PRESSURE-INDUCED 4f INSTABILITIES AND INSULATOR-METAL TRANSITIONS IN HEAVY RARE EARTH MATERIALS

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PRESSURE-INDUCED 4f INSTABILITIES AND INSULATOR-METAL TRANSITIONS IN HEAVY

RARE EARTH MATERIALS

K. Syassen

To cite this version:

K. Syassen. PRESSURE-INDUCED 4f INSTABILITIES AND INSULATOR-METAL TRANSI- TIONS IN HEAVY RARE EARTH MATERIALS. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-123-C8-139. �10.1051/jphyscol:1984824�. �jpa-00224324�

JOURNAL DE PHYSIQUE

Colloque C8, supplement au n°l 1, Tome *5, novembre 198* page C8-123

P R E S S U R E - I N D U C E D 4f I N S T A B I L I T I E S A N D I N S U L A T O R - M E T A L T R A N S I T I O N S IN HEAVY RARE EARTH M A T E R I A L S

K. Syassen

Physikalisehes Institut, Universitat Dusseldorf, D-4000 Dusseldorf 1, F.R.G

Abstract - The pressure-induced 4f instability in rare earth solids is dis- cussed with main emphasis on the pressure range above 100 kbar. The presenta- tion focuses on three materials: (i) Results related to the valence state of Yb metal are summarized, (ii) The optical response of intermediate-valent (IV) YbS is shown to be consistent with a localized mechanism of the valence instability, which is in sharp contrast to the behavior of IV Sm- and Tm- monochalcogenides. (iii) The effect of a structural B1-B2 transition on the electronic structure of IV SmTe is compared to the situation in SrTe.

I - INTRODUCTION

Most of the rare earth (RE) atoms are divalent in the gas phase, but change their valence state to trivalent in the solid state. The valence change results from a gain in total energy due to the transfer of an electron from the 4f shell into a bonding state. In the middle (Sm and Eu) and at the end (Tm and Yb) of the RE series Hund's rule correlation energy is in some cases sufficient to either stabilize the divalent state or cause the occurrence of intermediate (non-integral) ionic valence states. The pressure variable plays an important role in creating the intermediate valence (IV) state or/and in the investigation of its many unusual features, as is well documented in several review articles1"6 and conference proceedings.7"10

RE materials showing divalent or IV behavior are most easily recognized by their large molar volume (or the larger ionic size of the RE ion) as compared to similar compounds across the RE series. This is demonstrated in Fig. 1 for the RE metals and the RE monochalcogenides (including oxides). The anomalies near the middle and the end of the RE row, which correspond to divalent behavior, are immediately obvious.

In particular the Sm and Tm chalcogenides have attracted considerable interest, because their valence state can be changed significantly by the application of moderate pressures (7 kbar in SmS, 50 kbar in TmTe), by chemical alloying, or by temperature. Eu and Yb metal and their monochalcogenides (MC's) are more stable against a pressure-induced valence transition, and considerably higher pressures are required in order to possibly induce a significant valence change. Thus, in following the scope of this conference, which emphasizes the high pressure regime, we will discuss valence related properties of Yb and its monochalcogenides, but also

Résumé - On discute l'instabilité 4f induite par la pression en terres rares solides, en se concentrant sur le domaine de pression au dessus de 100 kbar.

Trois sujets sont abordés: (i) Les résultats concernant l'état de valence du Yb métallique sont sommairement exposés, (ii) On montre que la réponse op- tique du YbS des valences intermédiaires (IV) est en relation avec un méca- nisme localisé d'instabilité de valence, ce qui est en contradiction avec le comportement du IV-chalcogênure de Sm et de Tm. (iii) L'effet d'une transi- tion structurale B1-B2 sur la structure électronique du IV SmTe est comparé avec la situation en SrTe.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984824

JOURNAL DE PHYSIQUE

Ce Nd Sm Gd Dy Er Yb L a , P r I R n l E ~ I T b l H ~ I T m I L ~

10 1 1 ' 1 1 1 1 '

Fig. 1 - Molar volumes o f r a r e e a r t h metals and RE monochalcogenides a t normal pressure.

add some remarks r e g a r d i n g t h e corresponding Eu based m a t e r i a l s . A f u r t h e r aspect o f the h i g h pressure regime, brought about by the occurrence o f s t r u c t u r a l t r a n s i t i o n s i n I V MC's, i s t h e i n t e r p l a y between c r y s t a l s t r u c t u r e and e l e c t r o n i c s t r u c t u r e i n these m a t e r i a l s .

The o u t l i n e o f t h e paper i s as f o l l o w s : F i r s t , a b r i e f c h a r a c t e r i z a t i o n o f the I V s t a t e and o f t h e o v e r a l l f e a t u r e s o f t h e e l e c t r o n i c s t r u c t u r e o f t h e RE m a t e r i a l s o f i n t e r e s t here i s given. Second, the experimental and t h e o r e t i c a l evidence sup- p o r t i n g t h e ( s t i l l c o n t r o v e r s i a l ) occurrence o f a pressure-induced valence change i n Yb metal i s summarized. Third, the o p t i c a l response o f I V YbS i s discussed, which lends s t r o n g support t o a l o c a l i z e d mechanism o f the valence i n s t a b i l i t y i n t h i s m a t e r i a l . This unusual behavior i s i n sharp c o n t r a s t t o t h e common p i c t u r e o f the e l e c t r o n i c s t r u c t u r e o f o t h e r I V MC's. F i n a l l y , the e f f e c t o f a B1-B2 s t r u c t u r a l t r a n s i t i o n (NaC1-type t o CsC1-type) on the e l e c t r o n i c s t r u c t u r e o f I V SmTe i s compared t o t h e s i t u a t i o n i n the a l k a l i n e e a r t h compound SrTe.

I 1 - INTERMEDIATE VALENCE AND ELECTRONIC STRUCTURE

An exhaustive p r e s e n t a t i o n o f the many f a c e t s o f t h e valence f l u c t u a t i o n phenomenon i s given i n Refs. 1-10. Very b r i e f l y , t h e I V s t a t e can be thought o f as a m i x t u r e of RE i o n s w i t h two n e a r l y degenerate i o n i c c o n f i g u r a t i o n s 4 f n + l and 4fn5d. A t any given s i t e , charge f l u c t u a t i o n s between,the two c o n f i u r a t i o n s occur on a time s c a l e known as t h e valence f l u c t u a t i o n time ( t y p i c a l l y 10-1q t o 10-13 seconds). I n the I V ( o r homogeneously mixed-valent) s t a t e a l l t h e RE ions occupy c r y s t a l l o g r a p h i c a l l y e q u i v a l e n t s i t e s . This has t o be d i s t i n g u i s h e d from such inhomogeneously mixed- valence compounds, where two d i s t i n c t valence s t a t e s occupy i n e q u i v a l e n t l a t t i c e s i t e s . Quantum mechanically t h e s i t u a t i o n i s described i n terms o f a h y b r i d i z a t i o n o f t h e two c o n f i g u r a t i o n s . A common view i s t h a t the h y b r i d i z a t i o n i n v o l v e s 5d i t i n e r a n t s t a t e s o r "band e l e c t r o n s " . The I V s t a t e i s then described i n t e r n s o f a band s t r u c t u r e p i c t u r e .

