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A new pressure-induced phase in PbHfO3
Volume 30A, number 8
PHYSICS
i o n i z i n g s t a t e s . B e c a u s e of t h e high i n t e n s i t y of the absorption lines these states should give a l a r g e c o n t r i b u t i o n to the p h o t o i o n i z a t i o n c r o s s s e c t i o n of t h e CH 4 m o l e c u l e in t h i s s p e c t r a l r e gion. I g r a t e f u l l y a c k n o w l e d g e the c o o p e r a t i o n with t h e s y n c h r o t r o n r a d i a t i o n g r o u p ( D r . R. H a e n s e l , D r . C. Kunz, D i p l . - P h y s . G. K e i t e l , D i p l . - P h y s . P. S c h r e i b e r ) at the G e r m a n E l e c t r o n S y n c h r o t r o n (DESY. H a m b u r g ) .
A
NEW
PRESSURE-INDUCED
LETTERS
15 December 1969
References
1. H.-U. Chun, Tae-Kju Ha and R. Mann, Z. Phys. Chem. Frankfurt/M. [N. F.] 47 (1965) 313. 2. H.-U. Chun and H. Gebelein. Z. Naturforsch. 22a (1967) 1813. 3. H.-U. Chun, Z. lqaturforsch. 23a (1968) 1415. 4. H.-U. Chun, Dissertation. Frankfurt/M. 1963. 5. G Brogren. Nova Acta Regiae Soc. Sci. Upsaliensis Ser. IV. 14. No. 4 (1949).
PHASE
IN
PbHfO
3
G.A. SAMARA Sat~dia L a b o r a t o r i e s , Albuquerque, New Mexico 87115, USA
Received 3 November 1969
A new a n t i f e r r o e l e c t r i c phase becomes stable in PbHfO3 at high p r e s s u r e . The t e m p e r a t u r e - r a n g e of stability of this phase increases with increasing pressure.
T h e r e h a s b e e n m u c h r e c e n t i n t e r e s t [1] in the study of p h a s e t r a n s i t i o n s e x h i b i t e d by s u b s t a n c e s c r y s t a l l i z i n g in t h e p e r o v s k i t e s t r u c t u r e and in the i n t e r p r e t a t i o n of t h e s e t r a n s i t i o n s in t e r m s of i n s t a b i l i t i e s of v a r i o u s l a t t i c e v i b r a t i o n a l m o d e s . L e a d h a f n a t e , PbHfO3, is known [2] to e x h i b i t two t e m p e r a t u r e - i n d u c e d p h a s e t r a n s i t i o n s , at ~ 433OK and ~ 475OK. T h e low temperature orthorhombic phase is antiferroe l e c t r i c and i s o m o r p h o u s with t h e a n t i f e r r o e l e c t r i c p h a s e of P b Z r O 3. T h e i n t e r m e d i a t e t e t r a g o n a l p h a s e is a l s o a n t i f e r r o e l e c t r i c but w i t h a d i f f e r e n t t y p e of d i p o l e a r r a n g e m e n t . A b o v e ~ 475OK the c r y s t a l h a s t h e i d e a l c u b i c p e r o v s k i t e s t r u c t u r e and i s p a r a e l e c t r i c . W e h a v e r e c e n t l y i n v e s t i g a t e d t h e e f f e c t s of t e m p e r a t u r e (T) and h y d r o s t a t i c p r e s s u r e (P) on t h e d i e l e c t r i c p r o p e r t i e s and t r a n s i t i o n s in P b H f O 3. In a d d i t i o n to i n t e r e s t i n g c h a n g e s in t h e s e p r o p e r t i e s , the r e s u l t s show that a n e w p h a s e is i n d u c e d by t h e a p p l i c a t i o n of p r e s s u r e and that t h i s p h a s e i s m o s t l i k e l y a l s o a n t i f e r r o electric. T h e m e a s u r e m e n t s w e r e m a d e on p o l y c r y s * This work was supported by the US Atomic Energy Commission. 446
