RAMAN SCATTERING FROM PHONONS IN DISORDERED CsMg1-xCoxCl3

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RAMAN SCATTERING FROM PHONONS IN DISORDERED CsMg1-xCoxCl3

I. Johnstone, G. Jones, D. Lockwood

To cite this version:

I. Johnstone, G. Jones, D. Lockwood. RAMAN SCATTERING FROM PHONONS IN DIS- ORDERED CsMg1-xCoxCl3. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-90-C6-92.

�10.1051/jphyscol:1981628�. �jpa-00221564�

JOURNAL DE PHYSIQUE

Colloque C6, suppZ6ment

au nO1 2,

Tome

42,

d6cembre

1981

page

C6-90

RAMAN

SCATTERING FROM PHONONS IN DISORDERED CsMgl-,CoXCl3

I . W . Johnstone, G.D.

ones"

and D . J . Lockwood

Physics Division, National Research Council, Ottawa, Canada

KIA

OR6

"physics Department, Canterbury University, Christchurch, New ZeaZand

Abstract.- F i v e zone-centre phonons have been measured a t l o w temperatures and f o r a range of c o n c e n t r a t i o n s i n CsMgl,CoXCl3. Three E modes and t h e A

l g mode a l l e x h i b i t t h e expected one-mode behaviour. A newqheory i s r e q u i r e d t o e x p l a i n t h e unusual c o n c e n t r a t i o n dependence of t h e Elg phonon i n t e n s i t y , l i n e s h a p e and p o l a r i s a t i o n .

1. Introduction.- The l a t t i c e dynamics of AMX3 compounds have been widely s t u d i e d and many possess t h e CsMgC13 s t r u c t u r e ( s e e f i g u r e 1 ) of space group

~2~

^{with two}

f o r m l a u n i t s i n t h e p r i m i t i v e c e l l . A f a c t o r group a n a l y s i s p r e d i c t s A +El +3E l g g 2g Raman a c t i v e modes. The A and E l g modes comprise c h l o r i n e i o n motions,

l g

p e r p e n d i c u l a r and p a r a l l e l t o t h e c (2) a x i s r e s p e c t i v e l y , w h i l e t h e E modes 2g

involve halogen and cesium i o n displacements i n t h e hexagonal (X,Y) plane. We have s t u d i e d t h e mixed c r y s t a l C S M ~ ~ - ~ C O ~ C ~ ~ t o c l a r i f y assignments of phonons i n C S M ~ C ~ ~ ~ ' and CsCoC1 31y and thereby a s s i s t t h e i n t e r p r e t a t i o n of t h e h i g h l y s t r u c t u r e d magnon spectrum of CsCoC13.

2. Experiment and Results.- Large s i n g l e c r y s t a l s of were grown w i t h c o n c e n t r a t i o n s i n t h e ranges 0

<

x < 0.2 and 0.6 < x

<

1. Raman s p e c t r a were e x c i t e d with 100 mW of 476.5 nm argon l a s e r l i g h t i n t h e 90° s c a t t e r i n g geometry X( -*)Y, analysed w i t h a double monochromator (2 cmdl r e s o l u t i o n ) , and recorded under computer c o n t r o l . P o l a r i s e d Raman s p e c t r a f o r pure CsMgC13 recorded a t 5 K allowed unambiguous assignments of phonon l i n e s a t 55.0, 127.5, 132.0, 189.0 and 255.0 cm-l t o symmetry s p e c i e s EZg, Elg, EZg, Epg and A r e s p e c t i v e l y . The A mode showed

lg

' '

^g

no anomalous (YX) i n t e n s i t y i n c o n t r a s t t o t h a t found e a r l i e r . F i g u r e 2 p r e s e n t s p a r t of t h e r e s u l t s obtained f o r t h e mixed c r y s t a l s . The s c a t t e r i n g from E and

2g A phonons f o l l o w s DGh s e l e c t i o n r u l e s f o r a l l concentrations and only the A and

l g l g

E (132 cm-l) modes show a p p r e c i a b l e frequency s h i f t s with i n c r e a s i n g x. A l l of 2g

t h e s e phonons e x h i b i t one-mode behaviour a s p r e d i c t e d by t h e ~ r y . ~

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981628

Fig. 1: Details of the CsMgC13 crystal structure.

FREQUENCY (crn-1)

Fig. 2: Concentration dependence of the X(ZX)Y

(-1

and X(YX)Y

(--I

Raman spectra for phonons i n CsMgl-,Co,Cl3 at 5 K .

JOURNAL DE PHYSIQUE

F i g u r e 2 shows t h e d r a m a t i c changes i n band shape and i n t e n s i t y f o r t h e E g phonon a s a f u n c t i o n of x. The s i n g l e l i n e observed a t 127.5 cm-l i n (ZX)

p o l a r i s a t i o n f o r x = 0 becomes a p a i r of l i n e s by x = 0.19. By x = 0.63 t h i s f i n e s t r u c t u r e has smeared o u t and t h e continuum of s c a t t e r i n g t o h i g h e r frequency is now more prominent and has e q u a l i n t e n s i t y i n (ZX) and (YX) p o l a r i s a t i o n s . As x i s f u r t h e r i n c r e a s e d beyond x = 0.8 t h e E band d i s a p p e a r s and t h e continuum becomes

g

weaker, being b a r e l y v i s i b l e f o r x = 0.88. The i n t e n s i t y r a t i o IZX/IyX f o r t h e E g phonon a l s o changes w i t h c o n c e n t r a t i o n and IZX IyX f o r x > 0.63, c o n t r a r y t o D

6h s e l e c t i o n r u l e s .

3. Discussion.- The d e g e n e r a t e E mode has normal mode d i s p l a c e m e n t s of t h e form4 lg

Z l+Z 2-Z 4-Z c 2 Z 3+2Z and J3 (Z l-Z 2+Z ,+-Z 6) u s i n g t h e atom l a b e l s of f i g u r e 1

.

T h e r e f o r e t h e Raman c r o s s - s e c t i o n f o r t h i s mode should be r e l a t i v e l y i n s e n s i t i v e t o s u b s t i t u t i o n of t h e d i v a l e n t m e t a l ion, and t h e o t h e r Raman a c t i v e modes behave s e n s i b l y i n t h i s r e s p e c t . Thus a l t h o u g h t h e E mode frequency is c o n c e n t r a t i o n

l g

independent t h e l i n e s h a p e , p o l a r i s t i o n and i n t e n s i t y behave anomalously.

E x t r a p o l a t i o n of o u r d a t a i n d i c a t e s a frequency of

-

127 cm-' f o r t h e E mode i n l g

CsCoC13, as opposed t o an e a r l i e r assignment of 118 cm-l. l' 3 ' 4

R e f e r e n c e s

1. W. B r e i t l i n g , W. Lehmann, T.P. S r i n i v a s a n and R. Weber, S o l i d S t . C o m n . 20, 525 (19761. _{, -}

2. M.H. Brooker and C.-H. Huang, Mat. Res. B u l l .

15,

9 (1980).

3. W. B r e i t l i n g , W. Lehmann, T.P. S r i n i v a s a n , R. Weber and U. Durr, S o l i d S t . Comnun.

24,

267 (1977).

4. F u r t h e r d e t a i l s a r e given i n I . W . Johnstone, G.D. J o n e s and D.J. Lockwood, S o l i d S t . Commun.

39,

395 (1981).