F i g . 2 - Schematic energy l e v e l diagram ( a ) f o r a d i v a l e n t RE m e t a l , ( b ) f o r t h e c o r r e s p o n d i n g semiconducting RE-sulphide, and

1 '

As i n d i c a t e d i n F i g . 2, t h e 4 f w l mu1 t i - e l e c t r o n l e v e l o f a d i v a l e n t RE metal i s l o c a t e d below t h e Fermi l e v e l ( F i g . Za), w h i l e i n t h e d i v a l e n t RE-MC1s t h e 4 f n + l l e v e l f a l l s i n t h e gap between v a l e n c e band and 5d6s c o n d u c t i o n band s t a t e s ( F i g s . 2b and c ) . We emphasize t h a t t h e meaning o f t h e s e energy l e v e l schemes i s t o account f o r energy d i f f e r e n c e s measured i n o p t i c a l e x c i t a t i o n processes. Under p r e s s u r e t h e c o n d u c t i o n band broadens and t h e b o t t o m o f t h e 5d band i s lowered r e l a t i v e t o t h e 4 f s t a t e , w h i l e s - l i ke c o n d u c t i o n band s t a t e s a r e pushed up i n energy. T h i s b e h a v i o r i s suggested by more g e n e r a l t r e n d s i n b a n d s t r u c t u r e under pressure."-'5 As a r e s u l t , t h e o p t i c a l 4f-5d e x c i t a t i o n energy i s reduced under p r e s s u r e . The decrease o f t h e o p t i c a l f - d e x c i t a t i o n energy i s c l o s e l y r e l a t e d ( b u t n o t i d e n t i c a l ) t o t h e decrease i n t h e e n t h a l p y d i f f e r e n c e between d i v a l e n t and t r i v a l e n t ground s t a t e . 1 6 W i t h i n t h e g i v e n i n t e r p r e t a t i o n o f o u r energy l e v e l sche e, t h e I V s t a t e corresponds t o an o v e r l a p o f t h e Fernii l e v e l w i t h t h e l o c a l i z e d 4 f l e v e l , as i n d i c a t e d i n F i g . 2d f o r a R E - t e l l u r i d e . I n t h e I V s t a t e charge f l u c t u a t e s between t h e 4 f w 1 s t a t e and band s t a t e s , and t h e bonding p r o p e r t i e s change c o n s i d e r a b l y . The Fermi l e v e l remains p i n n e d t o t h e 4 f l e v e l , as l o n g as i t i s n o t exhausted by a complete t r a n s f e r o f one f u l l e l e c t r o n i n t o band s t a t e s . I n t h e I V s t a t e t h e 4 f l e v e l i s broadened w i t h t h e w i d t h g i v e n by t h e i n v e r s e o f t h e f l u c t u a t i o n time. We w i l l r e f e r t o t h e s i t u a t i o n d e s c r i b e d h e r e as t h e "band p i c t u r e 1 ' o f t h e I V s t a t e .

A change f r o m d i v a l e n t t o I V b e h a v i o r i n RE-MC's corresponds t o a semiconductor t o metal t r a n s i t i o n . Some o f t h e s e I V " m e t a l s " e x h i b i t s e v e r a l unusual f e a t u r e s l i k e , f o r example;7 t h e i n c r e a s e o f t h e e l e c t r i c a l r e s i s t a n c e a t l o w temperatures i n

" g o l d n SmS, which may be r e l a t e d t o t h e presence o f a s m a l l ( 7 meV w i d e ) h y b r i d i - z a t i o n gap a t t h e Fermi l e v e l ." F o r o u r p r e s e n t d i s c u s s i o n such more s u b t l e f e a t u r e s o f t h e I V s t a t e w i l l n o t be r e l e v a n t , s i n c e we a r e concerned w i t h I V b e h a v i o r a t normal t e m p e r a t u r e and a t p r e s s u r e c o n d i t i o n s , which a r e n o t t r u l y h y d r o s t a t i c .

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111 - YTTERBIUM METAL

The energy balance f o r t h e occurrence o f a valence change i n Yb metal was f i r s t i n v e s t i g a t e d by Johansson and ~osengren.' T h e i r model s t a r t s o u t from b i n d i n g energy considerations, which su gest t h a t a t zero pressure t h e d i v a l e n t s t a t e w i t h e l e c - t r o n i c c o n f i g u r a t i o n 4 f 1 $ ( 5 d 6 s ) ~ ( i n a d d i t i o n t o the Xe core) i s s t a b l e by about 0.5 eV/atom w i t h r e s p e c t t o a h y p o t h e t i c a l t r i v a l e n t s t a t e 4f13(5d6s13. S i m i l a r energy d i f f e r e n c e s have be n d r i v e d by de Chatel and ~ o e r ~ ' s t a r t i n g from cohesive energies and the a t o n i c 4fT46sq - lf135d6s2 e x c i t a t i o n energy. A schematic diagram o f t h e i r Born-Haber c y c l e i s i n c l u d e d i n F i g . 3. Johansson and Rosengren use an e m p i r i c a l pseudopotential model t o o b t a i n t h e " r e p u l s i v e " p a r t o f the p o t e n t i a l curves f o r d i v a l e n t and t r i v a l e n t Yb as a l s o shown i n F i g . 3. The r e p u l s i v e p a r t obtained from an i n t e g r a t i o n o f t h e experimental pressure-volume (PV) r e l a t i o n o f

LU," being a t r i v a l e n t neighbor t o Yb i n t h e P e r i o d i c Table, i s i n c l o s e agreement w i t h the t h e o r e t i c a l p r e d i c t i o n . We have added the a t t r a c t i v e p a r t u s i n g t h e e m p i r i c a l r e l a t i o n proposed i n Ref. 22. I n t h i s way one a r r i v e s a t Eexc = 0.9 eV/

atom f o r the energy needed t o e x c i t e d i v a l e n t Yb t o t h e t r i v a l e n t s t a t e a t f i x e d l a t t i c e spacing corres onding t o d i v a l e n t Yb a t normal pressure For comparison, photoemi s s i o n studiesie y i e l d an e x c i t a t i o n t h r e s h o l d t o the 4 f I 3 5 d c o n f i g u r a t i o n of about 1.2 eV. This i s c o n s i s t e n t w i t h our value o f Eexc i f one takes i n t o account f i n a l s t a t e and/or l a t t i c e r e l a x a t i o n e f f e c t s , which i n photoemission increases t h e energy of t h e 4f135d s t a t e by about 0.2 t o 0.3 eV (see, e.g., Ref. 24). Thus t h e approximate energy l e v e l diagram i n Fig. 3 appears t o be w e l l supported by e x p e r i - mental data.