t a l l i n e s a m p l e s of n e a r l y (> 99.5%) t h e o r e t i c a l d e n s i t y . T h e a p p a r a t u s and e x p e r i m e n t a l t e c h n i q u e s w e r e s i m i l a r to t h o s e u s e d e a r l i e r [3]. F i g . 1 s h o w s two i s o b a r s of t h e s t a t i c d i e l e c t r i c c o n s t a n t , E, v e r s u s T. With i n c r e a s i n g T at 1 b a r t h e two t r a n s i t i o n s a r e o b s e r v e d at 433OK and 476OK. In the p a r a e l e c t r i c p h a s e c o b e y s the C u r i e - W e i s s l a w , c = C / ( T - To) , with C = 1.65 × × 105 OK and T O = 377OK. T h e 3 . 3 - k b a r i s o b a r in fig. 1 s h o w s s e v e r a l f e a t u r e s . T h e t r a n s i t i o n s shift to h i g h e r t e m p e r a t u r e s , and the ~ a n o m a l y at the u p p e r a n t i ferro-paraelectric transition is sharply reduced by p r e s s u r e ; but one of the m o s t i n t e r e s t i n g f e a t u r e s is the a p p e a r a n c e of a new p h a s e w h o s e T - r a n g e of s t a b i l i t y i n c r e a s e s with i n c r e a s i n g P. T h i s p h a s e , l y i n g b e t w e e n the a r r o w s at A and B in fig. 1, i s c l e a r l y d e f i n e d f r o m both' c v e r s u s T and d i e l e c t r i c l o s s (tan 6) v e r s u s T data. T h e tan 5 d a t a s h o w a w e l l - d e f i n e d m i n i m u m at t h e p o i n t A. T h e i n s e r t in fig. 1 s h o w s the T - P p h a s e d i a g r a m . L a b e l l i n g the d i f f e r e n t p h a s e s a , fl, V, and 6, with d e c r e a s i n g t e m p e r a t u r e , the s l o p e s ( d T / d P ) of the p h a s e b o u n d a r i e s in O K / k b a r a r e : 14.3 ~: 1.0 f o r ~ - f i , 5.0 +0.5 f o r fl-~, and 5.9 + 0.6 f o r V - 6 . A l t h o u g h high p r e s s u r e X - r a y a n d / o r n e u t r o n
Volume 30A, number 8 I
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1600
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PHYSICS LETTERS I
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Fig. 1. Isobars of the dielectric constant versus T for PbHfO 3. The insert shows a portion of the T-P phase diagram. d i f f r a c t i o n and o t h e r a u x i l i a r y m e a s u r e m e n t s a r e needed to e s t a b l i s h the e x a c t n a t u r e of the new fl p h a s e , our p r e s e n t r e s u l t s i n d i c a t e that this ph as e is a l s o a n t i f e r r o e l e c t r i c . T h e e v i de nc e is b a s e d on p o l a r i z a t i o n v e r s u s e l e c t r i c fie ld m e a s u r e m e n t s which f a i l e d to r e v e a l any e v i d e n c e f o r f e r r o e l e c t r i c i t y , and t h i s i s f u r t h e r s u p p o r t e d by the i n c r e a s e in the Ta_fl t r a n s i t i o n t e m p e r a t u r e with P . In the p e r o v s k i t e s , with d e c r e a s i n g T, t r a n s i t i o n s to f e r r o e l e c t r i c p h a s e s a r e a c c o m p a n i e d by i n c r e a s e s in unit c e l l v o l u m e , w h e r e a s t r a n s i t i o n s to a n t i f e r r o e l e c t r i c p h a s e s a r e a c c o m p a n i e d by d e c r e a s e s in v o l u m e . Since p r e s s u r e f a v o r s the s m a l l e r v o l u m e , p r e s s u r e lowers f e r r o e l e c t r i c transition tern-
15 December 1969
p e r a t u r e s and r a i s e s a n t i f e r r o e l e c t r i c t r a n s i t i o n t e m p e r a t u r e s . Studies on a l a r g e n u m b e r of f e r t o - and a n t i f e r r o e l e c t r i c p e r o v s k i t e s have conf i r m e d this o b s e r v a t i o n . The fl p h a s e in PbHfO 3 is d e f i n i t e l y not r e l a t e d to the i n t e r m e d i a t e f e r r o e l e c t r i c p h a s e o b s e r v e d o v e r a n a r r o w Tr a n g e (a few d e g r e e s ) at 1 b a r in P b Z r O 3. In this l a t t e r c a s e the p a r a - f e r r o e l e c t r i c t r a n s i t i o n is a c c o m p a n i e d by a v o l u m e expansion, and T p e _ F e d e c r