Optical properties of the Cd1−xPbxF2 alloys above the fundamental absorption edge

Optical properties of the Cd1−xPbxF2 alloys above the fundamental absorption edge

5011d 5tate C0mmun1cat10n5, V01.76, N0. 11, pp. 1317-1321, 1990. Pr1nted 1n 6reat 8r1ta1n. 0038-1098/90 $3.00 + .00 Per9am0n Pre55 p1c 0 P 7 1 C A L...

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5011d 5tate C0mmun1cat10n5, V01.76, N0. 11, pp. 1317-1321, 1990. Pr1nted 1n 6reat 8r1ta1n.

0038-1098/90 $3.00 + .00 Per9am0n Pre55 p1c

0 P 7 1 C A L P R 0 P E R 7 1 E 5 0 F 7 H E C d , ~ P 6 x F2 A L L 0 Y 5 A 8 0 V E 7 H E F U N D A M E N 7 A L A850RP710N ED6E 8.J. K0wa15k1 and 8.A. 0r10w5k1 1n5t1tute 0f Phy51c5, P0115h Academy 0f 5c1ence5, A1. L0tn1k6w 32/46, 02 668 War5aw, P01and

(Rece1ved 4 Ju1y 1990 6y M. 8a1kan5k1) 7he ref1ect1v1ty0f Cd,~xP6xF2 a110y5 (x = 0.0, 0.071, 0.25, 0.42, 0.72, 1.0) wa5 mea5ured 1n the ener9y ran9e 0 f 4 ~< hv <. 10 eV. 0 n the 6a515 0f the exper1menta1 5pectra, 0pt1ca1 (n, k) and d1e1ectr1c (e,, e2) parameter5 were ca1cu1ated 6y mean5 0f the Kramer5-Kr0n19 pr0cedure. "Per515tence••-type character 0f the 0pt1ca1 5pectra 0f the cry5ta15 1nve5t19ated wa5 revea1ed, and 1nterpreted a5 cau5ed 6y the pr0pert1e5 0f the wave funct10n5 0f the 5tate5 1nv01ved 1n the 0pt1ca1 tran51t10n5 065erved, rather than 6y the ener9y 5eparat10n 0f the 6and5 0f the tw0 61nary c0n5t1tuent5.