- 2

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1.6 1.8 2.0 2.2

WIGNER-SEITZ RADIUS ( A )

Fig. 3 - The ground s t a t e energies o f d i v a l e n t and t r i v a l e n t Yb metal as a f u n c t i o n o f Wigner-Seitz radius. Energies a r e g i v e n r e l a t i v e t o t h e d i v a l e n t (4f146s2) f r e e atom s t a t e . Aticoh(14) i s t h e cohesive energy o f d i v a l e n t Yb. AHcoh(13) i s t h e cohesive energy o f h y p o t h e t i c a l t r i v a l e n t Yb. AE(14-13 1s t h e energy r e q u i r e d t o e x c i t e a f r e e Yb atom t o the ( s p h e r i c a l l y averaged) 4fl3;dds~ c o n f i g u r a t i o n . These energy d i f f e r e n c e s are taken from Ref. 20. See t e x t f o r more d e t a i l s .

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1.6 1.8 2.0 2.2

W I G N E R S E l T Z R A D I U S ( A )

F i g . 4 - Pressure as a f u n c t i o n o f Wigner-Seitz r a d i u s f o r Yb. S o l i d l i n e and c l o s e d c i r c l e s r e p r e s e n t r e c e n t experimental d a t a o f Ref. 26. The dashed l i n e corresponds t o p s e u d o p o t e n t i a l c a l c u l a t i o n s f o r d i v a l e n t Yb.19 The dash-dotted l i n e r e p r e s e n t s t h e r e s u l t s o b t a i n e d by ~ k r i v e r ' * ' ~ ~ u s i n g t h e f i r s t p r i n c i p l e LMTO-ASA method. The experimental PV r e l a t i o n s o f c a Z g y 3 O and LuZ1 a r e shown f o r comparison.

Based on t h i s energy l e v e l scheme and t h e c o r r e s p o n d i n g e n t h a l p y d i f f e r e n c e , a f i r s t o r d e r t r a n s i t i o n f r o m t h e d i v a l e n t t o a f u l l y t r i v a l e n t s t a t e i s p r e d i c t e d a t about 130 k b a r w i t h a r e l a t i v e volume change o f about 14%.19 A s i m p l i f i c a t i o n i n t h i s p i c t u r e i s t h e n e g l e c t o f quantum m i x i n g between d i v a l e n t and t r i v a l e n t c o n f i g u r a - t i o n s o f t h e Yb i o n s , which may t u r n t h e t r a n s i t i o n i n t o a more gradual process, as i s observed e x p e r i m e n t a l l y .

Experimental evidence f o r a pressure-induced v a l e n c e chan e i n Yb stems f r o m PV s t u d i e ~ ~ ~ ' ~ ~ and f r o m x - r a y a b s o r p t i o n edge

measurement^.^^

Here, we w i l l o n l y c o n s i d e r t h e o v e r a l l b e h a v i o r o f t h e p r e s s u r e - r a d i u s r e l a t i o n w i t h r e s p e c t t o d i v a l e n t and t r i v a l e n t r e f e r e n c e s .

The experimental d a t a f o r Yb a r e compared i n F i g . 4 t o c a l c u l a t e d r e s u l t s o b t a i n e d w i t h i n t h e e m p i r i c a l p s e u d o p o t e n t i a l approachlg and f r o m t h e f i r s t p r i n c i p l e 1 in e a r m u f f i n t i n o r b i t a l method (LMTO-ASA) i n combination w i t h t h e a t o m i c sphere a p p r o x i - m a t i ~ n . ' ~ ' ~ ~ I n these c a l c u l a t i o n s Yb i s t r e a t e d as p u r e l y d i v a l e n t and a pressure- induced 6s t o 5d e l e c t r o n t r a n s f e r i s , o f course, t a k e n i n t o account. I n a d d i t i o n , F i g . 4 a l s o shows t h e experimental p r e s s u r e - r a d i u s r e l a t i o n f o r C a . 2 9 ' 3 0 A compari- son between Yb and Ca appears reasonable, because t h e y have a si-milar c o n d u c t i o n band s t r u c t u r e and i o n i c and atomic r a d i i o f Ca and d i v a l e n t Yb a t normal c o n d i t i o n s a r e a l s o v e r y s i m i l a r . The c a l c u l a t e d r e s u l t s f o r d i v a l e n t Yb agree r e a s o n a b l y w e l l among each o t h e r and a l s o w i t h t h e experimental d a t a f o r Ca, t h u s e s t a b l i s h i n g an

C8-128 JOURNAL DE PHYSIQUE

approximate pressure-radius r e l a t i o n f o r f i c t i c i o u s d i v a l e n t Yb under pressure.

Comparison w i t h experimental data then suggests t h a t t h e compression o f Yb i s indeed s t r o n g l y anomalous. An i n t e r p r e t a t i o n o f t h e PV anomaly i n terms o f a p a r t i c i p a t i o n of 4 f e l e c t r o n s i n chemical bonding appears t o be n o t j ~ s t i f i e d . ~ ' Therefore, t h e PV r e l a t i o n provides s t r o n g support f o r a s i g n i f i c a n t continuous valence change i n Yb s t a r t i n g a t pressures w e l l below 100 kbar.

The compression curve o f L U ~ ' (hcp phase i s i n c l u d e d i n F i g . 4. The pressure-radius curves o f Yb and Lu s t a r t t o o v e r l a p a t around 200 kbar. I n the case o f I V RE monochal co- genidesand i n t e r m e t a l l i c s a l a t t i c e parameter s c a l i n g (Vegard's law o r m o d i f i c a t i o n s o f i t , see Refs. 32and 33) i s f r e q u e n t l y u s e d t o d e t e r m i n e t h e v a l e n c e . Suchsimple s c a l i n g p r o c e d u r e s a l w a y s y i e l d a c o m p l e t e v a l e n c e c h a n g e t o t r i v a l e n t , i f t h e e x p e r i m e n - ta1 volume approaches the l i m i t i n g v a l u e f o r t h e t r i v a l e n t r e f e r e n c e . Thus, one i s tempt- e d t o c l a i m a completevalence change t o t h e t r i v a l e n t s t a t e i n Ybnear200kbar. However, t h i s conclusion i s n o t c o n s i s t e n t wi t h r e s u l t s o f LIII-edge a b s o r p t i o n measurements. ^{2 7} The LIII-edge absor t i o n (near 8.94 keV i n Yb) corresponds t o dipole-allowed t r a n s i - t i o n s from the 2 ~ 3 1 5 c o r e l e v e l t o empty 56-1 i ke s t a t e s above t h e Fermi l e v e l . The t r a n s i t i o n energy i s about 7 eV l a r g e r f o r t r i v a l e n t Yb ions as compared t o d i v a l e n t ions, which a r i s e s from a d i f f e r e n c e i n t h e c o r e l e v e l b i n d i n g energy. S t r u c t u r e and p o s i t i o n o f t h e LIII-edge under pressure show t h a t Yb metal remains c l o s e t o d i v a l e n t i n the fcc-phase and t h a t i t undergoes a continuous valence change i n t h e bcc-phase reaching a mean valence o f o n l y about 2.6 + 0.1 near 200 kbar. Thus, we i n f e r from x-ray a b s o r p t i o n measurements, t h a t Yb i s c l e a r l y n o t i n a p u r e l y tri- v a l e n t s t a t e a t 200 kbar.

