OPTICAL ABSORPTION SPECTRA OF CAESIUM IODIDE (CsI ) AT PRESSURES UP TO 60 GPa

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OPTICAL ABSORPTION SPECTRA OF CAESIUM IODIDE (CsI ) AT PRESSURES UP TO 60 GPa

I. Makarenko, A. Goncharov, S. Stischov

To cite this version:

I. Makarenko, A. Goncharov, S. Stischov. OPTICAL ABSORPTION SPECTRA OF CAESIUM IODIDE (CsI ) AT PRESSURES UP TO 60 GPa. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-43-C8-45. �10.1051/jphyscol:1984808�. �jpa-00224306�

JOURNAL DE PHYSIQUE

Colloque C8, suppl6ment au n O 1 1, T o m e 45, novembre 1984 _{page C8-43}

O P T I C A L ABSORPTION SPECTRA OF C A E S I UM I O D I D E (CSI ) A T PRESSURES UP TO 6 0 GPa

I.N. Makarenko, A.P. Goncharov and S.M. Stischov

I n s t i t u t e o f L " r y s t a l Z o g m ~ h y , Academy o f S c i e n c e s o f t h e U.S.S.I?., Leninsky prospect 5 9 , 117333 E.loscow, U.S.S.R.

R6sum6 : Le spectre d'absorption o p t i q u e de monocristaux de C s I a 6 t 6 mesur6 dans une cellule diamant jusqu'8 des pressions de 60 G P a . P o u r la premisre f o i s , une structure

fine a 6 t 6 observ6e dans le flanc d'absorption de CsI 2 haute pression. On montre que cette structure est due 2 un effet excitonique. L a pression de mgtallisation de CsI est estimEe @tre e n v i r o n 110 GPa.

Abstract : T h e optical absorption spectra of C s I single crystals have been measured in a diamond anvil c e l l at pressures up to 6 0 GPa. F o r the first ti'me, the f i n e struc- ture of the a b s o r p t i o n e d g e of C s I h a s b e e n observed a t h i g h pressures. T h e exciton e f f e c t s are shown to be responsible for the f i n e s t r u c t u r e of the a b s o r p t i o n edge. T h e pressure of metallization of C s I estimated to be approximately 110 GPa.

T h e recent progress in the diamond anvil technique has made possible the experimental study of the insulator-to-metal transitions in rela- tively s i m p l e systems, s u c h a s rare-gas solids and a l k a l i h a l i d e c r y s - tals. X e and its isoelectronic a n a l o g C s I a r e o f particular interest f o r their metallization pressures a r e believed to be in the experimen- tally accessible region (1). T h e recent optical absorption s t u d i e s at ultrahigh pressures s h o w that the metallization pressure of X e is probably much h i g h e r than 100 GPa (2-4). The corresponding estimates f o r C s I look more encouraging and fall in the range 70-80 GPa (5).

H o w e v e r , there is a reason to b e l i e v e that the interpretation of the.

experimental data given in ref. 5 (2-5) is o v e r simplified and exciton effects, w h i c h are typical of those substances at normal pressure (6-8), w e r e n o t properly taken into account.

W e report h e r e the optical absorption s p e c t r a study of C s I a t pressures up to 6 0 G P a at room temperature. O n the basis of o u r experimental d a t a the metallic transition in C s I is expected to o c c u r at about

110 GPa.

T h e experiments were carried out by means of the gasketed anvil cell.

Solid X e w a s used a s a pressure transmitting medium (3). T h e pressure gradient in the cell did not exceed 0.5 GPa u p to the m a x i m u m pressure of 6 0 GPa. The optical density of C s I w a s obtained on the basis of the transmission spectra measurements of Xe and two C s I samples of diffe- rent thickness. The C s I samples w e r e f l a t monocrystalline chips with in-plane dimensions 30 x 2 0 U m 2 and 5-15 pm thick. T h e absor tion coefficient w h i c h could be measured w a s confined t o lo3 - ^{10' cm-'}

range. T h e transmission spectra w e r e recorded with the aid of d o u b l e grating monochromators (9) and a specially designed optical microsam- ple system. T h e pressure was measured by the well-known ruby fluores- c e n c e method (10) w i t h a precision of f 0.05 GPa.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984808

