BRI LLOUIN SCATTERING STUDY OF THE INCOMMENSURATE (ANTIFERROELECTRIC) PHASE TRANSITION IN SODIUM NITRITE

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BRI LLOUIN SCATTERING STUDY OF THE

INCOMMENSURATE (ANTIFERROELECTRIC)

PHASE TRANSITION IN SODIUM NITRITE

T. Yagi, Y. Hidaka, K. Miura

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque C6, suppl&ment au n o 12, Tome 42, dbcembre 1981 page C6-731

B R I

L L O U I N

S C A T T E R I N G

STUDY O F THE I N C O M M E N S U R A T E (ANTI

FERROELECTRIC)

PHASE TRANSITION IN SODIUM NITRITE

T. Yagi, Y. ~idaka*and K. Miura**

Kyushu University, Hakozaki, E'ukuoka, Japan 812

Abstract.- The Brillouin scattering spectra of NaNO2 have been observed as a function of temperature near the two phase transition points; normal (para- electric)-incommensurate (antiferroelectric) and incommensurate-commensurate (ferroelectric) phase transition points. A simple temperature dependence of the Brillouin frequency shifts has been found in the incommensurate phase indicating existence of a dispersion of the sound velocity in hypersonic frequency.

1. I n t r o d u c t i o n . - The phase t r a n s i t i o n i n f e r r o e l e c t r i c sodium n i t r i t e (NaN02) c r y s t a l h a s been r e g a r d e d a s a t y p i c a l example o f t h e o r d e r = d i s o r d e r t y p e t r a n s i t i o n . The f l i p - f l o p motion o f e l e c t r i c d i p o l e s =

e a c h of which i s composed of a NO; i o n and t h e n e a r e s t n e i g h b o u r i n g N: i o n - i s c a p a b l e o f b e i n g t r e a t e d t h e o r e t i c a l l y a s a motion of t h e I s i n g s p i n s . On t h e o t h e r hand t h e p h a s e t r a n s i t i o n of NaN02 i s n o t s o s i m p l e a s e x p e c t e d from t h e s i m p l e mechanism mentioned above. With l o w e r i n g t e m p e r a t u r e , t h e c r y s t a l symmetry changes from t h e d i s o r d e r e d

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p a r a e l e c t r i c (normal) DZh-Immm t o t h e s i n u s o i d a l - a n t i f e r r o e l e c t r i c (incommensurate) phase a t TN (437.7 K) and t h e n undergoes a 1 s t o r d e r t r a n s i t i o n t o t h e o r d e r e d - f e r r o e l e c t r i c (commensurate) c ~ : - I ~ ~ ~ a t Tc (436.3 K ) . I n t h e incommensurate p h a s e , t h e e l e c t r i c p o l a r i z a t i o n h a s an incommensurate wavenumber k

-

a * / 8 , where a * i s t h e magnitude o f t h e r e c i p r o c a l l a t t i c e v e c t o r p a r a l l e l t o t h e orthorhombic a - a x i s .

The o b s e r v a t i o n of sound wave p r o p a g a t i o n i s a p o w e r f u l method f o r i n v e s t i g a t i n g t h e dynamic p r o p e r t y o f t h e incommensurate p h a s e , because e l a s t i c waves a r e always c o u p l e d w i t h t h e o r d e r p a r a m e t e r . S e v e r a l s t u d i e s on t h e sound wave p r o p a g a t i o n i n NaNOZ have been r e - p o r t e d . The p u r p o s e of t h e p r e s e n t p a p e r i s t o o b s e r v e t h e sound wave p r o p a g a t i o n i n a g i g a h e r z r e g i o n by t h e B r i l l o u i n s c a t t e r i n g i n

o r d e r t o e x t e n d t h e f r e q u e n c y r e g i o n covered by t h e r e c e n t u l t r a s o n i c s t u d i e s i n t h e incommensurate p h a s e . 3 ' 4 )

2. Experiment.- The sample c r y s t a l s of NaN02 were grown by t h e B r i d g e - man method. During t h e c o u r s e o f t h e c r y s t a l growth, t h e t e m p e r a t u r e o f t h e e l e c t r i c f u r n a c e was c o n t r o l l e d t o keep t h e c o o l i n g s p e e d slow.

*

_{P r e s e n t a d d r e s s} _:_{I b a r a k i E l e c t r i c a l Communication Lab.. NTT. J a ~ a n}

, A 319-11

**present a d d r e s s : A t s u g i P l a n t , Sony Corp., A t s u g i , Japan 243

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C6-732 JOURNAL DE PHYSIQUE

A f t e r b e i n g c u t t o a c u b i c s h a p e w i t h a w e t - t h r e a d saw, t h e s u r f a c e s of t h e sample c r y s t a l were f i n e l y p o l i s h e d i n o r d e r t o r e d u c e t h e l i g h t s c a t t e r e d t h e r e . The sample c r y s t a l mounted i n a l i g h t s c a t t e r - i n g c e l l was i l l u m i n a t e d by a beam o f t h e l o n g i t u d i n a l s i n g l e - m o d e

~ r + i o n l a s e r o p e r a t e d a t an a v e r a g e o u t p u t power of about 100 mW. A 5145 l i n e , p o l a r i z e d v e r t i c a l l y , was mainly u s e d a s t h e i n c i d e n t beam. The s c a t t e r e d l i g h t was a n a l y z e d an o r d i n a r y 90' B r i l l o u i n s p e c t r o s c o p i c s y s t e m which i s e s s e n t i a l l y t h e same a s t h e one d e - s c r i b e d F i g u r e 1 shows a t y p i c a l example o f t h e s p e c - trum o f NaN02 o b s e r v e d a t room t e m p e r a t u r e .