The apparent discrepancy between volume and LIII-edge valence near 200 kbar may be r e s o l v e d by t a k i n g i n t o account t h e e f f e c t o f quantum m i x i n g between 4 f n + l and 4fn5d c o n f i g u r a t i o n s o f t h e r a r e e a r t h ion. A phenomenological approach i s proposed by Wohl leben and coworkers , 3 4 ' 3 who discuss t h e c o n f i g u r a t i o n m i x i n g i n t h e framework o f s t a t i s t i c a l thermodynamics w i t h a p p l i c a t i o n t o Ce metal. I n t h e i r model (see a l s o Refs. 36 and 37) a volume-dependent m i x i n g m a t r i x element between t h e i o n i c 4 f " l and 4 f W d c o n f i g u r a t i o n s determines the valence f l u c t u a t i o n r a t e ( o r f l u c t u a t i o n temperature), which i n t u r n p l a y s an important r o l e i n c a l c u l a t i n g the m i x i n g entropy term i n the Gibbs f r e e energy o f a valence f l u c t u a t i o n system. As a r e s u l t , the m i x i n g entropy term y i e l d s a negative i n t e r n a l pressure c o n t r i b u t i o n , which, a t a given valence and e x t e r n a l pressure, d r i v e s t h e volume towards s m a l l e r values as compared t o the s i t u a t i o n w i t h o u t quantum mixing. Furthermore, a t a s u f f i c i e n t l y h i g h f l u c t u a t i o n temperature, t h e valence i s pushed towards the entropy l i m i t , which i s c l o s e t o 2.9 f o r Yb. This value i s de rm'ned by t h on - and e i g h t f o l d degener- acy o f t h e Hundos r u l e ground s t a t e o f l f f 4 (IS0) and 4fs3 c o n f i g u r a t i o n s , r e s p e c t i v e l y . The entropy mechanism i s c o n s i d e r a b l y more important i n elemental Yb as compared t o I V compounds o r a l l o y s , because t h e o v e r l a p between neighboring Yb atoms i s much l a r g e r i n the pure metal. Therefore, t h e approach t o t h e t r i v a l e n t reference volume near 20 GPa does n o t imply t h a t Yb i s i n a f u l l y t r i v a l e n t s t a t e a t t h i s pressure. I t would even be conceivable t h a t t h e entropy mechanism and the u n d e r l y i n g c o n f i g u r a t i o n m i x i n g r e s u l t s i n a molar volume o f I V Yb, which i s s m a l l e r compared t o the t r i v a l e n t reference. A t present i t i s premature t o apply the s t a t i s - t i c a l thermodynamics model t o Yb i n a q u a n t i t a t i v e manner,because,in a d d i t i o n t o t h e PV r e l a t i o n , a more accurate determination o f t h e valence under pressure i s r e q u i r e d i n o r d e r t o determine the volume dependence o f t h e f l u c t u a t i o n temperature. These data may soon becomeavailableusing improved techniques f o r x-ray a b s o r p t i o n s t u d i e s a t h i g h pressures. ''

The temperature dependence o f the h i g h pressure e l e c t r i c a l r e s i s t a n c e o f yb3','0 does n o t show any evidence f o r anomalous s c a t t e r i n g processes, which a r e u s u a l l y observed i n I V i n t e r m e t a l 1 ic s . Based on t h e i r experimental r e s u l t s , aireckhoven and W i t t i g b 0 conclude t h a t Yb metal remains e s s e n t i a l l y d i v a l e n t up t o a t l e a s t 270 kbar.

The v a l i d i t y o f t h i s conclusion would, f o r instance, imply the complete f a i l u r e o f a f i r s t p r i n c i p l e c a l c u l a t i o n t o y i e l d the pressure-volume r e l a t i o n o f d i v a l e n t Yb metal i n reasonable agreement w i t h experiment. This i s d i f f i c u l t t o imagine i n view o f r e c e n t successes o f t o t a l energy c a l c u l a t i o n s . Furthermore, t h e energy l e v e l

diagram presented i n Fig. 3, which i s supported by experimental data, would be s e r i o u s l y i n e r r o r , a t l e a s t w i t h regard t o t h e " r e p u l s i v e p a r t s " . I n our opinion, the considerable d i f f e r e n c e i n e l e c t r o n i c s t r u c t u r e o f I V Yb metal as compared t o I V Y b - i n t e r m e t a l l i c s has t o be taken i n t o account, before any conclusions about the degree o f c o n f i g u r a t i o n m i x i n g i n Yb can be drawn from r e s i s t a n c e measurements.

Recent renormal i z e d atom c a l c u l a t i o n s by Herbst and U i l k i n s 4 ' a r e c o n s i s t e n t w i t h the onset o f a 4 f i n s t a b i l i t y i n Yb i n t h e 100 kbar range. These c a l c u l a t i o n s a l s o suggest t h a t a predominantly d i v a l e n t s t a t e o f Yb may become s t a b l e again a t pres- sures i n the order o f 3 Mbar. Such a r e v e r s a l o f t h e valence change may be i n t e r - p r e t e d by analogy t o t h e p o t e n t i a l and k i n e t i c energy considerations, which q u a l i t a - t i v e l y e x p l a i n t h e e l e c t r o n t r a n s f e r from extended s-states i n t o t h e h i g h e r angular momentum d - s t a t e s i n t r a n s i t i o n and RE metals under pressure (see, e.g., Refs. 11, 12,42). The o u t e r 5d e l e c t r o n s o f Yb a r e excluded from the core r e g i o n by o r t h o - g o n a l i t y requirements. A t l a r g e compression the increase o f t h e i r one-electron k i n e t i c energies w i l l dominate over t h e change i n p o t e n t i a l energy. Thus, the 5d l e v e l s should r i s e i n energy r e l a t i v e t o the more l o c a l i z e d 4 f s t a t e s . The p r e d i c t e d