a funct10n 0f the c0mp051t10na1 parameter perf0rmed 6y K05ack1 and Lan9er [4], 9ave new exper1menta1 CADM1UM f1u0r1de, 1ead f1u0r1de, and the a110y5 ar9ument5 f0r th15 m0de1. 6a5ed 0n them 6e10n9 t0 the fam11y 0f 10n1c cry5ta15 1n th15 paper, an attempt wa5 undertaken t0 w1den w1th the f1u0r1te 5tructure. 7he e1ectr0n1c and 0pt1ca1 the 5et 0f exper1menta1 data c0ncern1n9 the 0pt1ca1 pr0pert1e5 0f the5e mater1a15 have a1ready 6een 1nve5t1pr0pert1e5 0f the a110y5 1nve5t19ated, and t0 1nterpret 9ated [1-5]. Neverthe1e55, 5evera11ntere5t1n9 pr061em5 them 0n the 6a515 0f the re5u1t5 0f the m0de1 ca1curema1n 0pen. 0ne 0f them 15 the 4ue5t10n 0f the charac1at10n5 0f m1xed cry5ta1 0pt1ca1 pr0pert1e5 [7, 8], a5 ter 0f the 0pt1ca1 tran51t10n5 065erved 1n the funwe11 a5 0f the0ret1ca11nve5t19at10n5 0f the Cd, • ~P6~ F 2 damenta11nter6and a650rpt10n re910n 1n Cd~ • xP6~ F2. 6and 5tructure [6, 9]. 7hu5, the ref1ect1v1ty 5pectra 0f 7he re5u1t5 0f the ref1ect1v1ty mea5urement5 carr1ed the Cd,~ xP6~ F2 cry5ta1 1n the wh01e c0mp051t10n par0ut f0r the 61nary cry5ta15 (CdF 2 [1] and P6F 2 [2]) ameter ran9e (x = 0.0, 0.071, 0.25, 0.42, 0.72, 1.0) 1nd1cate a marked d1fference 1n the nature 0f the have 6een mea5ured at r00m temperature, f0r the 0pt1ca1 tran51t10n5 065erved 1n the 1nter6and a650rpener9y ran9e 0f 4 ~< hv <~ 10eV. 7h15 ener9y ran9e t10n re910n (1n part1cu1ar, 1n the 10wer ener9y part 0f c0ver5 the re910n 0f the exc1t0n1c tran51t10n5 and the the 5pectra). 7h15 c0rre5p0nd5 t0 the d1fferent 6and 0n5et 0f the 1nter6and exc1tat10n5 ran9e 1n CdF2, and 5tructure5 0f the 61nary mater1a15. 7he re5u1t5 0f the the wh01e re910n 0f the three 10w-ener9y tran51t10n5 ••t19ht 61nd1n9•• ca1cu1at10n5 0f Ve11cky and Ma9ek [6] a5cr16ed t0 at0m1c-11ke P62• 652 =~ 656p pr0ce55 1n 5h0w that 1n P6F2, the t0p 0f the va1ence 6and P6F2. A Kramer5-Kr0n19 ana1y515 ena61ed u5 t0 (der1ved fr0m the F-2/, 6 5tate5 f0r CdF2) 15 5tr0n91y 06ta1n the 5pectra1 dependence 0f the ext1nct10n c0efm0d1f1ed 6y the c0ntr16ut10n 0f the p62+6.d 5tate5. f1c1ent, the refract1ve 1ndex and the c0mp1ex d1e1ectr1c 51mu1tane0u51y, the c0nduct10n 6and der1ved fr0m c0n5tant. A c0mpar150n 0f the pr0pert1e5 0f the ~;2 the empty cat10n1c 5-5tate5 (1n CdF 2) 15 rep1aced 6y the 5pectra 06ta1ned w1th the pred1ct10n5 0f the the0ry 6and f0rmed 0f the cat10n1c p-5tate5 (1n P6F 2). 7hu5, 1nd1cate5 the fact0r5 0n wh1ch the 5hape5 0f the a110y 1t wa5 expected that the 5hape 0f the 10w ener9y part 5pectra depend. 0f the 1nter6and a650rpt10n 5pectrum 0f P6F2 mu5t 6e 90verned 6y at0m1c-11ke P62• 652 ~ 656p exc1tat10n5, 2. EXPER1MEN7AL P R 0 C E D U R E , RE5UL75 1n c0ntra5t w1th the 51tuat10n f0r CdF2, 1n wh1ch the AND K R A M E R 5 - K R 0 N 1 6 ANALY515 ••char9e tran5fer•• exc1t0n (a h01e 10ca112ed 0n the an10n 5tate and an e1ectr0n tran5ferred 1nt0 the 5tate 7he exper1menta1 5et-up c0n515ted 0f a hydr09en 51tuated 0n the neare5t cat10n5), a5 we11 a5 tran51t10n5 1amp, a vacuum m0n0chr0mat0r, and a mea5ur1n9 fr0m the an10n-der1ved va1ence 6and 1nt0 the cat10n- 5y5tem wh1ch ena61ed u5 t0 determ1ne the 1nten51ty 0f der1ved c0nduct10n 6and, are expected. 7he th0r0u9h the 1nc0m1n9 and ref1ected 119ht 6eam5. A d15char9e 1n 1nve5t19at10n 0f the a650rpt10n ed9e 0f Cd 1-x P6x F 2 a5 f10w1n9 hydr09en 15a 50urce 0f u1trav101et rad1at10n 1n 1. 1 N 7 R 0 D U C 7 1 0 N

