OBSERVATION OF COUPLED AMPLITUDE MODES IN THE RAMAN SPECTRUM OF INCOMMENSURATE γ-Na2CO3

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OBSERVATION OF COUPLED AMPLITUDE MODES IN THE RAMAN SPECTRUM OF

INCOMMENSURATE γ-Na2CO3

A. Maciel, J. Ryan

To cite this version:

A. Maciel, J. Ryan. OBSERVATION OF COUPLED AMPLITUDE MODES IN THE RAMAN SPECTRUM OF INCOMMENSURATEγ-Na2CO3. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-725-C6-727. �10.1051/jphyscol:19816212�. �jpa-00221291�

JOURNAL DE PHYSIQUE

CoZZoque C6, suppZ6ment au n o 22, Tome 42, de'eembre 1982 page C6-725

OBSERVATION O F COUPLED AMPLITUDE MODES IN THE RAMAN SPECTRUM OF INCOMMENSURATE Y-Na2C03

A. Maciel and J . F . Ryan

Czarendon Laboratory, University of Oxford, Oxford, U. K.

A b s t r a c t . - The Raman spectrum of incommensurate y-Na2C03 r e v e a l s a n addi- t i o n a l number of modes which a r e a c t i v a t e d by t h e d i s t o r t i o n . A complex coupled mode spectrum observed a t low f r e q u e n c i e s i s i n t e r p r e t e d a s a r i s i n g from an i n t e r a c t i o n of a c o u s t i c and l i b r a t i o n a l d i s p e r s i o n branches a t t h e i n c o m e n s u r a t e wavevector. The high-frequency i n t e r n a l mode " 1 of t h e C O ~ ' i o n s is observed a s a d o u b l e t whose s p l i t t i n g v a r i e s w i t h temperature a s

( T ~ - T ) ~ @ w i t h B

-

The d i s o r d e r e d 6 s t r u c t u r e of sodium c a r b o n a t e , which forms t h e b a s i c s t r u c - t u r e f o r t h e incommensurate phase, i s derived from t h e hexagonal a form by a mono- c l i n i c d i s t o r t i o n c o n s i s t i n g mainly of r o t a t i o n s of t h e C03 2- groups about t h e

a x i s ' ' ) . The incommensurafe d i s t o r t i o n which s e t s i n a t TI = 633 K i s known t o c o n s i s t of r o c a t i o n s of t h e c a r b o n a t e i o n s about axes normal t o t h i s d i - r e c t i o n and displacements of both t h e sodium and carbonate i o n s along t h i s d i r e c - tion"). The c r i t i c a l wavevector v a r i e s continuously w i t h temperature: f o r example, a t r o o m t e m p e r a t u r e i t has t h e value q* = (0.182a * + 0.318:*), w h i l e below * 130 K

I *

i t a t t a i n s t h e approximately c o n s t a n t v a l u e (

&'

FREUUENCY ICN-'! TEMPERATURE ( K I

F i g . 1: Unpolarised Raman s p e c t r a of ~ i g . 2: Temperature dependence of t h e Na2C03 a t v a r i o u s temperatures (K) . low-energy modes i n Na2C03.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816212

C6-726 JOURNAL DE PHYSIQUE

on d i s p e r s i o n branches with a p p r o p r i a t e symmetry may become a c t i v e and c a n provide i n f o r m a t i o n about t h e d i s t o r t i o n . I n t h i s paper we r e p o r t Raman d a t a f o r y-Na2C03 which show evidence of t h e s e modes both i n t h e low frequency spectrum and i n t h e high-frequency i n t e r n a l mode spectrum of t h e C03 2- i o n s .

