RAMAN SCATTERING SPECTRA OF Rb2ZnCl4 UNDER UNIAXIAL STRESS

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RAMAN SCATTERING SPECTRA OF Rb2ZnCl4 UNDER UNIAXIAL STRESS

N. Massa

To cite this version:

N. Massa. RAMAN SCATTERING SPECTRA OF Rb2ZnCl4 UNDER UNIAXIAL STRESS. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-593-C6-595. �10.1051/jphyscol:19816173�. �jpa-00221252�

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JOURNAL DE PHYSIQUE

CoZZoque C6, suppZ6ment au n012, Tome 42, de'cembre 1981 page C6-593

RAMAN SCATTERING SPECTRA OF RbZZnC14 UNDER UNIAXIAL STRESS

N . E . Massa

BehZen Laboratory o f Physics, University o f Nebraska, LincoZn, Nebraska 68588, Canada

Abstract. - Tne Rar~lan s c a t t e r i n g o f Rb2ZnC14 under u n i a x i a l s t r e s s up t o 300 bars on t i l e 5 and 5 a x i s (orthorhombic Dhb - Pna,) has been measured i n t h e incommensu- r a t e phase a t room temperature and a!. 82K. A downward s h i f t i n frequency has been observed i n those bands r e l a t e d w i t h bending and s t r e t c h i n g modes o f t h e ZnC14 f r e e molecule. U n l i k e K2Se04 i n the same s t r e s s range, t h e study o f t h e s o f t mode, assigned as amplitudon by Wada e t e l . does n o t show a s h i f t toward lower f r e - quencies.

Rb2ZnC14 i s among t h e f a m i l y o f ABX4 compounds t h a t a r e being i n t e n s i v e l y s t u d i e d and have a s t r u c t u r a l l y modulated incommensurate phase. It undergoes t h r e e phase t r a n s i t i o n s a t TI = 303 K, TII = 94 K, T I I l = 75 K ( ~ ) , from the p r o t o t y p i c o r - thorhombic p a r a e l e c t r i c ::(D - Pnam; Z = 4) i n t o t h e incommensurate phase a t room temperature t o t h e f e r r o e l e c t r i c ( f e r r i e l e ~ t r i c ? ( ~ ) ) (CZv 9 - Pna2,; Z = 12) t o a l i k e l y monoclinic phase, r e s p e c t i v e l y . - The axes are chosed such t h a t i n t h e o r t h o r - hombic phase b>a>c ( b -- 3c, 5: pseudo hexagonal a x i s ) .

C r y s t a l s were o b t a i n e d from slow evaporation o f an aqueous s o l u t i o n o f RbC1:

ZnC12 i n a 2 : l r a t i o . They r e s u l t e d i n o r i e n t e d s i n g l e c r y s t a l p l a t e s about 2 X 3 cm

(a

x 5 ) and 3 mm t h i c k from where samples o f about 3. x 1.2 x 3.5 ( 5 x b x c ) mm were c u t . These, a f t e r p o l i s h e d and under t h e microscope, showed good e x t i n c t i o n and no macroscopic defects.

l~leasurements were made w i t h t h e s t r e s s apparatus ( 4 ) i n s e r t e d i n a conventional dewar. Data were c o l l e c t e d on f l o p p y d i s k s and p l o t t e d on a d i g i t a l x - y recorder.

U n i a x i a l s t r e s s was a p p l i e d a t room temperature and 82 K i n steps o f about 40 bars along t h e p a r a e l e c t r i c

a

and

c

a x i s .

L a t t i c e and bending i n t e r n a l modes are found i n the 0-160 cm-' s p e c t r a span and those associated w i t h s t r e t c h i n g a r e i n t h e 260 t o 320 cm-' region. I n zero s t r e s s measurements a t 298 K and 82 K, t h e s p e c t r a l f e a t u r e s agree w i t h those r e - p o r t e d by Wada e t a l . ( ' ) and w i l l n o t be discussed here. F i g 1 (a'b) shows t h e room temperature r e s u l t s o f s p e c t r a under s t r e s s f o r t h e a(cc) b and c(aa) b o r i e n t a t i o n s . I t can be seen t h a t t h e most i n t e n s e bands, associated w i t h molecular bending, have a d e f i n i t i v e frequency downshift t h a t becomes more pronounced i n the s t r e t c h i n g region, shown i n f i g l b . A small b u t r e p r o d u c i b l e increase i n t h e s p l i t t i n g be- tween t h e l i n e a t 145 cm-' and i t s lower frequency shoulder a t about 124 cm-' seems

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816173

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C6-594 JOURNAL DE PHYSIQUE

t o occur a t about 30 - 50 bars as shown in f i g 2. This may be interpreted as i f Rb2ZnC14 goes from an e l a s t i c region a t lower s t r e s s t o a p l a s t i c l i k e behavior in which the downward frequency s h i f t occurs. Much weaker l a t t i c e modes show some increase in r e l a t i v e i n t e n s i t y but no change i n the apparent peak position.

