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STUDY OF ”PURE” KTaO3 AND KTN SINGLE
CRYSTALS BY THERMAL CONDUCTIVITY
MEASUREMENTS DOWN TO 50 mK
B. Salce, A. de Goër, L. Boatner
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C6, suppZ6ment au n o 12, Tome 4 2 , d6cembre 1981 page C6-424
STUDY OF
PURE^
K T a 0 3 AND K T N S I N G L E C R Y S T A L S B Y THERMAL C O N D U C T I V I T Y MEASUREMENTS DOWN TO 5 0 m KB. S a l c e , A.M. D e Go& and L.A. ~ o a t n e r *
Service d e s Basses Tempdratures, Laboratoire de Cryophysique, Centre d'Etudes NucZ6aires de GrenobZe, 85 X , 38041 GrenobZe Cedex, France.
*SOL. S t a t e Division, Oak Ridge NationaZ Laboratory, Post O f f i c e Box 10, Oak Ridge, Tennessee 37830, U. S. A.
ABSTRACT
-
We have measured t h e thermal c o n d u c t i v i t y o f pure and doped (Nb, Na, L i , Co, Ag, e t c . . . ) KTa03 s i n g l e c r y s t a l s t o i n v e s t i g a t e t h e induced f e r - r o e l e c t r i c phase t r a n s i t i o n mechanisms. The s t r o n g phonon s c a t t e r i n g observed i n t h e " p u r e s t 1 ' KTa03 i s t e n t a t i v e l y e x p l a i n e d as w e l l as t h e enhancement o f t h i s s c a t t e r i n g b y s u b s t i t u i n g Nb i n t h e c r y s t a l s . R e s u l t s o f p r e l i m i n a r y n u m e r i c a l a n a l y s i s o f t h e curves a r e g i v e n .I n r e c e n t y e a r s , a s u b s t a n t i a l amount o f work has been undertaken t o understand t h e mechanisms by which a f e r r o e l e c t r i c phase t r a n s i t i o n (FPT) can be induced by a d d i n g i m p u r i t i e s i n I ' i n c i p i e n t f e r r o e l e c t r i c s " such as KTa03 ( 1 ) . Several a u t h o r s have shown t h a t t h e mixed c r y s t a l s KTa Nbx O3 (KTN) can undergo a FPT a t a tempera- t u r e Tc depending on x f o r x > xc = 0.8 %, b u t a number o f q u e s t i o n s o f i n t e r e s t about n u c l e a t i o n and dynamic o f FPT a r e n o t answered y e t ( 2 ) . A l s o t h e o r i g i n o f t h e d i s o r d e r induced Raman s c a t t e r i n g observed i n pure KTa03 i s n o t w e l l u n d e r s t o o d ( 3 ) . Experimental
-
We have s t u d i e d s i n g l e c r y s t a l s o f KTa03 ( p u r e o r doped w i t h d i f f e r e n t i o n s ) and KTN c r y s t a l s ( 0 < x < 3 % ) by measuring t h e thermal c o n d u c t i v i t y K(T) i n t h e temperature range from 50 mK t o 200 K, as t h i s p r o p e r t y has been shown s u i t a b l e t o i n v e s t i g a t e samples w i t h b o t h i s o l a t e d i o n s and extended d e f e c t s .A l l c r y s t a l s were grown by a f l u x t e c h n i q u e and p r e l i m i n a r y r e s u l t s on KTa03, pure o r doped w i t h Ag o r Cu, and KTN c r y s t a l s , have been p r e v i o u s l y r e p o r t e d ( 3 ) . I n t h e p r e s e n t work, o t h e r c r y s t a l s doped w i t h Co, N i , Fe, Na and L i have been measured, and t h e e f f e c t o f h e a t t r e a t m e n t s on p u r e KTa03 has a l s o been s t u d i e d . The r e s u l t s a r e i l l u s t r a t e d i n f i g u r e s 1 and 2 and can be summarized as f o l l o w s :
( a ) The s t r o n g phonon s c a t t e r i n g observed i n "pure" KTa03 i s n o t s i g n i f i c a n t l y changed by ( i ) a d d i n g paramagnetic i m p u r i t i e s such as Co, N i , Ag o r Cu a t con- c e n t r a t i o n s l o w e r t h a n .5 % ( i i ) a n n e a l i n g i n a i r o r vacuum ( i i i ) a p p l y i n g a magnetic f i e l d u p t o 7 Teslas ( f i g . 2 ; n o t e t h e e n l a r g e d s c a l e compared to f i g . 1 ) ( b ) T h i s s c a t t e r i n g i s s l i g h t l y i n c r e a s e d , t o g e t h e r w i t h a change o f shape, by a d d i t q
1.5 % i r o n o r 0.3 % L i ; t h e curves a r e s i m i l a r t o t h a t o f a semiconducting sample ( f i g . 2 ) .