e a s e s with P , unlike T~_fi in PbHfO 3. In t e r m s of the d y n a m i c s of the l a t t i c e , the p r e s s u r e r e s u l t s on PbHfO 3 (and s i m i l a r r e s u l t s on PbZrO3) can be u n d e r s t o o d as follows [4j. In both c r y s t a l s t h e r e a r e two low f r e q u e n c y soft m o d e s : a zone c e n t e r (q = 0) f e r r o e l e c t r i c mode which c a u s e s the l a r g e • and its C u r i e - W e i s s b e h a v i o r in the p a r a e l e c t r i c p h a s e and a zone boundary ( q ¢ 0 ) a n t i f e r r o e l e c t r i c mode (possibly a coupled mode) which c a u s e s the p a r a - a n t i f e r r o e l e c t r i c t r a n s i t i o n . On l o w e r i n g T, each c r y s tal b e c o m e s unstable a g a i n s t its a n t i f e r r o e l e c t r i c mode just b e f o r e the i n s t a b i l i t y due to the f e r r o e l e c t r i c mode is r e a c h e d . P r e s s u r e r a i s e s the f r e q u e n c y of the f e r r o e l e c t r i c m o d e but l o w e r s (softens) that of the a n t i f e r r o e l e c t r i c mode and, as a r e s u l t , the l a r g e • an o m al y at the p a r a - a n t i f e r r o e l e c t r i c t r a n s i t i o n is s h a r p l y r e duced as o b s e r v e d e x p e r i m e n t a l l y . Th e s u c c e s sion of p h ase t r a n s i t i o n s in PbHfO 3 o b s e r v e d on l o w e r i n g T m u s t r e s u l t f r o m the s u c c e s s i v e c o n densation of v a r i o u s a n t i f e r r o e l e c t r i c modes. The identification of t h e s e m o d e s m u s t await neutron s c a t t e r i n g and R a m a n a n d / o r f a r i n f r a r e d studies. A m o r e d e t a i l e d acco u n t of the p r e s e n t work will be published e l s e w h e r e . I wish to thank C. A. Hall f o r p r o v i d i n g the s a m p l e and W. L. CA~risman f o r t e c h n i c a l a s s i s t ance.
References 1. G. Shirane and Y. Yamada, Phys. Hey. 177 (1969) 858, and references therein. 2. O. Shirane and H. Pepinsky, Phys. Rev. 91 (1953) 812. 3. G.A. Samara, J. Phys. Chem. Solids 26 (1965) 121. 4. G.A. Samara, Proe. 2nd Intern. Meeting on f e r r o electricity, Kyoto, September 1969 (to be published in J. Phys. Soc., Japan).
447
PHYSICS
i o n i z i n g s t a t e s . B e c a u s e of t h e high i n t e n s i t y of the absorption lines these states should give a l a r g e c o n t r i b u t i o n to the p h o t o i o n i z a t i o n c r o s s s e c t i o n of t h e CH 4 m o l e c u l e in t h i s s p e c t r a l r e gion. I g r a t e f u l l y a c k n o w l e d g e the c o o p e r a t i o n with t h e s y n c h r o t r o n r a d i a t i o n g r o u p ( D r . R. H a e n s e l , D r . C. Kunz, D i p l . - P h y s . G. K e i t e l , D i p l . - P h y s . P. S c h r e i b e r ) at the G e r m a n E l e c t r o n S y n c h r o t r o n (DESY. H a m b u r g ) .
A
NEW
PRESSURE-INDUCED
LETTERS
15 December 1969
References
1. H.-U. Chun, Tae-Kju Ha and R. Mann, Z. Phys. Chem. Frankfurt/M. [N. F.] 47 (1965) 313. 2. H.-U. Chun and H. Gebelein. Z. Naturforsch. 22a (1967) 1813. 3. H.-U. Chun, Z. lqaturforsch. 23a (1968) 1415. 4. H.-U. Chun, Dissertation. Frankfurt/M. 1963. 5. G Brogren. Nova Acta Regiae Soc. Sci. Upsaliensis Ser. IV. 14. No. 4 (1949).
PHASE
IN
PbHfO
3
G.A. SAMARA Sat~dia L a b o r a t o r i e s , Albuquerque, New Mexico 87115, USA
Received 3 November 1969
A new a n t i f e r r o e l e c t r i c phase becomes stable in PbHfO3 at high p r e s s u r e . The t e m p e r a t u r e - r a n g e of stability of this phase increases with increasing pressure.