3. R e s u l t s and D i s c u s s i o n . - The B r i l l o u i n f r e q u e n c y s h i f t (Av) o f t h e l o n g i t u d i n a l modes was o b s e r v e d a s a f u n c t i o n of t e m p e r a t u r e f o r e a c h o f t h e t h r e e d i r e c t i o n s [ l o o ] , [OlO] and [OOl] i n a t e m p e r a t u r e range from 360 t o 480 K . The t e m p e r a t u r e dependence of Av i n t h e [OlO] d i - r e c t i o n shows a q u a l i t a t i v e agreement w i t h t h e p r e v i o u s r e s u l t e x c e p t f o r t h e r e s u l t s v e r y n e a r t h e t r a n s i t i o n p o i n t s TC and

the

r e - s u l t s n e a r t h e t r a n s i t i o n p o i n t s TC and TN a r e shown i n F i g . 2 . I n t h e incommensurate p h a s e , a g r a d u a l d e c r e a s e o f Av a l o n g 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 i s s e e n i n e a c h of t h e t h r e e d i r e c t i o n s . Taking a c c o u n t o f t h e t e m p e r a t u r e dependence o f t h e r e f r a c t i v e i n d i c e s , we can s e e a r a t h e r smooth change of t h e sound v e l o c i t y a l o n g w i t h t e m p e r a t u r e i n

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t h e incdmmensurate p h a s e , b e c a u s e Av i s p r o p o r t i o n a l t o t h e h y p e r s o n i c v e l o c i t y . On t h e o t h e r hand, t h e d i s t i n c t a n o m a l i e s n e a r TC and

TN

were r e p o r t e d i n t h e r e c e n t u l t r a s o n i c s t u d y of t h e l o n g i t u d i n a l wave p r o p a g a t i n g a l o n g t h e [010] d i r e c t i o n . 3 7 4 ) The r e l a t i v e change of t h e sound v e l o c i t y a t TC i n t h e u l t r a s o n i c d a t a i s e s t i m a t e d t o be 3

-

4

%.

On t h e o t h e r hand, t h e r e l a t i v e change i n t h e p r e s e n t h y p e r s o n i c d a t a i s about 0 . 5 % a t TC. A t t h e normal-incommensurate p h a s e t r a n s i t i o n p o i n t TN, a c o n t i n u o u s minimum i n t h e v e l o c i t y o f t h e l o n g i t u d i n a l 4, However no minimum sound wave i n t h e [OlO] d i r e c t i o n was r e p o r t e d .

was o b s e r v e d i n t h e p r e s e n t d a t a a s shown i n F i g . 2 .

The d i f f e r e n c e between t h e u l t r a s o n i c and t h e p r e s e n t h y p e r s o n i c r e s u l t i n d i c a t e s c l e a r l y t h e f r e q u e n c y dependence of t h e sound v e l o c i - t y i n t h e incommensurate p h a s e . Thus i t i s concluded t h a t t h e d i s p e r - s i o n f r e q u e n c y o f t h e sound wave e x i s t s below a h y p e r s o n i c r e g i o n . T h e r e f o r e , t h e a m p l i t u d e mode which i s e x p e c t e d t o c o u p l e s t o t h e

sound wave i n t h e incom-

NaN02 L

-

Mode

I

I I I 1 I I 1 I

J

434 436 438 440 L4 i

TEMPERATURE

( K ) mensurate p h a s e becomes s u f f i c i e n t l y o v e r damped i n t h e f r e q u e n c y r e g i o n c o n s i d e r e d h e r e .

,

References.

1. I.Hatta, T.Ishiguro and N.Mikoshiba, Phys.Lett., A29 421 (1969).

1

2. &a, Y.1shibashi and Y.Takagi, J.Phys.Soc.Jpn.,

29, 1545 (1970).

3. KHatta, Y .Shimizu and K.Hamano, J.Phys.Soc.Jpn.,

44, 1887 (1978). 4. YHatta, M.Hanami and

K.Hamano, J.Phys.Soc.Jpn., 48,160 (1980)

5. m h i m i z u , M. Tsukamoto, Y

.

Ishibashi and M.Umeno, J.Phys.Soc. Jpn., 36,498

(1974).

,

6. T.Yagi, Y.Ridaka and

1 K.Miura, J.Phys.Soc.Jpn., 48, 2165 (1980).

7. E c h o and T.Yagi, J.Phys. Soc. Jpn., 50,543 (1981).

Fig. 2 : Temperature dependence of the Brillouin frequency shift of the three longitudinal acoustic modes in the incommensurate (antiferroelectric) phase of NaN02. The phase lies in the temperature region between T and T