1317

1318

V01. 76, N0. 11

0P71CAL PR0PER71E5 0 F 7 H E Cd,~,P6, F2 A L L 0 Y 5

the 1amp c0nnected d1rect1y (w1th0ut a w1nd0w) t0 the nece55ary t0 revea1 the rea1 6ehav10ur 0fthe rema1nder m0n0chr0mat0r. 7he m0n0chr0mat0r (6a5ed 0n the 0f the 5pectra. 5eya-Nam10ka 5cheme) w1th a c0ncave d1ffract10n 7he Kramer5-K1"0nJ9 9•r0cedu•r•e app11ed wa5 9rat1n9 (1200 11ne5mm-~, the curvature rad1u5 e4ua15 6a5ed 0n the meth0d pr0p05ed 6y R0e551er [10]. 7he 0.5m) f0rm5 the m0n0chr0mat12ed 119ht 6eam. 7he appr0ach 15 part1cu1ar1y 5u1ta61e f0r the exper1menta1 1nten51ty 0f the 0ut901n9 6eam 15 m0du1ated and then 5pectra mea5ured 1n a re1at1ve1y narr0w (w1th re5pect mea5ured 6y mean5 0 f a 1um1n0ph0r (dep051ted 0n the t0 the wh01e 1nter6and a650rpt10n re910n) ener9y 0Pa4ue 5ect0r5 0f the r0tat1n9 m0du1at0r) and a ran9e. 7he d15per510n re1at10n wa5 expre55ed 1n the ph0t0mu1t1p11er w1th a 4uart2 w1nd0w. 7he 119ht f0rm: tran5m1tted thr0u9h the m0du1at0r 15 ref1ected 6y ~00)dw - -rt1 1h f ( R , w0)dw the 5amp1e t0ward5 a 5ec0nd 1um1n0ph0r - ph0t0- 0 ( 0 J 0 ) n-• mu1t1p11er 5et. M0du1ated 519na15 fr0m the tw0 ph0t0mu1t1p11er5 are mea5ured w1th 10ck• 1n - nan0v01tw0)dw meter5. 7he 5urface5 0f the 5amp1e5 were mechan1ca11y p0115hed and r1n5ed w1th a1c0h01 ju5t 6ef0re the where exper1ment t0 av01d water a650rpt10n 6y the hy9r05c0p1c mater1a1. 1w - 09011" 7he re5u1t5 0f the mea5urement5 are 5h0wn 1n F19. 1. 7he curve5 06ta1ned f0r the 61nary cry5ta15 1n the 1nterva15 6e10w (w < a) and a60ve (~0 > 6) c0rre5p0nd we11t0 the 5pectra a1ready pu6115hed [1,2]. the exper1menta1 ran9e the 1nte9ra15 were rep1aced 6y F0r CdF2, an exc1t0n1c 5tructure 15 065erved w1th the the f0rmu1ae ma1n feature at 7.6 eV. F0r P6F 2, three max1ma 0ccur, at 5.5, 6.3 and 8.2 eV. 7he weak dependence 0fthe f1r5t A1n w a ~+. ~ 0 tw0 peak5 (5.5, 6.3 eV) 0n the cry5ta1 c0mp051t10n wa5 1a -- w01 c1ear1y detected, 6ut a Kramer5-Kr0n19 ana1y515 wa5 a

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F19. 1.7he ref1ect1v1ty 5pectra f0r Cd,~,P6,F2 (x = 0.0, 0.071, 0.25, 0.42, 0.72, 1.0).

F19. 2. 7he e1(hv) (da5hed 11ne) and ~2(hv) (5011d 11ne) curve5 f0r C d t ~ P 6 , F 2 , 06ta1ned 6y mean5 0f a Kramer5-Kr0n19 ana1y515 0f the ref1ect1v1ty 5pectra 5h0wn 1n F19. 1.

V01. 76, N0. 11

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and 8 1n 6 + t00, 6 - t00 re5pect1ve1y (acc0rd1n9 t0 the 9enera112ed mean va1ue the0rem f0r 1nte9ra15). A and 8 were determ1ned 6y f1tt1n9 0 t0 the va1ue 06ta1ned fr0m a650rpt10n ed9e mea5urement5 [4]. 7he d1e1ectr1c parameter5 re5u1t1n9 fr0m th15 pr0cedure are 5h0wn 1n F19. 2.