"reentrance pressure" o f roughly 3 Mbar i s s u r p r i s i n g l y low. A p o s s i l b e consequence o f t h i s p r e d i c t i o n i s t h a t Yb metal w i l l never completely transform i n t o a f u l l y t r i v a l e n t s t a t e . Furthermore, a s i m i l a r argument may apply t o many I V m a t e r i a l s . Eu i s the o t h e r d i v a l e n t RE metal being a candidate f o r a pressure-induced 4 f i n -

~ t a b i l i t y . ' ~ " ' ~ ' ~ ~ The experimental PV r e l a t i o n i n t h e 300 kbar rangez6 suggests t h a t the compression o f Eu i s a l s o anomalously l a r g e , t h u s p r o v i d i n g evidence f o r a major change i n e l e c t r o n i c s t r u c t u r e i n v o l v i n g 4 f e l e c t r o n s . Eu metal may, however, be more s t a b l e a g a i n s t a pressure-induced 4 f i n s t a b i l i t y r e s u l t i n g i n a s m a l l e r valence change as compared t o Yb a t the same pressure. This i s i n d i c a t e d by t h e l a r g e r f r e e enthalpy d i f f e r e n c e ( - 1 eV) between d i v a l e n t and h y p o t h e t i c a l t r i v a l e n t

a t normal pressure and a l s o by t h e l a r g e r 4fw1 - 4fn5d e x c i t a - t i o n energy (-1.5 eV) seen i n p h o t o e m i ~ s i o n . ~ ~

I V - THE 4 f INSTABILITY I N YbS

The o n l y experimental evidence f o r pressure-induced valence t r a n s i t i o n s i n Yb-MC's comes from x-ray d i f f r a c t i o n data, which reveal t h e onset o f continuous com r e s s i o n anomalies a t pressures r a n g i n g from 80 kbar i n YbO t o 170 kbar i n YbTe.45-4e Here we a r e concerned w i t h t h e o p t i c a l d e t e c t i o n and c h a r a c t e r i z a t i o n o f t h e I V behavior

i n YbS, which leads t o a very d i f f e r e n t p i c t u r e o f t h e I V s t a t e i n t h i s m a t e r i a l (and o t h e r Yb-MC's) as compared t o the Sm- and Tm-MC's. I n order t o c l a r i f y t h i s d i f f e r e n c e , i t i s a p p r o p r i a t e t o summarize some o p t i c a l p r o p e r t i e s o f I V Sm and Tm compounds and t h e commonly accepted i n t e r p r e t a t i o n .

One o f t h e most s t r i k i n g f e a t u r e s o f t h e valence change i n SmS near 7 kbar i s t h e c o l o r change from dark grey t o golden y e l l o w . As shown i n F i g . 5, the c o l l a p s e d

"metal 1 ic " phase e x h i b i t s h i g h r e f l e c t i v i t y i n the n e a r - i n f r a r e d and a pronounced r e f l e c t i v i t y edge i n t h e v i s i b l e p a r t o f the o p t i c a l spectrum."'" A very s i m i l a r behavior i s found f o r S~nse,~' SmTeS1 (see a l s o below), and ~ m i n the pressure- ~ e ~ ~ induced I V phase and a l s o i n TmSe,53 which shows I V behavior a t normal pressure.

Moreover, t h e gross f e a t u r e s o f t h e o p t i c a l response o f these I V compounds i s s i m i - l a r t o t h a t o f GdS (see, e.g., Ref. 4 9 ) , which i s a t r i v a l e n t RE m a t e r i a l showing normal metal1 i c behavior.

The h i g h r e f l e c t i v i t y i n I V SmS, SmSe and TmSe has been a t t r i b u t e d t o a Drude-like behavior a r i s i n g from a t r a n s f e r o f 4 f e l e c t r o n s i n t o one-electron conduction band

state^.'^'^^'^^ I t i s i n f a c t the o p t i c a l response, which lends s t r o n g support t o t h e "band p i c t u r e " (see s e c t i o n 11) o f a valence t r a n s i t i o n i n these m a t e r i a l s . h i th i n t h i s p i c t u r e , t h e p o s i t i o n of the r e f l e c t i v i t y edge i s approximately given by the screened plasma frequency

u p = (ne / m * ~ ~ ) 2 1/2

JOURNAL DE PHYSIQUE

Fig. 5 - O p t i c a l r e f l e c t i o n spectrum of " m e t a l l i c " SmS. T h e d a s h e d l i n e corresponds t o the sample-vacuum i n t e r f a c e , t h e s o l i d l i n e t o a sample-diamond i n t e r f a c e .

s

where i s t h e d e n s i t y o f band e l e c t r o n s and m* i s t h e i r o p t i c a l e f f e c t i v e mass. The background d i e l e c t r i c constant E, i s p r i m a r i l y determined by valence t o conduction band t r a n s i t i o n s (see Fig. 2), which have appreciable o s c i l l a t o r s t r e n g t h comnenc- i n g i n t h e photon energy range from 3 t o 6 eV near normal pressure. The a n a l y s i s o f the o p t i c a l response o f I V SmS4' and ~ m S e ' ~ suggests t h a t t h e d e n s i t y o f band e l e c - t r o n s i n these m a t e r i a l s i s r o u g h l y c o n s i s t e n t w i t h the f r a c t i o n a l occupation o f t h e 4fn5d c o n f i g u r a t i o n , i.e., the d e l e c t r o n f u l l y c o n t r i b u t e s t o a f r e e e l e c t r o n l i k e response.

- ^\

-..

\ SmS

\ n , = l

- <

\ \ (Batlogg et 01.1

With the h e l p o f Fig. 5, we p o i n t o u t a t e c h n i c a l d e t a i l . I n the h i g h pressure r e f l e c t i o n spectra r e p o r t e d below t h e r e f l e c t i v i t y (denoted R ) i s always measured a t the sample-diamond i n t e r f a c e ( f o r more d e t a i l s see Ref. 547. The l a r g e r e f r a c t i v e

index o f diamond (n, = 2.41) i s r e s p o n s i b l e f o r the f a c t t h a t the r e f l e c t i v i t y Rd i s i n general lower as compared t o measuring a t the usual sample-vacuum i n t e r f a c e . This i s demonstrated i n F i g . 5 which a l s o shows t h e r e f l e c t i o n spectrum o f SmS f o r the sample-diamond i n t e r f a c e as c a l c u l a t e d from t h e d i e l e c t r i c f u n c t i o n given i n Ref. 49.

-

2.

