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Submitted on 1 Jan 1981
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A RAMAN STUDY OF THE STRUCTURAL PHASE
TRANSITION IN DISORDERED KMn0.85Ni0.15F3
D. Lockwood, G. Coombs
To cite this version:
JOURNAL DE PHYSIQUE
ColZoque C6, suppldment au n o 12, Tome 42, ddcembre 1981 page C6-761
A RAMAN
STUDYO F
THE STRUCTURAL PHASE TRANSITION IN DISORDERED K M n 0 . 8 5 N i ~ . 1 5 F 3D.J. Lockwood and G.J. Coombs*
Physics Division, National ~ e s e a r c h Council, Ottawa, Canada
KIA
OR6 * ~ h ~ s i c s Department, University of New South Wales, Kensington, AustraliaA b s t r a c t
.-
KMnl-xNiz
has been s t u d i e d t o i n v e s t i g a t e t h e e f f e c t of d i s o r d e r on t h e s t r u c t u r a l phase t r a n s i t i o n a t 186 K i n KMnF3. T h i s t r a n s i t i o n was found a t-
130 K i n I(MnO 8 5 N i 0 but was not observed i n o t h e r c r y s t a l s having x > 0.5. D i s o r d e r mainly a f f e c t s Tc and n o t t h e phonon behaviour. P r e s e n t r e s u l t s a r e c o n s i s t e n t w i t h a l i n e a r e x t r a p o l a t i o n of Tc Ro 0 K a t x=
0.5.1. I n t r o d u c t i o n . - The p r e s e n c e of d e f e c t s can s u b s t a n t i a l l y a l t e r s t r u c t u r a l phase t r a n s i t i o n t e m p e r a t u r e s . Although t h e o r i g i n s of t h i s change a r e poorly u n d e r s t o o d i t is now b e l i e v e d t h a t d e f e c t s i n f l u e n c e phase t r a n s i t i o n s i n d i f f e r e n t ways depending on t h e s i g n of t h e s p e c i f i c h e a t exponent.' We have i n v e s t i g a t e d t h e d i s o r d e r e d system KMnl-xNixF3 where t h e c r i t i c a l p r o p e r t i e s s h o u l d be t h e same as
t h o s e of p u r e KMnF3, which p o s s e s s e s two s t r u c t u r a l phase
transition^.^
2. Experiment and Results.- Raman measurements were performed on s e v e r a l a l l o y s f o l l o w i n g t h e procedure given i n Ref. 3. The i n c i d e n t and s c a t t e r e d beaus were d i r e c t e d a l o n g t h e c u b i c p r i n c i p a l axes (X and Y, s a y ) .
A s t r u c t u r a l phase t r a n s i t i o n was observed i n only one ( x = 0.15) of t h e mixed c r y s t a l s s t u d i e d a t t e m p e r a t u r e s > 5 K. Low t e m p e r a t u r e s p e c t r a f o r t h i s c r y s t a l a r e shown i n f i g u r e 1. S p e c t r a i n o f f - d i a g o n a l p o l a r i s a t i o n s were e s s e n t i a l l y i d e n t i c a l because of domain induced mixing. The broad peaks n e a r 330 and 145 cm-l a r i s e from two-magnon e x c i t a t i o n s 3 w h i l e t h o s e t o lower frequency ( n e a r 25 and 70 cm-') a r e probably two-phonon bands. The narrow l i n e s a r e a s s i g n e d t o f i r s t - o r d e r phonon s c a t t e r i n g ( s e e t a b l e 1). The 127 and 238 cm-' l i n e s i n o f f - d i a g o n a l p o l a r i s a t i o n a r e t o o s t r o n g t o be a t t r i b u t a b l e t o d e p o l a r i s a t i o n of l i n e s a t s i m i l a r f r e q u e n c i e s i n t h e (22) spectrum. A l l of t h e s e l i n e s d i m i n i s h i n i n t e n s i t y and e v e n t u a l l y v a n i s h w i t h i n c r e a s i n g t e m p e r a t u r e a s i s shown, f o r example, i n f i g u r e 2. These r e s u l t s i n d i c a t e Tc = 130 -+ 5 K. The peak frequency of t h e s o f t mode a t 17.4 cm-' r e n o r m a l i s e s r e l a t i v e l y slowly at f i r s t w i t h i n c r e a s i n g
t e m p e r a t u r e but is overdamped by 60 K ( s e e f i g u r e 5 ( a ) of Ref. 3). T h i s , t o g e t h e r w i t h t h e l o w frequency, made i t d i f f i c u l t t o e x t r a c t a d e t a i l e d t e m p e r a t u r e dependence. The remaining modes s h i f t l i t t l e i n frequency w i t h i n c r e a s i n g temperature.
JOURNAL DE PHYSIQUE
I
100 200 300 400
FREQUENCY (em')
Fig.
I:
Raman spectrum of KMn0.85Ni0. F grecorded a t 6 K w i t h a resolution of 2 . 8
cm-
.
Table 1. Assignments and peak f r e q u e n c i e s (cm-'1 of Raman a c t i v e modes i n t e t r a g o n a l ICMnI-JiixF3. Assignment 20.5 (zX)
-
62 (22) 17.4 (ZZ) 116 (zX) 127 (XY+zY) 124 ( W ) 127.9 (ZZ) 227.5 (ZX) 238 (XY+zY) 231 (YY) 238.2 ( Z Z ) 327.5 (YX) 340.5 (XY+zY)3. Discussion.- Comparison w i t h r e s u l t s obtained from pure KMnF3 ( s e e t a b l e 1 )
i n d i c a t e s t h a t t h e 130
K
phase t r a n s i t i o n i n KMnO. 8 5 N i 0 . 15F3 d e r i v e s from t h e sameI
c u b i c - t e t r a g o n a l phase change a s s o c i a t e d with condensation of t h e cubic-phase R15 modes. The i n t e n s i t y of t h e hard m d e a t 238 c m - l h a s a s i m i l a r reduced
temperature dependence t o t h a t found f o r t h e analogous l i n e i n KMIIF~~ and provides a nreasure of t h e o r d e r parameter. The A s o f t modes i n KMnF and KMn0.85Ni0, 15F3
ig
a l s o have a s i m i l a r t e m p e r a t u r e dependence. Furthermore, t h e hard and s o f t m d e damping f o r x = 0.15 is comparable with t h a t found f o r x = 0. Thus t h e c r i t i c a l p r o p e r t i e s of t h e mixed c r y s t a l a r e t h e same a s t h o s e of pure KMnF3, a s p r e d i c t e d by
theory. The main e f f e c t of d i s o r d e r appears a s a lowering of t h e phase t r a n s i t i o n temperature and a s m a l l e r a n i s o t r o p y i n Bgg
-
E mode f r e q u e n c i e s , implying a nI g
i n c r e a s e d r e s i s t a n c e a g a i n s t R15 mode s o f t e n i n g . P r e s e n t r e s u l t s a r e c o n s i s t e n t w i t h a l i n e a r e x t r a p o l a t i o n of Tc from 186 K a t x = 0 t o 0 K a t x = 0.5, a s i s shown i n f i g u r e 3.
Acknowledgement.- The e x p e r i m e n t a l work was performed a t t h e Edinburgh U n i v e r s i t y Physics Department w i t h s u p p o r t from t h e SRC and t h e US Army O f f i c e of S c i e n t i f i c Research.
R e f e r e n c e s
1. A.B. H a r r i s , J. Phys. C
1,
1671 (1974).2. D.J. Lockwood and B.H. T o r r i e , J. Phys. C