e n e i t y
and d i s c u s s i o n
Range
The i s o t h e r m graph log (Po2/Pa) versus x in N d M n O 3 + x is s h o w n in Fig. I. N d M n O 3 d e c o m p o s e ~ into NdMnO 3 and MnO according to the f o l l o w i n g reaction, NdMnO 3 (s) = N d 2 0 3
(s) + M n O
(s) + 1/4 02
(i)
b e l o w 10 -7.79 Pa of o x y g e n partial p r e s s u r e at 1273 K. F r o m the p r e v i o u s results for the d e c o m p o s i t i o n p r e s s u r e (12) log (Po2 / Pa) = -7.68 h a s b e e n o b t a i n e d a n d in g o o d a g r e e m e n t with the present result. If it is a s s u m e d that Nd203 (15) a n d M n O (15,16) are n e a r l y s t o i c h i o m e t r i c at 1273 K and log (Po2/Pa) = 8.41, the o x y g e n c o n t e n t s in N d M n O 3 phase can be c a l c u r a t e d f r o m the r e s u l t s of w e i g h t c h a n g e u p o n t h e d e c o m p o s i t i o n in Fig. i, a n d N d M n O 3 0 0 1 3 w a s o b t a i n e d a t l o g ( P o 2 / P a ) = -7.58 at 1 2 7 3 K, which can 6e-cdnsidered to b e a l m o s t { £ o i c h i o m e t r i c . Then the
1203
N dMnO 3
V o l . 19, No. 9
free e n e r g y change for the rea3.25 ction (I) w a s calculated from z o o the decomposiL~ 3.00 I--tion pressure z and determined o 2.75 to be ~ % 1 ~ 3 o ° 77.9 i~ z Under the o x y g e n ILl partial pres2.50 ÷ sures above that of t h e d e c o m p o MnO s i t i o n the h y p e r stoichiometry was J , a ~) revealed in NdMnO 3 phase, while hypostoic h i o m e t r i c phase FIG. 1 was shown by M c C a r t h y et al O x y g e n c o n t e n t in N d M n O 3 + x as a f u n c t i o n of o x y g e n p a r t i a l p r e s s u r e at 1273 K. (9). However, as t h e e x i s t e n ces of the h y p e r s t o i c h i o m e t r y in some rare earth manganites L n M n O 3 (Ln = La, Sm, Dy, Y a n d Er) p h a s e a t 1 4 7 3 K h a v e b e e n c o n f i r m e d b y K a m a t a et al. (4,17) a n d N d M n O 3 s h o w s p - t y p e s e m i c o n d u c t i v i t y as d e s c r i b e d below, it can s u r e l y be c o n s i d e r e d t h a t N d M n O 3 a l s o has o x y g e n - e x c e s s n o n s t o i chiometry. The maximum a m o u n t of t h e o x y g e n d e v i a t i o n from s t o i c h i o m e t r y in NdMnO3+. w a s d e t e r m i n e d to be x= +0.065 at 1273 K and log(Po2/Pa) = 5, a n d it is a l i t t l e l e s s t h a n t h a t in L a M n 0 3 + x (5).