C 2 ^{4 0 -}

-

C 0

W _-I - -

LL

W z -

-

Oo 1 2 3 4

PHOTON ENERGY ( eV 1

O p t i c a l r e f l e c t i o n spectra o f YbS a t pressures between 6 and 390 k b a r S 5 a r e shown i n Fig. 6. I n t h e semiconducting phase below about SO kbar one observed two weak r e - f l e c t i o n bands se r a t e d by about 1.3 eV. These t r a n s i t i o n s correspond t o e x c i t a - t i o n s from the 4fP4 ground s t a t e t o an e x c i t o n i c ( o r atomic-1 i ke) 4f135d(tzg) e x c i t e d s t a t e , which i s b a s i c a l l y s p l i t i n t o two components 2 ~ 7 / 2 and 2 ~ due t o spin- ~ / ~ o r b i t i n t e r a c t i o n - (1.24 eV i n t h e f r e e i o n ) o f the 4f13 c o n f i g u r a t i o n (see a l s o Refs.

56 and 57). A f - d Coulomb i n t e r a c t i o n o r 5d e l e c t r o n s p i n - o r b i t s p l i t t i n g may i n t r o - duce f u r t h e r s p l i t t i n g s , which would account f o r t h e weak s t r u c t u r e observed i n t h e f i r s t r e f l e c t i o n peak (see F i g . 6 ) . E x c i t a t i o n s o f 4 f n + l i n t o l o c a l i z e d 4fn5d s t a t e s are always observed i n semic n d u c t i n g RE-HC's, b u t are n o t i n c l u d e d i n F i g . 2. The e x c i t a t i o n t h r e s h o l d from 4fP4 t o 5d conduction band s t a t e s i n YbS i s 1.27 eVsa a t normal pressure, which i s s l i g h t l y l e s s than t h e energy o f t h e maximum o f the f i r s t e x f i t o n i c peak. With i n c r e a s i n g pressure the c r y s t a l f i e l d a c t i n g on t h e d component 4 f 5 d ( t z g ) increases and t h e e x c i t a t i o n energy decreases a t a r a t e o f about 11 meV/

kbar. This pressure dependence i n d i c a t e s t h e onset o f I V behavior i n t h e v i c i n i t y o f 100 kbar.

I n accordance w i t h t h i s e x t r a p o l a t i o n , the r e f l e c t i v i t y s t a r t s t o increase consider- a b l y above 100 kbar reaching s a t u r a t i o n between 300 and 400 kbar. However, b e l o w l e v one f i n d s a sharp r e f l e c t i v i t y edge, which becomes more pronounced and s h i f t s t o

5 I I I

- (a1 Y b S -

- 0 1 2 3

PHOTON ENERGY ( e V 1

F i g . 6 - O p t i c a l r e f l e c t i o n s p e c t r a o f YbS under p r e s s u r e ( a ) i n t h e semiconducting phase and ( b ) i n t h e i n t e r m e d i a t e - v a l e n t phase.

h i g h e r energy w i t h i n c r e a s i n g p r e s s u r e . I t i s t h i s b e h a v i o r , which i s c o m p l e t e l y d i f f e r e n t f r o m t h e I V Sm and Tm MC's.

The r e f l e c t i v i t y o f YbO under p r e s s u r e qua1 i t a t i v e l y shows a s i m i l a r b e h a v i o r i n t h e semiconducting phase as w e l l as i n t h e p r e s s u r e range c o r r e s p o n d i n g t o t h e e x i s t e n c e o f a I V phase." Obviously, t h e pronounced n e a r - i n f r a r e d r e f l e c t i v i t y minimum i n I V YbO und YbS i s c h a r a c t e r i s t i c f o r t h e Yb i o n . A c r y s t a l s t r u c t u r e e f f e c t can be excluded, because t h e NaCl phase i n YbO and YbS has been shown t o be s t a b l e up t o a t l e a s t 350 and 250 kbar, r e s p e c t i v e l y ,' 6 " 7 and, f u r t h e r m o r e , t h e c r y s t a l s t r u c t u r e s y s t e m a t i c s i n monochalcogenides under p r e s s u r e 6 0 suggests a t r a n s i t i o n t o t h e most l i k e l y CsCl s t r u c t u r e t o o c c u r a t p r e s s u r e s above 400 kbar.

I n o r d e r t o f a c i 1 i t a t e t h e i n t e r p r e t a t i o n o f t h e o p t i c a l r e f 1 e c t i o n s p e c t r a o f YbS, Drude-Lorentz t y p e e x p r e s s i o n s were f i t t e d t o t h e e x p e r i m e n t a l s p e c t r a . As an exam- p l e , F i g . 7 shows an o p t i c a l c o n d u c t i v i t y spectrum ( r e a l p a r t 01) o f YbS a t 3 9 0 k b a r , as o b t a i n e d f r o m t h e f i t t i n g procedure. (The o p t i c a l c o n d u c t i v i t y a (w) i s p r o p o r - t i o n a l t o U E ~ ( U ) w i t h c ~ ( w ) b e i n g t h e i m a g i n a r y ( a b s o r p t i v e ) p a r t o # t h e d i e l e c t r i c f u n c t i o n . ) F o r t h e a n a l y s i s we have assumed t h a t t h e r e f l e c t i v i t y o f I V YbS below t h e p r e s e n t e x p e r i m e n t a l energy range i n c r e a s e s a g a i n towards h i g h values, due t o e i t h e r m e t a l l i c b e h a v i o r o r o t h e r s t r o n g low energy e x c i t a t i o n s (see d o t t e d l i n e i n F i g . 7 ) . Furthermore, t h e background d i e l e c t r i c c o n s t a n t L,, was chosen t o be 4.5, which i s s l i g h t l y l a r g e r compared t o t h e c o r r e s p o n d i n g values o f a l k a l i n e e a r t h s u l p h i d e s a t normal p r e s s u r e . A l t h o u g h t h e a b s o l u t e values o f a1 a r e somewhat uncer- t a i n due t o t h e s e assumptions, t h e s p e c t r a l s h a p e o f t h e c o n d u c t i v i t y spectrum i s n o t

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u- ^"

0 1 2 3 I,

PHOTON ENERGY l e v 1

Fig. 7 - O p t i c a l c o n d u c t i v i t y ( r e a l p a r t ) o f YbS a t 390 kbar. The d o t t e d l i n e i s the corresponding r e f l e c t i o n spectrum w i t h t h e assumed e x t r a p o l a t i o n below 0.5 eV.

a f f e c t e d . Thus, a dominant f e a t u r e i n the o p t i c a l response o f I V YbS i s a pronounced narrow a b s o r p t i o n peak i n t h e near-infrared, which e x h i b i t s a b l u e - s h i f t w i t h i n - creasing pressure.

