BRILLOUIN SCATTERING STUDY OF PHASE TRANSITIONS IN p-POLYPHENYLS

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BRILLOUIN SCATTERING STUDY OF PHASE TRANSITIONS IN p-POLYPHENYLS

C. Ecolivet, B. Toudic, M. Sanquer

To cite this version:

C. Ecolivet, B. Toudic, M. Sanquer. BRILLOUIN SCATTERING STUDY OF PHASE TRANSI- TIONS IN p-POLYPHENYLS. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-779-C6-781.

�10.1051/jphyscol:19816230�. �jpa-00221315�

JOURNAL DE PHYSIQUE

Colloque C6, suppZ6ment au n o 1 2 , Tome 42, d6cembre 1981 page c6-779

BRILLOUIN SCATTERING STUDY OF PHASE TRANSITIONS IN P-POLYPHENYLS

C . E c o l i v e t , B. Toudic and M. Sanquer

Groupe de Physique cristalline,

E.

R.A. ctc C . N.R.S. n007001 5 , UniversitB de Rennes, Campus de BeauZieu, 35042 Rennes Cedex, France

A b s t r a c t ^A B r i l l o u i n s c a t t e r i n g s t u d y of t h e v a r i o u s phase t r a n s i t i o n s i n p-polyphenyls r e v e a l s d i f f e r e n t k i n d s of e l a s t i c anomalies depending on t h e n a t u r e of t h e s e t r a n s i t i o n s .

INTRODUCTION

The f i r s t elements of t h e p-polyphenyls s e r i e s undergo phase t r a n s i - t i o n s through a change of t h e i r molecular conformation. This change haprSens by a n a n t i f e r r o d i s t o r s i v e r o t a t i o n of aromatic r i n g s about t h e molecular long a x i s , which e f f e c t i s t h e s t a b i l i z a t i o n a t low temperatures of a non-planar molecular conforma- t i o n . Despite t h e i r isomorphism, p-polyphenyls p r e s e n t d i f f e r e n t k i n d s o f t r a n s i - t i o n s and c r i t i c a l behaviours. W e r e p o r t h e r e some r e s u l t s of a B r i l l o u i n s c a t t e r i n g i n v e s t i g a t i o n of t h e s e t r a n s i t i o n s and t h e i r e f f e c t on t h e e l a s t i c p r o p e r t i e s of t h e s e c r y s t a l s .

INCOMMENSURABLE PHASES OF BIPHENYL

Biphenyl undergoes a f i r s t incommensurate phase

-+ -f

t r a n s i t i o n a t TI = 42 K w i t h modulation v e c t o r s a l o n g a and b (4 s a t e l l i t e s ) . T h i s d i s p l a c i v e t r a n s i t i o n i s r e l a t e d t o a s o f t mode which happens a t t h e s a t e l l i t e s near t h e Z p o i n t (0, 1/2, 0 ) o f t h e lrst B r i l l o u i n zone. A p a r t i a l l o c k - i n t r a n s i t i o n hap-

+

1

pens a t T2 = 17 K where t h e incommensurability a l o n g a d i s a p p e a r s ( ) .

The major sound v e l o c i t y anomaly o c c u r s f o r t h e q u a s i l o n g i t u d i n a l mode propa-

-f

g a t i n g a l o n g a ( f i g u r e 1 ) . T h i s kind o f e l a s t i c anomaly h a s been observed i n many o t h e r m a t e r i a l s p r e s e n t i n g such phases l i k e K2S,O4, Rb2ZnC14.

[b U[

^{' T '}

\ /

?

F i g . 1 ( l e f t )

-

Biphenyl q u a s i - l o n g i t u d i n a l sound v e l o c i t y (upped and its B r i l l o u i n t o t a l l i n e w i d t h Uowed v e r s u s temperature. _-+

Fig. 2 ( r i g h t ) - Spectrum o f biphenyl a t 25 K by phonons p r o p a g a t i n g a l o n g a . The a r -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816230

C6- 780 JOURNAL DE PHYSIQUE

rows a t t h e f o o t o f t h e R a y l e i g h i n d i c a t e t h e e x p e c t e d l o c a t i o n o f t h e phason l i n e , whereas "L", "T" a n d T d e s i g n a t e s t h e q u a s i - l o n g i t u d i n a l , q u a s i - t r a n s v e r s e and t r a n s -

v e r s e and modes.

A s a phason was p r e v i o u s l y o b s e r v e d by n e u t r o n s c a t t e r i n g ( 1 ) , we have t r i e d t o

s e e i t by l i g h t s c a t t e r i n g . Our e x p e r i m e n t f a i l e d t o f i n d this component and what we c a n s a y , i f t h e phason h a s r e a l l y a p r o p a g a t i o n v e l o c i t y o f 1000 m / s , is t h a t i t s c a t - t e r s l e s s t h a n 1/10 of t h e l i g h t s c a t t e r e d by t h e w e a k e s t t r a n s v e r s e mode.