C8-44 JOURNAL DE PHYSIQUE

F i g . 1 - O p t i c a l d e n s i t y

d d c u r v e s o f C s I a t v a r i o u s

p r e s s u r e s ( G P a ) . T h e d o t t e d l i n e s r e p r e s e n t t h e r e s u l t s o f t h e c a l c u l a t i o n s c a r r i e d o u t u s i n g t h e t h e o r y ( R e f . 1 5 ) . N o t e t h a t t h e t h e o r e t i c a l d e s - c r i p t i o n d o e s n o t i n c l u d e t h e s e c o n d a b s o r p t i o n m a x i m u m .

O ' ' . . . . ' . ~ ~ ~ ' s

1.5 2.0 2.5

T h e t y p i c a l a b s o r p t i o n s p e c t r a a t v a r i o u s p r e s s u r e s f r o m 4 0 . 4 - 6 0 . 3 GPa a r c s h o w n i n F i g . 1 . One c a n s e e t h a t t h e o p t i c a l d e n s i t y c u r v e s s h i f t m o n o t o n i c a l l y t o t h e l o w - e n e r g y s i d e a s p r e s s u r e i n c r e a s e s . A l l t h e s p e c t r a r e v e a l t h e e x i s t e n c e o f t h e w e l l - f o r m e d maximum ( 1 ) n e a r t h e a b s o r p t i o n e d g e . A p a r t f r o m t h a t t h e r e i s a s e c o n d b r o a d a b s o r p t i o n maximum ( 2 ) o n t h e h i g h - e n e r g y s i d e o f t h e s p e c t r a .

I n F i g . 2 t h e p r e s s u r e d e p e n d e n c e s o f t h e s p e c t r a l p o s i t i o n s o f t h e 1 a n d 2 m a x i m a a r e s c h o w n . I n t h e s a m e d i a g r a m we a l s o p l o t t h e c o o r - d i n a t e s o f t h e f i r s t t w o e x c i t o n a b s o r p t i o n p e a k s a t 0 GPa i 5 . 6 a n d 6 eV ( R e f s . 7 a n d 8 ) ) , c o r r e s p o n d i n g t o t h e Tg ^-+

Tt

we a s s i g n t h e 1 a n d 2 m a x i m a t o t h e (1'8 , l':) a n d ( T i , r:) e x c i t o n s ( F i g . 2 ) , r e s p e c t i v e l y ( 1 3 , 1 4 ) .

F i g . 2 - E n e r g y c h a r a c t e r i s t i c s o f t h e C s I a b s o r p t i o n s p e c t r a a t h i g h p r e s s u r e s . 0 , A : s p e c t r a l

6 p o s i t i o n s o f t h e f i r s t a n d t h e

s e c o n d a b s o r p t i o n m a x i m a ( F i g . 1 )

5 a t v a r i o u s p r e s s u r e s : A , . : t h e

s a m e a t r o o m t e m p e r a t u r e a n d a t - m o s p h e r i c p r e s s u r e ( R e f s 7 a n d 8 ) .

4 T h c d a s h e d a n d d o t - d a s h e d l i n e s

r e p r e s e n t t h e p r e s s u r e d e p e n d e n c e o f t h e e x c i t o n m a x i m a a n d t h e

3 c o r r e s p o n d i n g b a n d - g a p e n e r g i e s .

T h e s h a d e d a r e a c o r r e s p o n d s t o

2 t h e b a n d - g a p e n e r g y Eg e s t i m a t i o n s

o b t a i n e d i n R e f . 5 . T h e i n s e t s h o w s t h e e x c i t o n b i n d i n g e n e r g y

1 ^R c o r r e s p o n d i n g t o t h e l o w e s t

i n t e r b a n d t r a n s i t i o n v s , t h e b a n - g a p e n e r g y E g ( + ) a c c o r d i n g t o t h e t h e o r e t i c a l c a l c u l a t i o n s

0 20 40 60 80 100 120 ( R e f . 1 5 ) .

