RAMAN SCATTERING STUDY OF LOW FREQUENCY PHONON POLARITONS IN SEVERAL CRYSTALS

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RAMAN SCATTERING STUDY OF LOW

FREQUENCY PHONON POLARITONS IN SEVERAL

CRYSTALS

S. Kojima, T. Nakamura

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque C6, suppze'rnent au n o 1 2 , Tome 4 2 , de'cembre 1981 page C6-767

RAMAN SCATTERING STUDY O F LOW FREQUENCY PHONON P O L A R I T O N S I N SEVERAL CRYSTALS

S. K o j i m a and T. ~akarnura*

I n s t i t u t e of Applied Physics, University o f Tsukuba, Sakura, Ibaraki 305,

Japm

*

_{The I n s t i t u t e for Solid S t a t e Physics, The University of Tokyo, Roppongi,}

Minato-ku, Tokyo 106, Japan

Abstract.- Dispersion r e l a t i o n s of low-frequency phonon p o l a r - i t o n s have been i n v e s t i g a t e d by t h e near-forward Raman s c a t t e r - i n g technique. The r e s u l t s of measurements a r e given f o r t h e c r y s t a l s Sr2Nb207, LiTa03 and Gd2(MoO4I3.

1. I n t r o d u c t i o n . - Dispersion r e l a t i o n s of low-frequency phonon p o l a r - i t o n s i n t h e c r y s t a l s , which undergo s t r u c t u r a l phase t r a n s i t i o n s , g i v e important information about t h e mechanism of t h e t r a n s i t i o n s . However, i n f a c t , measurements of low-frequency p o l a r i t o n s r e q u i r e o b s e r v a t i o n s of near-forward Rarnan s c a t t e r i n g a t very s m a l l angles. So p o l a r i t o n peaks a r e obscured by t h e wing of t h e Rayleigh l i n e . For such a prob- lem, t h e i o d i n e f i l t e r technique h a s been used t o r e j e c t t h e Rayleigh component. I n t h a t c a s e complicated compensations a r e r e q u i r e d . On t h e o t h e r hand, t h e r e c e n t development of concave holographic g r a t i n g s e n a b l e s a double monochromator t o improve t h e s t r a y l i g h t r e j e c t i o n up t o ( u s u a l l y 10-lo- 1 0 - 1 2 ) . I n t h i s work, low-frequency p o l a r - i t o n s have been measured by t h e combination of t h e double monochromator and t h e o p t i c a l system designed f o r t h i s purpose.

2. Experimenta1s.- The samples were e x c i t e d by an i n c i d e n t beam a t 5145A from A r ion l a s e r . The s c a t t e r e d l i g h t from t h e saiiples was s e l e c t e d by a s t o p p e r w i t h two c o c e n t r i c arc-shaped symmetric h o l e s and was c o l l e c t e d o n t o t h e e n t r a n c e s l i t of a double monochromator

( J o b i n Yvon Ramanor Hg.2S). The d e t e c t o r used was cooled photomulti- plier(HTV-R649S) and t h e o u t p u t c u r r e n t was analysed w i t h t h e u s u a l photon counting technique.

3. R e s u l t s .

3-1. Sr2Nb207

Strontium Niobate Sr2Nb207 i s one of t h e A2B207 type oxide com- pounds w i t h a p e r o v s k i t e s l a b s t r u c t u r e . I t undergoes 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 a t about 1342OC w i t h ' a p o i n t group change from mmm t o

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C6-768 JOURNAL DE PHYSIQUE

mm2 and h a s a spontaneous p o l a r i z a t i o n along t h e c-axis.' A t about 215OC, a n o t h e r t r a n s i t i o n occurs and transforms i n t o an incommensurate

-t

phase modulated w i t h t h e wave v e c t o r k=t (1-S)f;* (where

fa*

i s a r e c i p - r o c a l l a t t i c e v e c t o r ) . 2

A s t o t h e l a t t i c e i n s t a b i l i t y , Kojima e t a l . h a s found an amplitude mode r e s p o n s i b l e f o r t h e normal-incommensurate phase t r a n s i t i o n a t 215'C by t h e backward Raman s c a t t e r i n g t e c h n i q u e V 3 Namely, a mode l o c a t e d a t 44cm-1 i n b ( c c ) b s c a t t e r i n g s p e c t r a shows s o f t e n i n g a t f i r s t w i t h i n c r e a s i n g temperature from room temperature. Next, l e v e l r e p u l s i o n and i n t e n s i t y t r a n s f e r t a k e p l a c e between t h e s o f t mode and a n o t h e r mode l o c a t e d a t 28cm-l. With f u r t h e r i n c r e a s i n g temperature, t h e lower mode b e g i n s t o s o f t e n toward 215OC.