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3. D15CU5510N 7he 5pectra1 dependence 0f the 1ma91nary part 0f the d1e1ectr1c c0n5tant e215 the m05t c0nven1ent 0pt1ca1 funct10n t0 ana1y5e, due t0 1t5 c105e c0nnect10n w1th the j01nt den51ty 0f 5tate5 and 0ther e1ectr0n1c 5tructure character15t1c5 that are ava11a61e a5 6and 5tructure ca1cu1at10n re5u1t5. 1n 0rder t0 065erve chan9e51n the 5tren9th5 and ener9y p051t10n5 0f the 5pectra1 feature5, the e2 curve5 were dec0mp05ed 1nt0 a 5et 0f 6au551an max1ma. 7h15 5hape wa5 a55umed t0 re5u1t 1n v1ew 0f the 6r0aden1n9 cau5ed 6y the 1ar9e 1att1ce re1axat10n and the a110y1n9 effect5 expected 1n the mater1a15 1nve5t19ated. 7he5e phen0mena 90vern a 5hape 0f the at0m1c-11ke 0r exc1t0n1c 11ne5. 50me Fan0-type 1nteract10n c0u1d 6e expected 6etween the tw0 h18h-cncr9y, at0m1c-11ke tran51t10n5 1n P6F~ (6.3, 8.2eV) and exc1tat10n5 t0 5tate5 0f the c0nduct10n 6and c0nt1nuum. H0wever, the 5pectrum d0 n0t rcvca1 any marked a5ymmetry 0r ••w1nd0w•• charactcr u5ua11y 065ervcd undcr 5uch c1rcum5tance5 [11]. 1n part1cu1ar,th1515c1earf0r thc we11 5eparated m a x 1 m u m at 8.2 cV. 1t wa5 5h0wn [12]that thc 1nterfcrcncc0fthe d15crete Eu 2+ 5tate5w1th thc c0nt1nuum 0fc0nduct10n 6and 5tate51n Cd~ ~xCa~ F21cad5 t0 the Fan0 6r0aden1n9 065erved f0r 5harp Eu 2+ 1ntra-10n a650rpt10n 5pectrum. H0wever, th15can 6e d155em61ed 6y a60vemcnt10ned phen0mena 1cad1n9 t0 the marked 6r0adcn1n9 0f the 0ther 11ne5 1n m1xed Cd1~xP6~ F2.7hu5, the 6au551an 5hape wa5 ch05en a5 the 6e5t appr0x1mat10n 0f the rea1 5hape 0f the tw0 h19h-ener9y max1ma 0f P6F2, t 0 0 . 7 h e extrap01ated 0n5et 0f the h19h ener9y a650rpt10n (c0rre5p0nd1n9 t0 1nter6and tran51t10n5) wa5 5u6tracted fr0m the a11 5pectra 6ef0re the dec0nv01ut10n pr0cedure. 7he curve5 06ta1ned a5 a 5um 0f the 6au551an max1ma are drawn, t09ether w1th the exper1menta1 5pectra, 1n F19. 3a. 7he arr0w5 1nd1cate the ener9y p051t10n5 0f the max1ma re501ved. 1n F19. 36, the ener9y p051t10n5 0f the A, 8, c, D, e, and f p e a k 5 are 5h0wn a5 a funct10n 0f the cry5ta1 c0mp051t10n. A, 8, andfc0rre5p0nd t0 the 1ntra-10n exc1tat10n 065erved 1n P6F2, wherea5 D and e are feature5 a5cr16ed t0 the exc1t0n1c tran51t10n5 1n CdF2. 7he c c0ntr16ut10n (w1th 5tren9th and ener9y p051t10n a1m05t 1ndependent 0f the cry5ta1 c0mp051t10n) can

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F19.3. (a): 7he curve5 (d0tted f1ne), 06ta1ned a5 a 5um 0f 6au551an max1ma, drawn t09ether w1th the exper1menta1 5pectra (5011d 11ne) (after h19her ener9y 6ack9r0und 5u6tract10n). 7he arr0w5 1nd1cate the p051t10n5 0f the 6au551an max1ma. 7he1r ener91e5 arc 5h0wn 1n F19. 36 a5 a funct10n 0f the cry5ta1 c0mp051t10n.

pr06a61y 6e c0nnected w1th the pr0pert1e5 0f the an10n1c 5u61att1ce. A11 the ma1n feature5 065erved f0r the 61nary cry5ta15 can 6e d15cerned 1n the a110y 5pectra, the1r 5tren9th decrea5e5 and 1ncrea5e51n acc0rdance