I n F i g . 8 the o p t i c a l t r a n s i t i o n energies o f YbS (maxima o f c2(w)) a r e p l o t t e d as a f u n c t i o n o f pressure. Results f o r t h e I V phase a r e given as negative energies. By r e p r e s e n t i n g t h e data i n t h i s way, an e x p l a n a t i o n o f the unusual a b s o r p t i o n f e a t u r e i n I V YbS becomes immediately obvious. A f t e r the "crossover" between t h e two 4 f con- f i g u r a t i o n s near 100 kbar, a c o r r e l a t e d ( o r l o c a l i z e d ) 4f135d s t a t e becomes the ground s t a t e o f the system. n t i s p i c t u r e , the s t r o n g absor t i o n peak i n t h e I V phase $arresponds t o o p t i c a l a x c i k a t i o n s from t h e l o c a l i z e d 4fy35d ground s t a t e t o the 4 f s t a t e , which now i s t h e e x c i t e d s t a t e . This d e s c r i p t i o n o n l y gives the very basic f e a t u r e s o f our i n t e r p r e t a t i o n . The actua s i t u a t i o n i s c e r t a i n l y more compl i- cated. F i r s t , h y b r i d i z a t i o n between 4f14 and 4 f I 3 5 d ( i n d i c a t e d by d o t t e d l i n e s i n Fig. 8 ) i s p o s s i b l e according t o a mechanism proposed by Kaplan and ah anti^' and a l s o d' cussed i n Refs. 62-64. Thus, the valence hange i s a gradual t r a n s i t i o n w i t h

I

5

the 4 f l a ground s t a t e a c q u i r i n g more and more 4 f 5d c h a r a c t e r and, simul taneoulsy, w i t h t h e e x c i t e d s t a t e becoming of predominant 4f14 character. The r e l a t i v e admix- t u r e o f t h e two c o n f i g u r a t i o n s i n the ground s t a t e determines the valence s t a t e . Second, s i n c e d conduction band s t a t e s o v e r l a p t h e l o c a l i z e d ground s t a t e , some o f the e l e c t r o n s o f 4f135d go i n t o band s t a t e s . Thus, t h e system becomes " m e t a l l i c " , which then j u s t i f i e s o u r above e x t r a p o l a t i o n o f the experimental r e f 1 e c t i v i t y below 0.5 eV photon energy. A schematic p i c t u r e o f t h e s i t u a t i o n near the Fermi l e v e l i s given i n Fig. 9, which a l s o demonstrates t h e d i f f e r e n c e between t h e l o c a l i z e d p i c - t u r e and t h e "band p i c t u r e " o f the I V s t a t e i n RE K ' s . I t seems t h a t the o p t i c a l response o f the I V Yb-MC's provides the , f i r s t c l e a r experimental evidence f o r a l o c a l i z e d mechanism o f a 4 f i n s t a b i l i t y i n Re-MC's.

The l o c a l i z e d mechanism o f the valence t r a n s i t i o n i n Yb-MC's r e q u i r e s a s t r o n g Coulomb i n t e r a c t i o n f o r the formation o f the l o c a l i z e d 4f135d c o n f i g u r a t i o n . I n t u i - t i v e l y , i t may be p l a u s i b l e t h a t an almost f i l l e d 4 f s h e l l has a s t r o n g tendency t o b i n d an e x t r a 5d e l e c t r o n i n o r d e r t o form a s p i n s i n v l e t s t a t e . However, i t remains t o be i n v e s t i g a t e d i n more d e t a i l , why a predominantly l o c a l i z e d ground s t a t e i s formed i n I V Yb-MC's, b u t n o t i n t h e corresponding Sm and Tm compounds. I t should

PRESSURE ( k b a r ) 3

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0 [r

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Fig. 8 - Optical t r a n s i t i o n e n e r g i e s in YbS a s a f u n c t i o n of p r e s s u r e .

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0 100 200 300 40 0

Fig. 9 - Schematic r e p r e s e n t a t i o n of t h e d i f f e r e n c e between ( a ) "band p i c t u r e " and ( b ) l o c a l i z e d p i c t u r e of t h e I V s t a t e i n r a r e e a r t h monochalcogenides.

' I ENERGY ( b )

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C8-134 JOURNAL DE PHYSIQUE

be mentioned, however, t h a t weak l o c a l c o r r e l a t i o n e f f e c t s have v e r y r e c e n t l y been p o s t u l a t e d i n t h e i n t e r p r e t a t i o n o f e x p e r i m e n t a l r e s u l t s r e l a t e d t o t h e valence i n s t a b i l i t y i n t h e TmSe-TmTe a1 l o y system. 6 5 Furthermore, Kaplan e t a1. " argue t h a t a l o c a l i z e d model would even be c o n s i s t e n t w i t h t h e o p t i c a l response o f " g o l d " SmS.

An i n t e r e s t i n g q u e s t i o n now i s , how l a r g e a s c r e e n i n g by band e l e c t r o n s i s t o l e r a b l e f o r t h e s t a b i l i t y o f a l o c a l i z e d ground s t a t e . I n o t h e r words, i s a l o c a l i z e d mecha- nism t o some e x t e n t r e l e v a n t t o I V Y b - i n t e r m e t a l l i c s o r even i n Yb m e t a l ?

A comparison w i t h Eu-MC's i s d i f f i c u l t , because EuO appears t o be t h e o n l y compound i n t h i s group, which undergoes a s i g n i f i c a n t valence change i n t h e p r e s s u r e range o f 400 k b a r . 6 6 ' 6 7 O p t i c a l r e f l e c t i o n measurements and x - r a y d i f f r a c t i o n d a t a r e v e a l a c o n t i n u o u s o n s e t o f t h e 4 f - i n s t a b i l i t y s t a r t i n g a t t h e unexpectedly l o w p r e s s u r e o f 130 k b a r , 6 7 which i s i n marked c o n t r a s t t o t h e p r e v i o u s l y r e p o r t e d t r a n s i t i o n a t about 300 k b a r . 6 6 E s s e n t i a l l y based on t h e v a l i d i t y o f t h e "band p i c t u r e " f o r IVEuO, t h e l o w t r a n s i t i o n p r e s s u r e was t e n t a t i v l e y i n t e r p r e t e d i n terms o f an i n t e r p l a y between f e r r o m a g n e t i c o r d e r i y , exchange s p l i t t i n g o f t h e c o n d u c t i o n band, and t h e o n s e t o f t h e 4 f i n s t a b i l i t y . The new r e s u l t s f o r Yb-MC's suggest t o a l s o c o n s i d e r t h e p o s s i b i l i t y o f a p r e d o m i n a n t l y l o c a l i z e d mechanism o f t h e v a l e n c e i n s t a b i l i t y i n EuO. T h i s m i g h t a f f e c t t h e i n t e r p r e t a t i o n o f t h e unusual b e h a v i o r o f EuO near 130 kbar. Although t h e o p t i c a l d a t a a r e p o s s i b l y c o n s i s t e n t w i t h a more l o c a l i z e d ground s t a t e i n I V EuO, t h e evidence i s n o t as s t r o n g compared t o t h e s i t u a t i o n i n Yb-MC's.