( c ) A s t r o n g enhancement o f t h e s c a t t e r i n g depending o f t h e c o n c e n t r a t i o n o f t h e dopant, and w i t h o u t change i n shape, i s observed i n KTN and K I - ~ N ~ , TaOj ( f i g . 1 ) One c r y s t a l , namely 1 % l i t h i u m - d o p e d KTa03, cannot f i t i n t h i s scheme : i t s beha- v i o u r around 7 K i s q u i t e t h e same t h a n t h e . 3 % L i doped c r y s t a l , b u t below
.7
KI I I 1 1 10 10 F i g u r e 2 ( K )
-
-0 Y 7 Er'
-
x
- 0 3 % N b-
3Y0 F i g u r e 1i t e x h i b i t s a K(T) % T 2 l a w which i m p l i e s a d d i t i o n a l phonon s c a t t e r i n g i n t h i s tem-
p e r a t u r e range s i m i l a r t o t h a t observed i n g l a s s e s .
D i s c u s s i o n
-
From t h e s e r e s u l t s , i t i s c l e a r t h a t s c a t t e r i n g i n " p u r e " KTa03 cannot be due t o i s o l a t e d d e f e c t s such as paramagnetic i m p u r i t i e s o r oxygen vacancies o r hydrogen. The o t h e r p o i n t i s t h a t Nb and Na d o p i n g l e a d t o q u a l i t a t i v e l y comparable e f f e c t s ; t h i s i s n o t t h e case f o r L i , c o n f i r m i n g t h e p e c u l a r i t y o f t h i s system (es- p e c i a l l y a t c o n c e n t r a t i o n s around 1 %),which has been c l a i m e d by s e v e r a l authors (5)(6). T h e r e f o r e we suggest t h a t phonon s c a t t e r i n g i n " p u r e " KTa03 a r i s e s from v e r y s m a l l q u a n t i t i e s o f e l e c t r i c d i p o l e s induced by r e s i d u a l and u n a v o i d a b l e Nb o r Na (namely% 10 ppm, f r o m t h e more a c c u r a t e a v a i l a b l e a n a l y s i s ) . These d i p o l e s can l o c a l l y
p o l a r i z e t h e 1 a t t i c e o v e r s e v e r a l a t o m i c d i s t a n c e s v i a l o n g range i n t e r a c t i o n . I t can be expected t h a t these extended d e f e c t s a r e s t r o n g l y c o u p l e d w i t h phonons. T h i s s c a t t e r i n g must be n e a r l y c o n c e n t r a t i o n independent as l o n g as no o v e r l a p b e t - ween t h e p e r t u r b e d r e g i o n s t a k e s p l a c e . From t h e l e s s c o n c e n t r a t e d Nb-doped c r y s t a l s measured ( - 1 5 % and . 3 % ) we can r o u g h l y e s t i m a t e a t 1000 ppm t h e mean c o n c e n t r a t i o n t o a c h i e v e o v e r l a p so t h a t t h e r a d i u s o f one p e r t u r b e d r e g i o n would be 5-10 a t o m i c d i s t a n c e s . I n c r e a s i n g t h e Nb c o n c e n t r a t i o n r e s u l t s i n an e x t e n s i o n o f t h e " o r d e r e d " p a r t of c r y s t a l , u n t i l x = xc where t h e whole sample becomes f e r r o e l e c t r i c o v e r t h i s
C6-426 JOURNAL DE PHYSIQUE
c r i t i c a l c o n c e n t r a t i o n . Supplementary Nb do n o t p l a y any r o l e o t h e r w i s e a c t i n g as p o n c t u a l d e f e c t s a t h i g h temperature. A l t h o u g h t h e measurements on K1-xNaxTa03 c r y s - t a l s a r e n o t so comprehensive and t h a t t h e d e t e r m i n a t i o n o f r e l i a b l e Na c o n c e n t r a t i o n i s s t i l l a m a j o r problem, i t can be expected t h a t a q u i t e s i m i l a r i n t e r p r e t a t i o n h o l d s f o r t h i s system.
