ANOMALOUS TEMPERATURE DEPENDENCE OF A LO MODE IN LiKSO4

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Submitted on 1 Jan 1981

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ANOMALOUS TEMPERATURE DEPENDENCE OF

A LO MODE IN LiKSO4

M. Bansal, S. Deb, Arnaud Roy, V. Sahni

To cite this version:

M. Bansal, S. Deb, Arnaud Roy, V. Sahni. ANOMALOUS TEMPERATURE DEPENDENCE

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JOURNAL DE PHYSIQUE

ColZoque C6, suppldment au n o 1 2 , Tome 42, ddcembre 1981 page C6-902

ANOMALOUS TEMPERATURE DEPENDENCE OF A LO MODE IN LiKSOq

M.L. Bansal, S.K. Deb, A.P. Roy and V.C. Sahni

Nuclear Physics Division, Bhabha Atomic Research Centre, Trombay, Bombay 400

085, India

A b s t r a c t

.-

Raman s c a t t e r i n g meaaure meots f o r LiKSOq over t h e temperature range 200 K t o 725 K are r e p o r t e d . I b s r e e u l t e corroborate t h e occurrence of a phaae t r a n s i t i o n at 700 K aab

a l s o e x h i b i t two unusual l a t t i c e dynamical f e a t u r e s v i a an invee- t e d LO-TO s p l i t t i n g and an i n c r e a e e i n t h e frequency of a n

-x e with

rice

i n temperature. A phenomenclogi a1 explanation f o r t h e s e f e a t u r n s is given.

1

1. Iatroduct1on.- We had r e c e n t l y e s t a b l i s h e d a s u b t l e firet order 6 4

low temperature phase t r a n s i t i o n i n L I K S O ~ ( C ~

+

C3v) ( a t 201 K) using Raman s c a t t e r i n g meaeurements and we have extended our measurements t o high temperature region e i n c e a d d i t i o n a l phaae t r a n s i t i o n ( - 7 0 0 ~ ) had

2

been proposed f o r t h i s c r y s t a l i n the l i t e r a t u r e

.

In extending our measurements upto 725K while we have found evidence t h a t a phase t r a n s i t i o n does occur around 700K, i n a d d i t i o n , we have found two i n t e r e s t i n g f e a t u r e s of v i b r a t i o n a l modes i n t h e

6 temperature s a x e between t h e t r s n sit ion p o i n t s (cry8 tal syimne try a6). These f e a t u r e e a r e t (1) t h e M frequency of a mode of SO4 appears below i t s TO counterpart and ( i i ) the frequency of t h i s

M

mode

-

i n c r e a s e s with t h e r i s e i n temperature. Since t h e s e are somawhat unusual f e a t u r e e we propoee t o discuea t h e s e i n g r e a t e r d e t a i l here and except f o r a b r i e f mention, omit the discussion of tbe changes p e r t a i - ning t o high temperature phase t r a n s i t i o n .

I n s e c t i o n 2 we deacribe experimental d e t a i l s and p r e s e n t t h e s p e c t r a . Diecuseion of t h e r e e u l t e and a p o s s i b l e explanation f o r t h e

LO-TO frequency i n v e r s i o n and anomalous temperature depeodence of t h e B 1 ( ~ ) 3 2 mode a r e given i n e e c t i o n 3.

2. Experimental d e t a i l s

.-

Polarized Raman S p e c t r a were recorded using a horn made 40 mW He-Cd l a s e r and a g r a t i n g double monochromator 3

1

w i t h s p e c t r a l bandpaes of 3.5cm-

.

F o r h i g h temperature s t u d i e s , we used a c e l l , wherein t h e eam- p l e is mounted mechanically and i t s t h r e e f a c e s a r e i n contact w i t h

copper t o ensure uniformity of temperature. The temperature was

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monitored ueing a oopper conetant thermocouple and a p r o p o r t i o n a l temperature c o n t r o l l e r was ueed t o maintain it con a t a n t w i t h i n +0.5°C.

The r e p o r t e d temperature values r e f e r t o t h e temperature of t h e

E

aopper block; t h e eample temperature

f

is somewhat lower (by d l O K a t 600K).

t

$ 3. Result8 and D1ecueeion.'- Fig. 1

5 U)

t ehowe the temperature dependence

w

+

f (210~-6T5K) of Y(ZY)X and Y(X2)X

epeotra over the frequency range 400 cdl-520 om-'. 9 ,t and 92

l a b e l modes involving predominantly L i t r a n e l a t i o n and SO4 i n t e r n a l v i b r a t i o n r e ape o t ive ly

.

F u r t h e r a8 ~ i g . 1 Raman s p e c t r a a t d i f f e r e n t i e of%en the caae with Iii s a l t a , t h e temperaturee f o r two o r i e n t a t ions.