P

In order to elucidate the behavior of the band-gap energy Eg and the exciton binding energy R a t h i g h pressures w e tentatively use the theory ( 1 5 ) , describing the optical absorption in the vicinity of the absorption edge.

The use of theory ( 1 5 ) in the case of complex exciton absorption spectrum of C s I may be justified on the basis of s i m p l e considera- tion (16), which shows that at high pressures the lowest band g a p , corresponding t o t h e r o + r z direct transition, i s very c l o s e to the first exciton m a x i m u m , o r , in other words, the exciton binding ener- gy R is small as compared with the energy difference b e t w e e n the two stated exciton maxima. T h e results of the corresponding calcu- lations are presented in Figs. 1 and 2 .

As seenfrom Fig. 2, the exciton binding energy R rapidly decreases with the pressure w h i c h results in the fact that the first exciton maximum energy and the band-gap energy become very c l o s e at high pressures. T h u s , the metallization pressure of C s I may be estimated by the extrapolation of the E (PI dependence which c a n be reliably obtained f r o m the e x ~ e r i m e n t . ~ T h i s extrapolation y i e l d s the pressure of the metallic transition in CsI of approximately 1 1 0 G P a , which is surprisingly close to the theoritical estimate ( 1 7 1 , but differs considerably from the results reported by Asaumi and K o n d o ( 5 ) .

REFERENCES -

1 - Ross and M. and Hc M a c h a n ; in "Physics of Solids under high Pressure" edited by J.S. Schilling and R.N. Shelton (North - H o l l a n d , Amsterdam, 1 9 8 1 1 , 1 6 1 .

2 - Sysassen K. Phys. Rev. E , ( 1 9 8 2 ) 6 5 4 8 .

3 - Makarenko I, Weill G. ItiC J.P., Besson J.M., Phys. Rev. B26

( 1 9 8 2 ) 7 1 1 3 .

4 - Asaumi K., Mori T., Kondo Y., Phys. Rev. Lett. K ( 1 9 8 2 ) 8 3 7 . 5 - ^{Asaumi K . ,} K o n d o Y., Solid. State C o m m u n ,

40,

128

7 - Eby E . , T e e g a r d e n K.J., D u t t o n D.B., Phys. Rev. 1 1 6 ( 1 9 5 9 ) 1 0 9 9 . 8 - F i s h e r F., Hilsh R., Nachr. Acad. Wiss.GoeLtingen,~ath. Phys.

K1, 2 ( 1 9 5 9 ) , 2 4 1 .

9 - The a u t h o r s a r e grateful to G.N. Z h i r z h i n and N.N. Melhik f o r giving the opportunity to use their infrared monochrosmator.

1 0 - Mao H.K., Bell P.M., Shaner J.W., S t e i n b e r g D.J., J. Appl. Phys.

4 9 , ( 1 9 7 8 ) 3 2 7 6 .

1 1 - K o d e r a Y . , J. Phys. Soc. J a p a n

5,

1 2 - Lync D.W. and Brothers A.D., Phys. Rev. Lett. 21, 1 0 , ( 1 9 6 8 ) 6 8 9 .

1 3 - It i s interesting to note that the transformation of the c o n d u c t i o n

band of CsI with the pressure is analogons to the behavior of the S - and d - electronic states in metallic Cs. Glatzel D. and Mc Mahan A.K, Phys. Rev. B20 ( 1 9 7 9 ) 3 2 1 0 .

1 4 - Recently two second - order phase transitions were reported in

C s I in the range 3 5 - 6 0 G P a (Asaumi K . , P h y s . Rev. B 2 9 ( 1 9 8 4 ) 1 1 1 8 ; Khittle E. and J e a n l o z R., Science

223

1 5 - Elliott R.J., Phys. Rev. 1 0 8 ( 1 3 5 7 ) 1 3 3 4 .

1 6 - Red shift the absorption edge of C s I with pressure certainly means

the decrease of Eg which in its turn should lead to the rapid de- crease of the e x c i t o n b i n d i n g e n e r g y ( ~ % E ~ h ~ % ~ - & C " ' ~ . E ~ - ~ , where i s the dielectric constant).

1 7 - Aidun J. and Bukowinski M.S.T., Solid. State Commun.

47