1320

0 P 7 1 C A L P R 0 P E R 7 1 E 5 0 F 7 H E Cd1~P6~F2 A L L 0 Y 5

V01. 76, N0. 11

w1th the cry5ta1 c0mp051t10n, and chan9e5 1n the1r ener9y p051t10n5 are 5ma11 1n c0mpar150n w1th the ener9y 5eparat10n 6etween them. 7he chan9e5 1n peak p051t10n5 are m05t c1ear f0r the e and f peak5.7he5e feature5 are re1at1ve1y weak and c105e t09ether 1n ener9y. Due t0 the ana1y5150fhe19ht 0fthe max1ma we c0u1d f0110w them 1n the 5u65e4uent 5pectra. 7he e c0ntr16ut10n wa5 a5cr16ed t0 the h19h-ener9y exc1t0n1c tran51t10n5 f0r x = 0.0, 0.71, and 0.25. 7he f peak appear5 f0r Cd0.929P60.07 • F2 at 8.7 eV and 5urv1ve5 f0r the 5amp1e5 0f h19her P6 c0ntent (at decrea51n9 ener9y). 7hu5, 1t wa51nterpreted a5 the th1rd max1mum fr0m the three peak5 character15t1c 0f P6F2. Acc0rd1n9 t0 th15, the 0pt1ca1 5pectra 0f the C d ~ P 6 ~ F 2 cry5ta15 turn 0ut t0 have ••per515tence••type character (acc0rd1n9 t0 the cate90r12at10n pr0p05ed 6y 0n0dera and 70y02awa [7]). 1n the1r appr0ach, the m05t 1mp0rtant parameter determ1n1n9 t0 wh1ch cate90ry a cry5ta1 6e10n95 15 the rat10 0f the d1fference (A) 0f the 6and ener91e5 0f the tw0 c0mp0nent5 t0 the w1dth (7) 0f the ener9y 6and. When A / 7 15 1ar9e, the