T h e r e h a s b e e n m u c h r e c e n t i n t e r e s t [1] in the study of p h a s e t r a n s i t i o n s e x h i b i t e d by s u b s t a n c e s c r y s t a l l i z i n g in t h e p e r o v s k i t e s t r u c t u r e and in the i n t e r p r e t a t i o n of t h e s e t r a n s i t i o n s in t e r m s of i n s t a b i l i t i e s of v a r i o u s l a t t i c e v i b r a t i o n a l m o d e s . L e a d h a f n a t e , PbHfO3, is known [2] to e x h i b i t two t e m p e r a t u r e - i n d u c e d p h a s e t r a n s i t i o n s , at ~ 433OK and ~ 475OK. T h e low temperature orthorhombic phase is antiferroe l e c t r i c and i s o m o r p h o u s with t h e a n t i f e r r o e l e c t r i c p h a s e of P b Z r O 3. T h e i n t e r m e d i a t e t e t r a g o n a l p h a s e is a l s o a n t i f e r r o e l e c t r i c but w i t h a d i f f e r e n t t y p e of d i p o l e a r r a n g e m e n t . A b o v e ~ 475OK the c r y s t a l h a s t h e i d e a l c u b i c p e r o v s k i t e s t r u c t u r e and i s p a r a e l e c t r i c . W e h a v e r e c e n t l y i n v e s t i g a t e d t h e e f f e c t s of t e m p e r a t u r e (T) and h y d r o s t a t i c p r e s s u r e (P) on t h e d i e l e c t r i c p r o p e r t i e s and t r a n s i t i o n s in P b H f O 3. In a d d i t i o n to i n t e r e s t i n g c h a n g e s in t h e s e p r o p e r t i e s , the r e s u l t s show that a n e w p h a s e is i n d u c e d by t h e a p p l i c a t i o n of p r e s s u r e and that t h i s p h a s e i s m o s t l i k e l y a l s o a n t i f e r r o electric. T h e m e a s u r e m e n t s w e r e m a d e on p o l y c r y s * This work was supported by the US Atomic Energy Commission. 446
t a l l i n e s a m p l e s of n e a r l y (> 99.5%) t h e o r e t i c a l d e n s i t y . T h e a p p a r a t u s and e x p e r i m e n t a l t e c h n i q u e s w e r e s i m i l a r to t h o s e u s e d e a r l i e r [3]. F i g . 1 s h o w s two i s o b a r s of t h e s t a t i c d i e l e c t r i c c o n s t a n t , E, v e r s u s T. With i n c r e a s i n g T at 1 b a r t h e two t r a n s i t i o n s a r e o b s e r v e d at 433OK and 476OK. In the p a r a e l e c t r i c p h a s e c o b e y s the C u r i e - W e i s s l a w , c = C / ( T - To) , with C = 1.65 × × 105 OK and T O = 377OK. T h e 3 . 3 - k b a r i s o b a r in fig. 1 s h o w s s e v e r a l f e a t u r e s . T h e t r a n s i t i o n s shift to h i g h e r t e m p e r a t u r e s , and the ~ a n o m a l y at the u p p e r a n t i ferro-paraelectric transition is sharply reduced by p r e s s u r e ; but one of the m o s t i n t e r e s t i n g f e a t u r e s is the a p p e a r a n c e of a new p h a s e w h o s e T - r a n g e of s t a b i l i t y i n c r e a s e s with i n c r e a s i n g P. T h i s p h a s e , l y i n g b e t w e e n the a r r o w s at A and B in fig. 1, i s c l e a r l y d e f i n e d f r o m both' c v e r s u s T and d i e l e c t r i c l o s s (tan 6) v e r s u s T data. T h e tan 5 d a t a s h o w a w e l l - d e f i n e d m i n i m u m at t h e p o i n t A. T h e i n s e r t in fig. 1 s h o w s the T - P p h a s e d i a g r a m . L a b e l l i n g the d i f f e r e n t p h a s e s a , fl, V, and 6, with d e c r e a s i n g t e m p e r a t u r e , the s l o p e s ( d T / d P ) of the p h a s e b o u n d a r i e s in O K / k b a r a r e : 14.3 ~: 1.0 f o r ~ - f i , 5.0 +0.5 f o r fl-~, and 5.9 + 0.6 f o r V - 6 . A l t h o u g h high p r e s s u r e X - r a y a n d / o r n e u t r o n
Volume 30A, number 8 I
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1600
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PHYSICS LETTERS I