The spectrum of y-Na2C03 (Fig. 1) shows two q u i t e h e a v i l y damped modes below 50 cm-I which s o f t e n w i t h i n c r e a s i n g temperature. The p l o t of t h e i r e n e r g i e s v e r s u s temperature given i n F i g . 2 s u g g e s t s t h a t they a r e s o f t modes f o r t h e a-8 t r a n s i - t i o n a t T = 763 K. A t low temperatures e x t r a l i n e s appear i n t h e spectrum which a r e a s s o c i a t e d w i t h t h e incommensurate phase. The energy of l i n e D i n F i g . ! i s r e l a t i v e l y temperature independent (Fig. 2) up t o about 160 K a t which p o i n t it i s obscured by t h e wings of the much more i n t e n s e h i g h e r frequency modes. Line C i s a l s o very weak a t low temperature b u t i t s s t r e n g t h i n c r e a s e s with temperature a s t h a t of B d e c r e a s e s ; t h e l a t t e r i s n o t r e s o l v e d c l e a r l y above room temperature. The s p e c t r a p r e s e n t e d i n F i g . 1 a r e u n p o l a r i s e d and i t i s n o t p o s s i b l e t o determine t h e symmetry o f t h e l i n e s . However, an i n t e r p r e t a t i o n of t h e B-phase d a t a can be suggested s i n c e on synrmetry grounds two Raman-active s o f t modes (Ag+Bg) are pre- d i e t e d c 4 ) corresponding t o r o t a t i o n s of t h e C03 2- i o n s about ( and ( 120)hex

r e s p e c t i v e l y . Because of t h e complexity of t h e mode i n t e r a c t i o n s and t h e l a c k of p o l a r i z a t i o n d a t a t h e s p e c t r a of t h e incommensurate phase a r e more d i f f i c u l t t o i n t e r p r e t ; however, t h e temperature dependence of t h e e x t r a modes i s c o n s i s t e n t with t h e model i n which a coupling of a c o u s t i c and l i b r a t i o n a l d i s p e r s i o n branches a t 9* gives r i s e t o two coupled amplitude modes i n t h e i n c o m e n s u r a t e phase. This l i b r a t i o n a l branch i s expected t o be low l y i n g i n Na2C03 j u s t a s i n K 2 CO 3 where an a n t i f e r r o d i s t o r t i v e t r a n s i t i o n o c c u r s ( 5 ) due t o a s o f t e n i n g of t h i s branch a t t h e B r i l l o u i n zone boundary.

I I I I I I I ~ J

100 200 300 400 500 600 7W

TEMPERATURE L K

F i g . 3: Raman spectrum of t h e t o t a l l y symmetric Fig. 4: Temperature dependence of v1 mode of t h e ~ 0 3 - i o n i n Na2C03. The d a t a Av, t h e s p l i t t i n g of t h e v l mode i n a r e u n p o l a r i z e d . Spectrometer bandpass i s t h e incommensurate phase.

1 cm-1.

A d d i t i o n a l l i n e s a r e a l s o o b s e r v e d i n t h e s p e c t r u m o f t h e incommensurate p h a s e i n t h e n e i g h b o u r h o o d o f t h e C03 2- i n t e r n a l modes. The most obvious o f t h e s e i s t h e appa-

r e n t s p l i t t i n g o f t h e nondegenerate symmetric b r e a t h i n g mode v l ( F i g . 3 ) . The temp- e r a t u r e dependence of t h e s p l i t t i n g shown i n F i g . 4 shows t h a t i t t e n d s t o z e r o n e a r TI a l t h o u g h we a r e i n f a c t u n a b l e t o r e s o l v e t h e components above * 500 K. I n t h e

6 phase we o b s e r v e o n l y a s i n g l e l i n e , a l b e i t q u i t e broad. A l o g - l o g p l o t of Av

v e r s u s (T - TI) y i e l d s a s l o p e 0.70 - 0.76 depending on t h e c h o i c e o f TI a s 619 K ( ~ ) o r 633 K ( ~ ) r e s p e c t i v e l y . Assuming a r e l a t i o n s h i p Av ~ ( T ~ - T ) ~ ' y i e l d s a c r i t i c a l exponent B

-

R e f e r e n c e s

1. C . J . d e P a t e r , P h y s i c a

z,

2 . W. v a n A a l s t , J. den H o l l a n d e r , W.J.A.M. P e t e r s e and P.M. de Wolff Acta. C r y s t . 32B 47 (1976)

3 . C.J. P a t e r and R.B. Helmholdt Phys. Rev. B 19 5735 (1979) 4. Angela Maciel and J . F . Ryan. J. Phys. C . 2 L 509 (1981) 5. Angela M a c i e l and J.F. Ryan. J. Phys. C. _-. 14, 1611 (1981)