I i o n i c conductivity increaeea conei- derably w i t h t h e r i e e i n temperaturq So t h e former mode oan be expe cted t o broaden more r a p i d l y with

i n c r e a s i n g temperature. The assign- ment of LO modee i n Fig.1 is based on t h i e premiee and is d i f f e r e n t

I

from e a r l i e r aeeignmente. Fig.2

I

ehowe p l o t o f frequenciee vereue

P-ig.3 Dependence of LO frequen- zeros of e q n . ( l ) f o r a given e e t of

ciee O n 1' & '2- Input _frequ- _TO_{frequencies and o e c i l l a t o r}

e n c i e s a r e a l e 0 ehown.

--*-r... V,,(TO)

I

temperature. Note t h a t f a r t h e

d 2

400 mode 50-TO e p l i t t i n g is inverted.

200 400 600 The behaviour can be understood

TEMP(K) using a p i c t u r e of two nearby o s c i l -

Fia.2 Frequency v a r i a t i o n of mode^ l a t o r a where one o f them c a r r i e d a of Fig. 1 with t e m p r a t u r e . _{much l a r g e r d i p o l e moment. An}

-

!!i' approximate expreeeion f o r t h e di-

a

e l e c t r i a f ~ c t i o n , , , i n t h i n d

5

frequency range i s

-

a' I S , ~ ~ [ T O I S2v;(~0j

f

E,(g)8

ewt

+

t. 9

-

u:cro)

-fi:(')

0.a-

l

X 0 100 150

FREOUENCY(C~~) 500 T h e I1) frequenciee obtained from t h e

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C6-904 JOURNAL DE PHYSIQUE

strengthfa S1 and S2 a r e g i v e n i n Table 1. H l r a i s h i e t 814 g i v e value8 of S1 and S2 from an a n a l y e i s of t h e l r room temperature

IR

r e f l e c t i o n s p e c t r a . The v a l u e e chosen by ue at 210K a r e c l o s e t o t h e b e .

The most notable f e a t u r e about t e m p e r a t u r e v a r i a t i o n o f v a r i o u s modee ( ~ i g s . 1&2) is t h a t b o t h $*(TI)) and (LO) converge towards %(TO) a s temp6rature is i n c r e a s e d ; t h e former decreaeea by 17cm-' whereae t h e l a t t e r increases by 6cm-l over t h e temperature i n t e r v a l 210K-420K. This behaviour can be q u a n t i t a t i v e l y reproduced using eq. ( 1 ) asawning a d e c r e a s e i n t h e v a l u e of S2 from 0.065 t o 0.015 (Table 1). I n e q n ( l ) , t h e damping h a s been n e g l e c t e d and t o t h a t e x t e n t choice o f t h e i n i t i a l

v a l u e s o f

3

and S2 i s somewhat u n c e r t a i n . A glance a t Fig .3 c l e a r l y shows t h a t f o r p r e d i c t i n g t h e obeerved r e l a t i v e s h i f t s of 3 2 ( ~ ) and

3 t r ( ~ ) ,

s2

has t o d e c r e a s e w i t h i n c r e a s i n g temperature, irre s p e c t i v e of t h e u n c e r t a i n t y i n t h e i n i t i a l choice of S1 and S2. V a r i a t i o n of S1 keeping S2 f i x e d l e a d s t o eimultaneous i n c r e a e e o r decreaee of b o t h j 2 ( ~ 0 ) and 3 t,(M), whereae experimentally 32(11)) i n c r e a s e s dnd 3 * ( ~ ) ) d e c r e a s e s with i n c r e a s i n g temperature. A t s t i l l h i g h e r temperature

( 7 420K) L i modes can not be t r a c e d e x p e r i m e n t a l l y but d 2 (ID) moves g r o g r e e s i v e l y c l o s e r t o

s2

(TO;. Since t h e

32

mode i s

IR

i n a c t i v e i n t h e f r e e i o n s t a t e , t h i s continuous d e c r e a s e i n t h e o s c i l l a t o r s t r e n g t h impliea t h a t t h e SO4 d i s t o r t i o n (from t e t r a h e d r a l rahape ) dimin i s h e s w i t h i n c r e a s i n g temperature. Across 700K changes i n t h e Raman s p e c t r a occur over the e n t i r e frequency range. F u r t h e r , t h e widths of most o f t h e modes change a b r u p t l y a c r o e e t h e t r a n s i t ion point. However, a s t h e widthe o f t h e peake a r e r a t h e r l a r g e (FmWI even a s much ae 40cm-l) d e t a i l e d examination of the t r a n s i t ion by Raman s c a t t e r i n g alone , s e e m u n f e a e i b l e and o t h e r t e c h n i q u e @ may be necessary f o r e l u c i d a t i o n o f t h e t r a n s i t ion.

Table 1 1 Observed and c a l c u l a t e d M f r e q u e n c i e e ( i n a n - l l obs(T0) oba(LO1 c a l ( I D 1

References

.-

1. M.L. Bansal, S o l . S t . Cornmu.,

Pramana,

2,

223( t h e r e i n . T.R.Rao,V.C.Sahni &A.P.Roy, 1nd.J.Phye. 0,199(1

B.Taniguchi and H. Takahashi, J. Chsm. P&s.