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Fig. 1. Isobars of the dielectric constant versus T for PbHfO 3. The insert shows a portion of the T-P phase diagram. d i f f r a c t i o n and o t h e r a u x i l i a r y m e a s u r e m e n t s a r e needed to e s t a b l i s h the e x a c t n a t u r e of the new fl p h a s e , our p r e s e n t r e s u l t s i n d i c a t e that this ph as e is a l s o a n t i f e r r o e l e c t r i c . T h e e v i de nc e is b a s e d on p o l a r i z a t i o n v e r s u s e l e c t r i c fie ld m e a s u r e m e n t s which f a i l e d to r e v e a l any e v i d e n c e f o r f e r r o e l e c t r i c i t y , and t h i s i s f u r t h e r s u p p o r t e d by the i n c r e a s e in the Ta_fl t r a n s i t i o n t e m p e r a t u r e with P . In the p e r o v s k i t e s , with d e c r e a s i n g T, t r a n s i t i o n s to f e r r o e l e c t r i c p h a s e s a r e a c c o m p a n i e d by i n c r e a s e s in unit c e l l v o l u m e , w h e r e a s t r a n s i t i o n s to a n t i f e r r o e l e c t r i c p h a s e s a r e a c c o m p a n i e d by d e c r e a s e s in v o l u m e . Since p r e s s u r e f a v o r s the s m a l l e r v o l u m e , p r e s s u r e lowers f e r r o e l e c t r i c transition tern-
15 December 1969
p e r a t u r e s and r a i s e s a n t i f e r r o e l e c t r i c t r a n s i t i o n t e m p e r a t u r e s . Studies on a l a r g e n u m b e r of f e r t o - and a n t i f e r r o e l e c t r i c p e r o v s k i t e s have conf i r m e d this o b s e r v a t i o n . The fl p h a s e in PbHfO 3 is d e f i n i t e l y not r e l a t e d to the i n t e r m e d i a t e f e r r o e l e c t r i c p h a s e o b s e r v e d o v e r a n a r r o w Tr a n g e (a few d e g r e e s ) at 1 b a r in P b Z r O 3. In this l a t t e r c a s e the p a r a - f e r r o e l e c t r i c t r a n s i t i o n is a c c o m p a n i e d by a v o l u m e expansion, and T p e _ F e d e c r e a s e s with P , unlike T~_fi in PbHfO 3. In t e r m s of the d y n a m i c s of the l a t t i c e , the p r e s s u r e r e s u l t s on PbHfO 3 (and s i m i l a r r e s u l t s on PbZrO3) can be u n d e r s t o o d as follows [4j. In both c r y s t a l s t h e r e a r e two low f r e q u e n c y soft m o d e s : a zone c e n t e r (q = 0) f e r r o e l e c t r i c mode which c a u s e s the l a r g e • and its C u r i e - W e i s s b e h a v i o r in the p a r a e l e c t r i c p h a s e and a zone boundary ( q ¢ 0 ) a n t i f e r r o e l e c t r i c mode (possibly a coupled mode) which c a u s e s the p a r a - a n t i f e r r o e l e c t r i c t r a n s i t i o n . On l o w e r i n g T, each c r y s tal b e c o m e s unstable a g a i n s t its a n t i f e r r o e l e c t r i c mode just b e f o r e the i n s t a b i l i t y due to the f e r r o e l e c t r i c mode is r e a c h e d . P r e s s u r e r a i s e s the f r e q u e n c y of the f e r r o e l e c t r i c m o d e but l o w e r s (softens) that of the a n t i f e r r o e l e c t r i c mode and, as a r e s u l t , the l a r g e • an o m al y at the p a r a - a n t i f e r r o e l e c t r i c t r a n s i t i o n is s h a r p l y r e duced as o b s e r v e d e x p e r i m e n t a l l y . Th e s u c c e s sion of p h ase t r a n s i t i o n s in PbHfO 3 o b s e r v e d on l o w e r i n g T m u s t r e s u l t f r o m the s u c c e s s i v e c o n densation of v a r i o u s a n t i f e r r o e l e c t r i c modes. The identification of t h e s e m o d e s m u s t await neutron s c a t t e r i n g and R a m a n a n d / o r f a r i n f r a r e d studies. A m o r e d e t a i l e d acco u n t of the p r e s e n t work will be published e l s e w h e r e . I wish to thank C. A. Hall f o r p r o v i d i n g the s a m p l e and W. L. CA~risman f o r t e c h n i c a l a s s i s t ance.
References 1. G. Shirane and Y. Yamada, Phys. Hey. 177 (1969) 858, and references therein. 2. O. Shirane and H. Pepinsky, Phys. Rev. 91 (1953) 812. 3. G.A. Samara, J. Phys. Chem. Solids 26 (1965) 121. 4. G.A. Samara, Proe. 2nd Intern. Meeting on f e r r o electricity, Kyoto, September 1969 (to be published in J. Phys. Soc., Japan).
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