1 !

ZERO 2 C 5 Cn-1 i

i

I

243 248 258 233 273 23: 2% 2?G , 325 3.5 373 31: 2"3 F R E O U E N C Y , cl.-i ;

Fig 1.

( a ) Effect of uniaxial s t r e s s on Ag phonons a t room temperature in the a(cc)b geometry.

s t r e s s along a_

(b) Same but c(aa)b geometry i n the stretching region.

s t r e s s along c_

To discuss t h e e f f e c t of s t r e s s on the internal modes one should f i r s t remem- ber t h a t , due t o the chlorine natural isotopic abundance, the spectra w i l l contain the normal modes of i n c l i 5 , Zn c1i5 ~ 1 Zn clZ5 ~ ~C I ; ~ , Zn , ~1 35 c l i 7 , Zn c l i 7 . A basic internal mode spectrum can be thought as a number of Raman components of an envelope, t h a t i s r e s u l t a n t from the Raman spectra of the most abundant v a r i e t i e s . (9

Thus, any interaction of the ZnC14 s u b l a t t i c e will be disrupted by the e f f e c t of s t r e s s on the s h o r t range interaciions. An increase i n uniaxial s t r e s s will tend t o lower the symmetry resulting in Raman frequencies closer t o those found f o r glassy

z ~ c $ .

⁽⁶⁾

This point i s emphasized by s i m i l a r band p r o f i l e of the stretching region in the glass (vv) and t h a t f o r the a ( c c ) b geometry i n the c r y s t a l . Three weak fea- tures observed with crossed polarizers in the g l a s s a l s o appear in Rb2ZnC14 as

*Mu and Sutherland (5) estimated the r e l a t i v e percent age of various isotopic com- positions f o r C C14 t o be 31.6, 42.2, 21.2, 4.7 and 0.4 respectively.

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weak overlapping bands r a i s i n g the l i k e l i h o o d t h a t t e t r a h e d r a l Zn C14 u n i t s form a polymer-chain-like s t r u c t u r e i n Rb2ZnC14.

Further, Rb2ZnC14 i s known t o c o n t a i n commensurate domains and incommensurate domain w a l l s ( 7 ) t h a t w i l l j u s t i f y p l a s t i c behavior and a lower y i e l d t o s t r e s s i n the incommensurate phase.

On t h e o t h e r hand, the Ag and 929 s p e c t r a have been s t u d i e d i n t h e f e r r o e l e c - t r i c phase a t 82 K. No change i n frequency has been found. P a r t i c u l a r l y , t h e broad f e a t u r e s assigned as amplitudon and a temperature dependent phonon (shown i n f i g . 3) do n o t go down i n frequency e i t h e r as s t r e s s i s a p p l i e d along

a

o r

c

i n con- t r a s t w i t h t h e K2Se04 amplitudon behavior (8), a t 82 K and i n t h e same s t r e s s range.

This r e s u l t i s c o n s i s t e n t w i t h t h e view t h a t i n Rb2ZnC14 long range i n t e r a c t i o n s a r e weak and t h e f o l d i n g o f t h e B r i l l o u i n zone, i . e . r e d u c t i o n o t the coherent l e n g t h by t h e t r i p l i n g o f t h e u n i t c e l l below 94 K, i s s t i 11 o p e r a t i v e ~t 82 K and precludes the observation o f any s i g n i f i c a n t s t r e s s induced

m o n i c a l l y coupled phonons.

F i g 2 U n i a x i a l s t r e s s induced 145 cm-I band.

s t r e s s a l o n g c_

changes on the shoulder o f

F i g 3 Ariiplitudon - phonon coupled band from zero t o 300 bars s t r e s s a l o n g 2

-4s -30 -:3 0 15 33 45

FSEC*CXCY I C"-i I

change on those anhar-

FIG. 2 ^I

I . I I I . ~ . ! I J .i I

1C3 :1? 123 13% :LC 1EZ :E3 r-.C^' 'YJ" _scr-1 ;

Acknowledgments: The author i s t h a n k f u l t o Professor R. K i r b y f o r f r u i t f u l discus- sions d u r i n g t h e development o f t h e present i n v e s t i g a t i o n and t o Professor S.S.

Jaswal f o r reading the manuscript.

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