CRYSTAL POTENTIAL AND SIZE EFFECTS IN XANES K-SPECTRA OF ALKALI HALIDES

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CRYSTAL POTENTIAL AND SIZE EFFECTS IN XANES K-SPECTRA OF ALKALI HALIDES

L. Bugaev, I. Gegusin, A. Novakovich, R. Vedrinskii

To cite this version:

L. Bugaev, I. Gegusin, A. Novakovich, R. Vedrinskii. CRYSTAL POTENTIAL AND SIZE EFFECTS

IN XANES K-SPECTRA OF ALKALI HALIDES. Journal de Physique Colloques, 1986, 47 (C8),

pp.C8-101-C8-104. �10.1051/jphyscol:1986818�. �jpa-00226085�

JOURNAL D E PHYSIQUE

Colloque C8, suppl6rnent au n o 12, Tome 47, decembre 1986

CRYSTAL POTENTIAL AND SIZE EFFECTS IN XANES K-SPECTRA OF ALKALI HALIDES

L.A. BUGAEV, 1.1. GEGUSIN, A.A. NOVAKOVICH and R.V. VEDRINSKII

Institute of Physics, Rostov State University, Engels str. 105, 344006 Rostov-on-Don, U.S.S.R.

XRPJT;:S

IC-spectra of LIP, %aF,

LiCl, XaC1, 1x1 c r y s t a l s exhibit several, well resolved features, u n t i l now not sdequa- t e l y explained. ile present the calculations of these spectra by metins of band approach,

well a s c l u s t e r one. I n the l a s t case tho core hole potential is token i n t o account. Both app- roaches employ D-model.

band r e s u l t s ore compared with the c l u s t e r predictions, which are carried out by m e a m of mul- t i p l e s c a t t e r i n g technique.

this Fnvestlgation the c l u s t e r potential is equnl t o the c r y s t a l one inside the c l u s t e r , and i a equal t o W-zero outeide. The c l u s t e r c ~ l c u l ~ t i o n s give t h e r e s u l t s close t o ones obtained by r n e w of band approach

the c l u s t e r s used involve more than 3 coordination s h e l l s of neigh-

atoms (Fig.1). fPherefore i t s e e m , t h a t

r e s u l t s . obtained f o r l i t t l e clustera ( p a r t i c u l a r l y , 7 a t o m ) are ambf- guous .

Uhen

s t r u c t u r e is subatantially affected by a collap- sing o r b i t u l

- ^{o r}

^{€d 1,} c l u s t e r calculations demon- s t r a t e large serzaibility t o changes of the potential. Among the spectra studied the most expreesive exemple represents the kt.-formation of LiK-upectrum i n LIE'

consequence of tho core hole p o t e n t i a l (Fig.2). The m a l l enough core hole influ- ence may be i l l u s t r a t e d

the example of HaK-spectrum i n NaF (Fig.3). The reexalting spectra alao depend on whether the con-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986818

C8-102 JOURNAL DE PHYSIQUE

tinuum s t a t e o r b i t s l s a r e obtained by m e a n s of X s -exchange o r Hortroe-Pock one (Fig.4). Theoe f i n d i n g s evidence t h a t i t i s

d i f f i c u l t t a s k t o define t h e n e a r neighboursf geometry from

spectra.

Various models of p o t e n t i a l s were studied. It i s found t h a t t h e b e s t r e s u l t s i n t h e case of a l k a l i h a l i d e s may be ob- t a i n e d when t h e following contributiona t o e f f e c t i v a p o b e n t i a l a r e taken i n t o account: i ) t h e c r y s t a l e l e c t r o n d e n s i t y is con- s t r u c t e d as superposition of t h e f r e e i o n e l e c t r o n d e n s i t i e s , i i ) Xadelung c o r r e c t i o n i s added, i i i ) core hole p o t e n t i a l is properly acreened, i v ) t h e nonlocal Hartree-Pock exchange poten- t i a l is used i n s i d e I2.P-spheres, v) 122-zero is a f r e e parameter, and v i ) t h e p o t e n t i a l outside t h e c l u s t e r is equal t o XI!-zero.

The proposed c l u s t e r p o t e n t i a l model i s used i n t h e course of t h e systematic

examination of K-apectra of a l k a l i

l i d e s .

r e s u l t s presented were computed f o r 57 a t o m clus- t e r s . The conduction band bottom p o s i t i o n is used t o b r i n g to- g e t h e r t h e o r e t i c a l and experiment a 1 spectra. The a p e c t r a cal- c u l a t e d w i t h a s well a s without core hole p o t e n t i a l are a l s o compared.

The following conclusions are drawn. The i d e a l c r y s t a l

c l u s t e r r e s u l t s agree w e l l w i t h band predictions. P a r t i c u l a r l y

t h e i n t e n s i t y suppression f o r t h e f i n i t e s i z e c l u s t e r s i n t h e

gap region is found. It implies t h a t f o r c r y s t a l s s t u d i e d t h e

gap is primRrily fonned owing t o multiple s c a t t e r i n g in a suf-

f i c i e n t l y small region of t h e c r y s t a l . I n most cases except

LiK-spectra, t h e band r e s u l t s agree q u a l i t e t i v e l y w i t h t h e

p r i m e n t o 1 spectra. Wen core h o l e p o t e n t i a l i s taken i n t o ac-

count, one reaches ~ e m i q u a n t i t a t i v e agreement (Fig.2 and 3 ) .

The core hole p o t e n t i a l e x h i b i t s a s q u i t e s m a l l peak p o s i t i o n s

s h i f i s and r e l a t i v e i n t e n s i t i e l s changes. The dramatic trans-

formation is found

t h e case of LiK-spectre only.

Fig.?.

EL-spectra

( a ) and C1 (b) ia gC1. The 70, 19-9 27-, 33- and

clusters are used

cluster ca1culat;ians.

The histogram represents the band results.

Fig020 Li &spectra in U F calcu- lated for 27-atom cluster. The foll-

owing atomic

configurations of the central Li+-ion are usedr 4 -

2 - ^{4 ~ ~ 3 *} - ^{~ Q5, ~ ,}

- ^{18 1.25,}

5 - ^IS'*

05 $ 0 45

The daA8d line - emerimsnt, f u l l

4.0

and dashed-dotted lines represent

O'

the T7-atom cluster results, obtai-

,, ^{ned for two models of} crystalUse

l ' ' l ' l ' l l r l ' l ' l l l l " ' ' '

3080 4090 Lo

p~ter&iEll.

JOURNAL DE PHYSIQUE

Fig.4.

&-(left) and L ( r i g h t ) spectra of K i n GI.

cafculations are carried out f o r 27-atom clugter both i n Xoc

0.75;

and Hartroe-Fock approaches.