NdMnO3. x
Nd203~ -10
(b)
Lattice
-5 0 LOG(Po2/Pa)
constant
The n o n s t o i c h i o m e t r i c p h a s e c o u l d be i n d e x e d w i t h the o r t h o rhombic symmetry. The c h a n g e of l a t t i c e c o n s t a n t s in n o n s t o i chiometric NdMn03+ x is p l o t t e d a g a i n s t the e x c e s s o x y g e n cont e n t x i n Fig. 2, f r o m w h i c h it is s e e n t h a t t h e a - a x i s is a l m o s t c o n s t a n t and the c-axis g r a d u a l l y increases, w h i l e the baxis d e c r e a s e s w i t h i n c r e a s i n g x, w h i c h leads to the c o n t r a c t i o n of v o l u m e of the unit cell as the d e v i a t i o n f r o m s t o i c h i o m e t r y b e c o m e s large. (c)
Electrical
conductivity
T h e v a l u e s of l o g ( ~ T ) versus reciprocal temperatures of nonstoichiometric NdMnO 3 specimens a r e p l o t t e d in Fig. 3, in w h i c h the c o n d u c t i v i t y i n c r e a s e s as i n c r e a s i n g o x y g e n c o n t e n t at lower temperature region. This characteristic shows that N d M n O 3 is a p - t y p e s e m i c o n d u c t o r and that h o l e h o p p i n g m e c h a n i s m is d o m i n a n t in N d M n O 3 as w e l l as L a M n O 3 (7)'. A t h i g h t e m p e r a ture the e l e c t r i c a l c o n d u c t i v i t y b e c o m e s a l m o s t i n d e p e n d e n t of o x y g e n content, w h i c h w a s c o n f i r m e d by m e a s u r i n g the v a r i a t i o n of the e l e c t r i c a l c o n d u c t i v i t y w i t h p a r t i a l p r e s s u r e of o x y g e n
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5.90
...7f,k
"~<_~ ~8°t
|~..~
5.7o~
A : 0.064 B:o.o47
l\~.
-~ 5
)
s3ot
'
C : 0.029
% \ ~
%%, % ~ . ~
D : 0.017
~..
°%
%, ' , °
e....... D'"-...
"%
%
103KIT .
.
.
1100 0.02
.
0.04
0.06
X in NdMn03. x
FIG. 3 E l e c t r i c a l c o n d u c t i v i t y as a f u n c t i o n of r e c i p r o c a l t e m p e r a t u r e in N d M n 0 3 + x.
FIG. 2 V a r i a t i o n of l a t t i c e p a r a m eters w i t h o x y g e n c o n t e n t in N d M n O 3 + x.
at 1 2 7 3 K a s w a s s h o w n in p r e v i o u s s t u d y (12). F o r all o x y g e n c o m p o s i t i o n s t h e r e is seen a sharp j u m p and a f t e r w a r d s a gradual increase in conductivity c u r v e as t e m p e r a t u r e increases, socalled Z-type jump, which would reflect the existence of some kind of s t r u c t u r a l c h a n g e and in this case the p h a s e t r a n s i t i o n f r o m o r t h o r h o m b i c s y s t e m at l o w t e m p e r a t u r e to r h o m b o h e d r a l f o r m at high temperature (8), as is t h e c a s e in L a M n O 3 (7). T h e t r a n s i t i o n t e m p e r a t u r e w a s d e t e r m i n e d f r o m the t e m p e r a t u r e corr e s p o n d i n g to the b r e a k p o i n t in c o n d u c t i v i t y curve. (d)
D T A study
DTA studies also clearly show thermal anomalies, although peaks are c o n s i d e r a b l y small. W h e n the h e a t of t r a n s i t i o n of Ni at 626 K w a s c h o s e n as t h e r e f e r e n c e , t h e n t h e e n t h a l p y c h a n g e of the p h a s e t r a n s i t i o n in N d M n O 3 near 1100 K w e r e a p p r o x i m a t e l y e v a l u a t e d to be 1 K J / m o l for stoichiometric composition. DTA studies a l s o s h o w e d a d e c r e a s e in the t r a n s i t i o n t e m p e r a t u r e and peak a r e a w i t h an i n c r e a s e in x. (e) T r a n s i t i o n
temperature
The t r a n s i t i o n t e m p e r a t u r e of N d M n O 3 w i t h o x y g e n n o n s t o i c h i o m e t r y is s h o w n i n Fig. 4, w h e r e t h e v a l u e s T t w e r e d e t e r m i n e d u s i n g t w o k i n d s of m e t h o d s , DTA and electrical conductivity
NdMnO 3
Vol. 19, No. 9
m e s u r e m e n t , respectively. DTA peak b e c o m e s s m a l l e r as x i n c r e a s e s , a n d so the t r a n s i t i o n temperature could not be d e t e c t e d in the r a n g e of x > 0.04, while that c o u l d be m o r e e a s i l y detected with electrical conductivity method, alt h o u g h t h e j u m p of e l e c trical conductivity at the t r a n s i t i o n temperature g r a d u a l l y d e c r e a s e s with increasing x and a larger uncertainty is included for the d e t e r m i n a t i o n of t h e t r a n s i t i o n temperature. Thus the phase t r a n s i t i o n o c c u r s in N d M n O 3 at a l i t t l e higher t e m p e r a t u r e t h a n t h a t in L a M n O 3, a n d the larger the d e v i a t i o n from stoichiometry, the l o w e r the t r a n s i t i o n temperature.