tAh, t8a, and tAa (1n the neare5t ne19h60ur appr0x1mat10n), 1n acc0rdance w1th the type 0f at0m5 0ccupy1n9 51te5 1 andj. /AAand t88 determ1ne the w1dth 0f the 6and5 1n the pure c0n5t1tuent5. 1f they have d1fferent 519n5, the k = 0 c0mp0nent 0f the exc1t0n 6and 15 p051t10ned 0n the 60tt0m 0f the 6and 1n 0ne c0n5t1tuent, and 0n the t0p 1n the 0ther. 1n th15 ca5e, the a650rpt10n 5pectrum 0f the a110y c0n515t5 0f tw0 d15t1nct peak5, even 1f A/715~a1~ and the 1~-1nte9rated 5tate d15tr16ut10n f0rm5 0ne 6and [8]. F0r exc1t0n5, the pr0pert1e5 0f tran5fer-matr1x e1cmcnt5 are 90verned-6y the wave funct10n5 0f the 9r0und and exc1ted 5tate5 0f the e1ectr0n5 [14]. 1n the 5y5tem 1nve5t19ated, the at0m1c-11ke P62~ 65 =~ 656p tran51t10n 1n P6F2 may 6e de5cr16ed a5 a Frenke1 exc1t0n and the exc1tat10n at 7.6 eV 1n CdF2, a5 a ••char9e tran5fer•• exc1t0n. 7hu5, the 5tate5 1nv01ved 1n the f1r5t pr0ce55 5h0u1d 6e de5cr16ed w1th at0m1c-11ke 5- and p-type funct10n5, wh11e f0r the 5ec0nd type exc1t0n, the h01e 5tate c0rre5p0nd5 t0 the an10n p-5tate5, and the funct10n 0f the e1ectr0n mu5t 6e a c0m61nat10n 0f the 5-5tate5 0f the ................................. ener9y6andca~5pf1t..~nt0tW06and51nthem~xedneare5tcat~0n50(0fth~0vera11 5ymmetry a110wed 6y cry5ta1, and th15 1ead5 t0 the 5p11t (••per515tence••-type) the 5e1ect10n ru1e5 f0r 0pt1ca1 tran51t10n5). 7h15 5eem5 a650rpt10n 5pectrum 0f the a110y. 1n the 0pp051te ca5e, t0 6e re5p0n5161e f0r the tran5fer-matr1x e1ement5 the 6and w1dth 15 re1at1ve1y 1ar9e, and the tw0 6and5 6e1n9 d1fferent en0u9h t0 prevent 61end1n9 0f the un1te t0 91ve a 51n91e feature 1n the 6and 5tructure, a5 0pt1ca1 5pectra. 51mu1tane0u51y, the re5u1t5 0f CPA ca1cu1at10n5 we11 a5 1n the 0pt1ca1 5pectra (••ama19amat10n••-type [6, 9] 5h0w that the 5pectra1 den51ty 1n the 6and5 0f the character, after [7]). Acc0rd1n9 t0 th15, the ••per515tence••Cd~ ~P6xF2 a110y d0e5 n0t 0n1y 5h1ft 1n ener9y, 6ut type character 065erved 1n C d ~ P 6 x F 2 15 an unu5ua1 re5u1t, 51nce the 0pt1ca1 pr0pert1e5 0f m05t cat10n- chan9e5 1t5 0r61ta1 character and d15tr16ut10n 1n the 8r1U0u1n 20ne, acc0rd1n9 t0 the chan9e 0f the cry5ta1 5u65t1tuted 10n1c ternary cry5ta15 are 0f ••ama19amat10n•• character, due t0 c0n910merat10n 0f the c0mp051t10n, and the c0ntr16ut10n5 fr0m the tw0 c0ncat10n-der1ved c0nduct10n 6and5 5uperp05ed 1n 5t1tuent5 can 6e de5cerned even 1f they 5uperp05e 1n the a110y [7.]. A 51m11ar effect c0u1d 6e expected 1n ener9y~ 1n the va1ence 6and, the 5tr0n9 c0va1ent Cd~ ~ P6x F2, 1n wh1ch the c0rre5p0nd1n9 6and5 0f the c0up11n9 6etween- the P6 65 and-F 210 5tate5d0e5 n0t 61nary c0n5t1tuent5 are c105e 1n ener9~(tak1n9 1nt0 marked1y a1fectthe 1nte9ra1 5paf1a1 d15~661/t10n 0f-the acc0unt the1r w1dth5) [6]. A pre11m1nary 5u99e5t10n e1ectr0n1c wave funct10n (the char9e 5tate5 0f the 10n5 [13] wa5 that the 5pectra 0f Cd~.~P6xF2 5h0u1d 6e are pre5erved). 7hu5, the 1mp0rtant feature5 0 f the 1nterpreted 0n the 6a515 0f the the0ret1ca1 re5u1t5 0f e1ectr0n1c 5tructure, determ1n1n9 the 0pt1ca1 pr0per0n0dera and 70y02awa [7], a5 we11 a5 0f the t1e5 0f CdF2 and P6F2, are 5t111 pre5ent 1n the a110y. 9enera112ed treatment 0f Yu1 and 5ak0da [8]. Yet, 7h15 1ead5 t0 a 51tuat10n 1n wh1ch we may 065erve the 50me attr16ute5 0f••ama19amat10n••-type 6ehav10ur 0f 61nary c0n5t1tuent5• 0pt1ca1 tran51t10n5 1n the 5pectra the 5tructure 1n the ener9y re910n 0f 7.6-8.3 eV c0u1d 0f the m1xed cry5ta15 1nve5t19ated. 0 n the 0ther hand, the 51m11ar 6ehav10ur 0f the n0t 6e exc1uded [13]. 51nce the d0u6t5 are rem0ved 6y the re5u1t5 0f the further, m0re accurate dec0nv01ut10n three peak5 character15t1c 0f P6F2 (5.5, 6.3, 8.2eV) and ana1y515 pre5ented 1n th15 paper, the appr0ach 6y 5upp0rt5 the 1nterpretat10n acc0rd1n9 t0 wh1ch they are Yu1 and 5ak0da turn5 0ut t0 6e part1cu1ar1y 5u1ta61e 0f the c0mm0n 0r191n fr0m at0m1c-11ke P62+ 652 ~ 656p f0r Cd~ ~xP6x F 2. 7hey 5h0w that a 5ma11 va1ue 0f A/7 pr0ce55e5. d0e5 n0t 5uff1ce a5 a c0nd1t10n f0r the 0ccurrence 0f 4. C 0 N C L U 5 1 0 N 5 ••ama19amat10n••-type 5pectra. 7hey de5cr16e a 5y5tem 0 n the 6a515 0f the 5pectra1 dependence 0f the w1th the Ham11t0n1an 0f the f0rm: 0pt1ca1 and d1e1ectr1c parameter5, 06ta1ned 6y mean5 H = ~ E1a+ a, + ~ t,ja~-aj. 0f the Kramer5-Kr0n19 ana1y515 0f the ref1ect1v1ty 1 1,j character 7he h0pp1n9 (0r tran5fer) 1nte9ra15 t~jtake three va1ue5 5pectra 0f Cd,~xP6~F2, ••per515tence••-type