V - 81-82 TRAiqSITION I N SmTe AND COMPARISON TO SrTe

H i g h p r e s s u r e x - r a y i n v e s t i g a t i o n s k 5 ' 5 1 suggest t h e occurrence o f a c o n t i n u o u s v a l e n c e change i n SmTe a t pressures below 100 kbar. From a l a t t i c e parameter s c a l i n g a mean valence o f r o u g h l y 2.7 a t 100 k b a r can be d e r i v e d . High p r e s s u r e r e f l e c t i o n s p e c t r a o f S m ~ e ~ ' a r e shown i n F i g . 10. The two sharp peaks below 2 eV observed i n t h e semiconducting phase ( F i g . 10a) can be a t t r i b u t e d t o 4 f 6 - 4 f 5 5 d ( t 2 ) t r a n s i -

t i o n s . The d o u b l e t o r i g i n a t e s f r o m t h e ~H,J and ~F,J terms o f t h e 4 f 5 ~ o q ? i ~ u r a t i o n . ~ ~ The t h i r d peak near 3 eV can n o t be i n t e r p r e t e d as a s i m i l a r atomic t r a n s i t i o n . We

t e n t a t i v e l y a s s i g n t h i s peak t o an e x c i t o n i c s t r u c t u r e a s s o c i a t e d w i t h zone c e n t e r v a l e n c e t o c o n d u c t i o n band ( p t o d ) t r a n s i t i o n s . T h i s assignment i s suggested by a comparison t o t h e h i g h p r e s s u r e o p t i c a l response o f SrTe, which, w i t h r e g a r d t o t h e valence-conduction band s t r u c t u r e , s h o u l d be v e r y s i m i l a r t o SmTe.

The f i r s t s t r o n g a b s o r p t i o n f e a t u r e i n SrTe a t normal p r e s s u r e occurs a t 3.75eV.E859 T h i s e x c i t o n i c t r a n s i t i o n i s c l e a r l y observed i n h i g h p r e s s u r e r e f l e c t i o n s p e c t r a as shown i n F i g . 11. Below 120 kbar, one f i n d s two sharp s t r u c t u r e s near 3 eV, which a r e separated by about 0.6 eV. Both peaks undergo a r e d - s h i f t w i t h i n c r e a s i n g p r e s - sure, w h i c h suggest t h e i n t e r p r e t a t i o n as e x c i t o n i c p t o d t r a n s i t i o n s . The 0.6 eV s e p a r a t i o n i s due t o s p i n - o r b i t s p l i t t i n g o f t h e valence band. The f a c t t h a t t h e c o r r e s p o n d i n g peak i n SmTe occurs a t about 0.8 eV l o w e r energy as compared t o SrTe a r i s e s f r o m t h e l a r g e r n u c l e a r charge o f Sm, which p u l l s t h e d s t a t e s down r e l a t i v e t o t h e t o p o f t h e valence band. F o r comparison, t h e f i r s t e x c i t o n i c peak i n BaTe i s s h i f t e d b y 0.65 eV t o l o w e r ener y r e l a t i v e t o SrTe. 6 8 ' 6 9 We note, t h a t a number o f band s t r u c t u r e c a l ~ u l a t i o n " - ~ 5 ' s 0 f o r a1 k a l i n e and RE MC1s p r e d i c t an i n d i r e c t band gap f o r these m a t e r i a l s i n t h e NaCl phase, i . e . , t h e t o p o f t h e valence band i s a t t h e zone c e n t e r and t h e bottom o f t h e c o n d u c t i o n band a t t h e zone edge ( X - p o i n t ) . Thus, weak valence-conduction band e x c i t a t i o n s a r e expected below t h e f i r s t e x c i - t o n i c peak. E x p e r i m e n t a l l y , t h e e x c i t a t i o n t h r e s h o l d f o r t h e s e band t o band t r a n s i - t i o n s seems t o be unknown.

The continuous e v o l u t i o n t o a D r u d e - l i k e m e t a l l i c r e f l e c t i v i t y i n SmTe ( F i g . l o b ) w i t h s a t u r a t i o n near 100 k b a r supports t h e occurrence o f a m a j o r valence change i n t h i s p r e s s u r e range. The p r e s s u r e r e q u i r e d t o i n i t i a t e t h e e l e c t r o n i c t r a n s i t i o n (-40 k b a r ) i s c o n s i s t e n t w i t h an f - d band gap o f 0.62 eV a t normal p r e s ~ u r e . ' ~ The s p e c t r a l c h a r a c t e r i s t i c s o f t h e r e f l e c t i v i t y o f SmTe near 100 kbar a r e qua1 i t a t i v e l y s i m i l a r t o t h e o p t i c a l response o f SmS. The r e f l e c t i v i t y edge i s observed a t l o w e r energy i n SmTe, m a i n l y because t h e p t o d t r a n s i t i o n s o c c u r a t l o w e r energy i n SmTe.

Thus t h e screened plasma frequency (Eq. 1 ) i s reduced.

SmTe

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-

-

-

-

-

-

0 1 2 3 4

PHOTON ENERGY (eV 1

Fig. 10 - O p t i c a l r e f l e c t i o n spectra o f SmTe a t h i g h pressures. ( a ) semiconducting phase; ( b ) I V phase w i t h NaCl s t r u c t u r e ; ( c ) I V phase below (120 kbar) and above (147 and 220 kbar) t h e NaC1-CsC1 s t r u c t u r a l t r a n s i t i o n .

A t about 130 kbar, the r e f l e c t i v i t y edge i n SmTe i s suddenly pushed back by about 0.7 eV i n t o the n e a r - i n f r a r e d and then remains almost s t a b l e up t o 220 kbar ( F i g . 10c). This discontinuous change i n r e f l e c t i v i t y can be r e l a t e d t o a s t r u c t u r a l B1-B2 t r a n s i t i o n o c c u r r i n g a t 130 k b a r 4 5 ' 5 ' w i t h a r e l a t i v e volume change o f 9%. A t t h i s s t r u c t u r a l t r a n s i t i o n t h e p-d gap becomes d i r e c t o r c l o s e t o d i r e c t a c c o r d i n 2 ^to

several r e c e n t band s t r u c t u r e c a l c u l a t i o n s f o r S m ~ e ' ~ and r e l a t e d compounds? " 5 ' 7 '

A corresponding observation can be made i n SrTe, which undergoes a 81-82 t r a n s i t i o n a t 120 k b a r . 6 0 The r e f l e c t i o n spectra o f SrTe i n the B2 phase ( a l s o shown i n F i g . 11) c l e a r l y e x h i b i t a n e a r - i n f r a r e d r e f l e c t i v i t y edge, which may be i n t e r p r e t e d as a s i g n a t u r e o f d i r e c t band t o band t r a n s i t i o n s . For comparison, from a b s o r p t i o n