I n t h e p r e s e n t work, near-forward Raman s c a t t e r i n g measurements have been made f o r s t u d y i n g A ( z ) p o l a r i t o n s . I n t h e low frequency

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r e g i o n , t h e p o l a r i t o n e f f e c t has been observed f o r t h e amplitude mode l o c a t e d a t 44cm-l and t h e mode l o c a t e d 54cm-I a t b ( c c ) b+Aa s c a t t e r i n g geometry. A s t h e peak a t 54cm-I approaches t o t h e peak a t 44cm-I i n t h e s p e c t r a , an i n t e n s i t y t r a n s f e r between t h e two modes occurs. Then t h e peak a t 44cm-I t e n d s t o z e r o i n d e c r e a s i n g q. No anomaly of po- l a r i t o n l i n e shapes induced by a coupling t o a n o t h e r lower mode a t 28cm-' h a s been d e t e c t e d , although, i n t h e c a s e of temperature dependence, a n t i - c r o s s i n g between t h e mode 44cm-1 and t h e mode a t 28cm-' occurs. The absence of a n t i - c r o s s i n g i n p o l a r i t o n d i s p e r s i o n curves s u g g e s t s t h a t t h e mode a t 28cm-I of which o s c i l l a t o r s t r e n g t h i s ex- tremely weak does n o t i n t e r a c t w i t h photon.

The high-temperature behavior of' t h e low frequency p a r t s i n b ( c c ) b s c a t t e r i n g s p e c t r a has a l s o been i n v e s t i g a t e d . The remarkable

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s o f t e n i n g and t h e line-broadening of t h e mode a t 54cm have been c l e a r l y observed i n approaching t h e t r a n s i t i o n p o i n t 1342OC.

3-2. LiTa03

Litium T a n t a l a t e LiTa03 i s one c r y s t a l of t e c h n o l o g i c a l impor- tance. I t undergoes 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 a t about 620°C. Recently, Penna e t a l . r e p o r t e d t h e anomalous l e v e l i n g i n t h e low wave number r e g i o n of t h e Al(z) p o l a r i t o n s , which was a t t r i b u t e d t o

f i n i t e propagation l e n g t h s of p o l a r i t o n s . The propagation l e n g t h of p o l a r i t o n s was i n t r o d u c e d by Penna e t a l . and i s determined by t h e d e s t r u c t i o n of p o l a r i t o n s due t o domain w a l l d i s t r i b u t i o n s .

The lowest branch of t h e A ₁( z ) p o l a r i t o n s has been a c c u r a t e l y measured. P o l a r i t o n peaks were c l e a r l y observed i n t h e r e g i o n w >

P 14cm-la The o b t a i n e d r e l a t i o n w v s . q i s n e a r l y s t r a i g h t i n t h e

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region q > 100cm-~ and such an effect reported by Penna et al. has not been observed.

3-3. Gd2 (Moo4)

Gadolinium Molybdate Gd2(Mo04)3 undergoes a ferroelectric phase transition at about 160°C.5 Neutron scattering studies have shown that above T there is a doubly degenerate soft phonon at Brillouin

C

zone-boundary (M point) associated with the transition. Fleury re- -1

ported a soft mode with Al(z) symmetry at about 46cm at room temperature.6 He found by the single damped oscillator fitting that the temperature dependence of the mode frequency is not remarkable, however, the line-width of the mode increases rapidly toward T with

C increasing temperature.

In the present study, the polariton effect for the soft mode has been investigated at room temperature. Near-forward Raman scattering measurements have been made at a(cc)a+Ab geometry. With decreasing the polariton vector q, the anomalous line-broadening of the polaritons at about 46cm-' has been observed. The result suggests the strong frequency dependence of the damping factor of the soft mode.

The detailed experimental procedure and analysis will be pub- lished elsewhere.

REFEREINCES

1. S.Nanamatsu, 14.Kimura and T.Kawamura, J. Phys. Soc. Jpn. 38 (1975) 817

2. N.Yamamoto, K.Yagi, G-Honjo and M.Kimura, J. Phys. Soc. Jpn. 48 (1980) 185

3. S.Kojima, K.Ohi, M.Takashige, T.Nakamura and H.Kakinuma, Solid State Commun. 31 (1979) 755

4. A.F.Penna, S.P.S.Porto and E-Wiener -Avnear, Solid State Commun. 23 (1977) 377