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V01. 76, N0. 11

0P71CAL PR0PER71E5 0 F 7 H E Cd1~xP6xF2 ALL0Y5

0f the a110y wa5 pr0ved 1n the ener9y ran9e 0f 4 ~< 3. hv <<. 10 eV. 7ak1n9 1nt0 acc0unt the ava11a61e re5u1t5 4. 0f the0ret1ca15tud1e5 [6-9] 0f m1xed cry5ta1 pr0pert1e5, a marked d1fference 1n the nature 0f the 0pt1ca1 5. exc1tat10n5 065erved 1n the 61nary c0n5t1tuent5 wa5 p01nted 0ut a5 the rea50n f0r the ••n0n-ama19amated•• 6. 0pt1ca1 5pectra 0f Cd~xP6xF2, 1n c0ntra5t w1th the 51tuat10n f0r many 0ther cat10n-5u65t1tuted 10n1c 7. cry5ta15, 1n wh1ch 5uperp051t10n 1n ener9y 0f the e1ec8. tr0n1c 6and5 1ead5 t0 ••ama19amat10n••-type 0pt1ca1 pr0pert1e5. 9. 7he c0mm0n 0r191n 0f the 0pt1ca1 tran51t10n5 at 5.5, 6.3, and 8.2 eV wa5 c0nf1rmed. 10.

Ackn0w1ed9ement5 - 7he auth0r5 apprec1ate 5t1mu1at- 11. 1n9 d15cu5510n5 w1th J.M. Lan9er and 8. Ve11cky. 7hey are a1501nde6ted t0 1. K05ack1 f0r cry5ta1 preparat10n. 12. 7h15 w0rk ha5 6een 5upp0rted 6y the pr09ram5 CP8P 01.04 and CP8P 01.08. 13. REFERENCE5 1. 2.

A.J. 80urd1110n • J.H. 8eaum0nt, J. Phy5. C 9, L973 (1976). J.H. 8eaum0nt, A.J. 80urd1110n • J. 80rda5, J. Phy5. C 10, 761 (1977).

14.

1321

P.R. F1nd1ey, 2. Wu • W.C. Wa1ker, Phy5. Rev. 828, 4761 (1983). 1. K05ack1 • J.M. Lan9er, Phy5. Rev. 835, 5972 (1986). V. Cha6, 8.J. K0wa15k1 • 8.A. 0d0w5k1, 5011d 5tate C0mmun. 58, 667 (1986). 8. Ve11cky• J. Ma~ek, 5011d 5tate C0mmun. 58, 663 (1986). Y. 0n0dera • Y. 70y02awa, J. Phy5. 50c. Jpn. 24, 341 (1968). K. Yu1 • 5.5ak0da, J. Phy5. 50c. Jpn. 37, 598 (1974). 8. Ve11cky • A. P1e~0nk0va, Phy51ca 5cr1pta 719, 558 (1987). D.M. R0e551er, 8r1t. J. AppL Phy5. 16, 1119 (1965). U. Fan0, Phy5. Rev. 124, 1866 (1961); U. Fan0 • J.W. C00per, Phy5. Rev. A137, 1364 (1965). K. 5w1~tek, M. 60d1ew5k1, 2. Ka11f15k1 • H. Pr2y6y11f15ka, Acta Phy5. P01. (1n pre55). 8.J. K0wa15k1, Acta Phy5. P0L A73, 381 (1988); 8.J. K0wa15k1 • 8.A. 0d0w5k1, 1n Phy51c5 0f 5011d 5urface5 1987, Pr0ceed1n95 0f the F0urth 5ymp051um 0n 5urface Phy51c5, 8echyn6, C2ech0510vak1a, 7-11 5eptem6er 1987, 5tud1e5 1n 5urface 5c1encc and Cata1y515 (Ed1ted 6y J. K0uka1), 36, 315, E15ev1er (1988). R.5. Kn0x, ••7he0ry 0f Exc1t0n5••, 5011d 5tate Phy51c5, 5upp1. 5, Academ1c Pre55 (1963).