IMPROPER FERROELASTIC TRANSITION OF p-TERPHENYL p - t e r p h e n y l u n d e r g o e s a t 193 K a n improper f e r r o e l a s t i c t r a n s i t i o n l a b e l l e d 2/m 1' F

i'

a c c o r d i n g to Aizu. T h i s t r a n s i - t i o n is c h a r a c t e r i z e d by a s u p e r s t r u c t u r e which a p p e a r s a t t h e C p i n t (1/2, 1/2, 0 ) o f t h e l r s t B r i l l o u i n zone l e a d i n g t o t h e e x i s t e n c e o f two c r i s t a l l o g r a p h i c a l l y d i f - f e r e n t k i n d s o f domains ( F i g . 3 ) depending o n t h e o r i e n t a t i o n o f t h e m o l e c u l e c e n t r a l r l n g ( 2 1.

Domain a Domain

F i g . 3 - P r o j e c t i o n on t h e (001) p l a - n e o f t h e c e n t r a l r i n g o r i e n t a t i o n s i n b o t h p h a s e s ; d a s h e d l i n e s f o r t h e h i g h t e m p e r a t u r e p h a s e a n d f u l l l i n e s f o r t h e low t e m p e r a t u r e p h a s e .

One o f t h e e l a s t i c a n o m a l i e s c r e a t e d by t h i s o r d e r - d i s o r d e r t r a n s i t i o n i s r e l a - t e d t o t h e d i f f e r e n t i a t i o n o f t h e 11101 and

I

li01 d i r e c t i o n s a s it c a n b e s e e n on t h e f i g u r e above. F o r o n e o f t h e s e d i r e c t i o n s d e p e n a i n g on t h e domain, c e n t r a l r i n g s a r e p e r p e n d i c u l a r t o t h e i r n e i g h b o u r s whereas a l o n g t h e o t h e r d i r e c t i o n t h e a n g l e between c e n t r a l r i n g s is much l e s s . T h i s d i f f e r e n c e and t h e p r e s e n c e o f s e v e r a l domains i n t h e s c a t t e r i n q volume a r e a t t h e o r i g i n o f t h e p r e s e n c e o f two d i s t i n c t and s i m u l t a - neous l o n g i t u d i n a l B r i l l o u i n l i n e s i n t h e s p e c t r u m ( F i g . 4 ) .

FFg.4

-

R l g n t a n g l e B r i l l o u l n s c a t t e r i n g s p e c t r u m i n t h e a ( c , c ) b geometry a t 3 d i f f e - r e n t t e m p e r a t u r e s showing t h e " l o n g i t u d i n a l ^" B r i l l o u i n l i n e s (from t h e n e x t o r d e r l a n d t h e " t r a n s v e r s e " B r i l l o u i n l i n e s n e a r t h e R a y l e i g h l i n e ; n o t i c e a l s o a q u a s i e l a s t i c component which g o e s through a maximum a t T, = 193 K. The f r e q u e n c y i n t e r v a l shown is 35.5 GHz.

A f i t performed on t h e s e s p e c t r a between 90 K and 193 K shows a ( T ~ - T ) ' - ~ ' va- r i a t i o n of t h i s s p l i t t i n g ; it corresponds t o t h e numerical v a l u e o f 0.30 o b t a i n e d f o r t h e s u p e r s t r u c t u r e i n t e n s i t y recorded by neutron s c a t t e r i n g ('1

.

T h i s v a l u e r e - f l e c t s a s l i g h t d i s c o n t i n u i t y o f t h e t r a n s i t i o n .

THE

p-QUATERPHENYL TRANSITION p-quaterphenyl uhdergoes a t r a n s i t i o n s i m i l a r t o t h e p-terphenyl i n t h e sense i t i s a l s o an o r d e r - d i s o r d e r t r a n s i t i o n between t h e P 2 / a

1 high temperature phase and probably a P

-

low temperature phase w i t h a l s o c o e x i s t e n -

3 1

c e o f domains ( ) . But h e r e two c e n t r a l r i n g s a r e moving i n o p p o s i t e d i r e c t i o n s i n s i - de each molecule and t h e d i f f e r e n c e between t h e 11101 and t h e 11i01 i s n o t a s obvious a s f o r p-terphenyl and two d i f f e r e n t l o n g i t u d i n a l B r i l l o u i n l i n e s a r e n o t observed.

Generally t h e e l a s t i c anomalies a r e a l s o weaker. The f i g u r e 5 shows an anomaly of t h e

" l o n g i t u d i n a l " sound v e l o c i t y recorded a l o n g a which shows a small change of s l o p e a t +

Tc = 240 K ; t h e r e i s no complete e x p l a n a t i o n f o r t h i s kind o f behaviour, b u t it i s could b e due to some r e l a x a t i o n mechanism.

References 30..

2.7,.

1. H. CAILLEAU, T h e s i s Rennes 1981.

2. J.L. BAUDOUR, Y. DELUGEARD, H . CAIUEAU, Acta C r y s t . (1976),

e,

150.

3 . J.L. BAUDOUR, Y . DELUGEARD, P. RIVET, ACta C r y s t . (19781,

G,

^625.

. ^VLa

\ \

0 100

F i g . 5 - Sound v e l o c i t y o f t h e q u a s i l o n g i t u d i n a l mode propaga- t i n g a l o n g

d.