1150
1205
o Electrical conductivity' • DTA
% o
1100
o 0
,,t"
,r.
0
lO5O 0.02 ' 0.04 ' 0 .'o 6 X in NdMn03.x
0.00
FIG. 4 Transition temperature oxygen nonstoichiometry NdMnO3+ x •
0.08
vs in
Acknowledgement The author would express for his helpful discussion.
their
thanks
for
Prof.
T. T a k a i s h i
References i.
A. W o l d a n d R.J. A r n o t t , J. P h y s . C h e m . S o l i d s ~ , 176 (1959). 2. B.C. T o f i e l d a n d W.R. S c o t t , J. S o l i d S t a t e Chem. i0, 183 (1974). 3. N.N. S i r o t a , A.P. K a r a v a y a n d V.I. P a v l o v , Kristall u. T e c h n i k ii, 861 (1976). 4. K. K a m a t a , T. N a k a j i m a , T. H a y a s h i a n d T. N a k a m u r a , Mat. Res. Bull. i~, 49 (1978). 5. T. N a k a m u r a , G. P e t z o w a n d L.J. G a u c k l e r , Mat. Res. Bull. 14, 649 (1979). 6. J.B. G o o d e n o u g h , Phys. Rev. 100, 564 (1955). 7. J.B. G o o d e n o u g h , P r o g r e s s in S o l i d S t a t e C h e m i s t r y , vol. 5, (H. R e i s s , Ed.) p.145. P e r g a m o n P r e s s , O x f o r d (1971). 8. N. K a m e g a s h i r a a n d Y. M i y a z a k i , Phys. Stat. Sol., (a) 76 K39 (1983). 9. G.J. M c C a r t h y , P.V. G a l l a g h e r a n d C. Sipe, Mat. Res. Bull. 8 , 1 2 7 7 (1973). i0. S. Q u e z e l - A m b r u n a z , Bull. Soc. fr. M i n e r a l . C r i s t a l l o g r . 91, 339 (1968). Ii. T. A r a k a w a , A. Y o s h i d a a n d J. S h i o k a w a , Mat. Res. Bull. 15, 269 (1980). 12. N. K a m e g a s h i r a , Y. M i y a z a k i a n d Y. H i y o s h i , M a t e r . Chem.
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Phys. 10, 299 (1984). 13. " J A N A F T h e r m o c h e m i c a l Tables" (D.R. S t a l l Ed.). D o w Chemical Co., Midland, Mich. (1965). 14. K. Naito, N. K a m e g a s h i r a and N. Sasaki, J. S o l i d State Chem. 35, 305 (1980). 15. P. Kofstad, N o n s t o i c h i o m e t r y , D i f f u s i o n , and E l e c t r i c a l C o n d u c t i v i t y in B i n a r y Oxides, p.213 ans p.265 Wiley, I n t e r s c i e n c e , (1972). 16. C. P i c a r d and P. G e r d a n i a n , J. S o l i d S t a t e Chem., ii, 190 (1974). 17. K. K a m a t a , T. N a k a j i m a and T. N a k a m u r a , Mat. Res. Bull. 14, 1007 (1979).