P r e l i m i n a r y computer c a l c u l a t i o n s have been done w i t h i n t h e Callaway model. A l l t h e Nb and Na-doped c r y s t a l s can be f i t t e d u s i n g a t o t a l r e l a x a t i o n t i m e
.r-I (w,T) = v / a L
+
Aw4+
( w , ~ )+
T-i
+ T - 1R N
where v/aL i s t h e boundary s c a t t e r i n g t e r m c a l c u l a t e d f r o m t h e s i z e o f t h e samples, A U ~ t h e p o i n t d e f e c t s c a t t e r i n g and I--; i s g i v e n by t h e phenomenological e x p r e s s i o n :
exp [-a [(T-TO(X))/TO(X)I . f T<TO(X)
~ - i
(w,T) = D ( x ).
w2.
f ( T ) w i t h f ( T ) =3
exp [-B [T
-
ToThe parameters needed t o f i t t h e adjustements shown i n f i g u r e 1 a r e l i s t e d i n t h e t a b l e 1.
Table 1
It can be seen t h a t , a l t h o u g h x i s changing b y a f a c t o r 20 f o r t h e KTN samples, D(x) ( s - 1 )
a
B To (K)
a and B a r e n e a r l y c o n s t a n t w i t h i n a f a c t o r o f 2. The parameter To decreases c o n t i - n o u s l y down t o
-
OK as x i n c r e a s e s . I f we suppose t h a t To d e f i n e s v e r y c r u d e l y t h e wavelength o f t h e most s t r o n g l y s c a t t e r e d phonons, t h i s wavelength i n c r e a s e s w i t h xNa 2
l o 7
3.2
.
22 8and can be r e l a t e d t o t h e e x t e n s i o n o f t h e "ordered" p a r t o f t h e c r y s t a l . A l s o t h e v a l u e To = 8 K f o r p u r e KTa03 l e a d s t o a p e r t u r b e d volume o f about 8 u n i t c e l l s , i n
"Pure" 3 . 1 0 ~ 2.6
.
15 8 agreement w i t h t h e q u a l i t a t i v e e s t i m a t i o n g i v e n above. .3 % Nb 3 xl o 7
2.2.
22 6 . I 5 % Nbl o 7
2.6 . 3 7.2 References( 1 ) HOCHLI U.T., WEIBEL H.E. and BOATNER L.A., Phys. R e v - L e t t e r s ,
2,
1158 (1977). ( 2 ) YACOBI Y., Z - P h y s i k B,3,
275 (1978).( 3 ) PRATER R.L., CHASE L.L. andaBOATNER L.A., Phys. Rev.B,
23,
221 (1981).( 4 ) DE GOER A.M., SALCE B. and B0ATNERL.A- Proceeding o f 3 r d I . C . on Phonon S c a t t e - r i n g i n Condensed M a t t e r , (providence-US), Plenum Press N.Y.,243 (1980).
3 % Nb 2.5 x
l o 8
1.3
.
18.
35( 5 ) PRATER R.L., CHASE L .L. and BOATNER L.A., Phys. Rev. B,
