LOCAL STRUCTURES ABOUT SOME TRANSITION ELEMENTS IN OXIDE GLASSES USING X-RAY ABSORPTION SPECTROSCOPY

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LOCAL STRUCTURES ABOUT SOME TRANSITION ELEMENTS IN OXIDE GLASSES USING X-RAY

ABSORPTION SPECTROSCOPY

J. Petiau, G. Calas

To cite this version:

J. Petiau, G. Calas. LOCAL STRUCTURES ABOUT SOME TRANSITION ELEMENTS IN OXIDE

GLASSES USING X-RAY ABSORPTION SPECTROSCOPY. Journal de Physique Colloques, 1982,

43 (C9), pp.C9-47-C9-50. �10.1051/jphyscol:1982909�. �jpa-00222402�

JOURNAL DE PHYSIQUE

Colloque C9, supplément au n°12, Tome 43, décembre 1982 page C9-47

LOCAL STRUCTURES ABOUT SOME TRANSITION ELEMENTS I N OXIDE GLASSES USING X-RAY ABSORPTION SPECTROSCOPY

J . Petiau and G. Calas

Laboratoire de Minéralogie-Cristallographie, Universités Paris VI et VII, 752SO Paris, Franoe et L.V.R.E., 91405 Orsay, France

+

et Ecole Normale Supérieure J.F. , Montrouge, France

Résumé.- La s t r u c t u r e locale autour de c e r t a i n s métaux de t r a n s i t i o n (Cr-Fe-Co-Ni) présents dans des verres s i l i c a t e s ou borates e s t étudiée à p a r t i r des r é s u l t a t s de la spectrométrie d'absorption X. La complémentarité des informations qui peuvent ê t r e obtenues à p a r t i r des o s c i l l a t i o n s "EXAFS" (distance des atomes voisins) et de la s t r u c t u r e du s e u i l (symétrie, é t a t chimique) e s t mise en évidence.

A b s t r a c t . - Absorption X-ray spectroscopy i s used to study the local s t r u c t u r e around t r a n s i t i o n elements (Cr-Fe-Co-Ni) in s i l i c a t e and borate g l a s s e s . The complemen- t a r i t y of EXAFS, giving the neighbour d i s t a n c e s , and of the near edge s t r u c t u r e giving informations about symmetry and chemical s t a t e i s emphasized.

X-Ray absorption spectroscopy i s an adequate method for studying local s t r u c - ture about one d e f i n i t e element in multicompoTient glasses if one uses i t s two complementary aspects, XANES (X-Ray Absorption Near Edge Structure) and EXAFS (Extended X-Ray Absorption Fine S t r u c t u r e ) . The former gives the same type of i n f o r - mations as the other spectroscopic techniques : with the same atomic s e l e c t i v i t y , i t informs about local symmetry and chemical e f f e c t s . On the other hand, EXAFS i s more similar to diffraction methods but i t s atomic s e l e c t i v i t y i s a clear advantage and allows to get informations on short range order around minor components of complex g l a s s e s . Some r e s u l t s are presented as an i l l u s t r a t i o n of the various kinds of i n - formations that can be obtained using XANES and EXAFS. The measurements are made using the synchrotron r a d i a t i o n of LURE-DCI (Orsay) at the K-edges of chromium, i r o n , cobalt and n i c k e l , present as minor components (one to several Wt%) in s i l i c a t e and borate g l a s s e s .

I . - Near-edge s t r u c t u r e (XANES) . I t extends from the beginning of the edge to s e - veral tens e l e c t r o n - v o l t s above. The successive increases of the absorbance in the K-shell ionization threshold region correspond to t r a n s i t i o n s of an Is e l e c t r o n to a more or less bound empty l e v e l . The shape and position of the absorption edge are determined by the o x i d a t i o n - s t a t e of the studied element ("chemical-shift"), the metal-oxygen covalency and the local symmetry (including the s i t e d i s t o r t i o n ) . In fact things are far more complicated than this over simple presentation : many body problems, multiple s c a t t e r i n g have important effects which are not always completely understood.

1 . 1 . - Oxidat^oi^state : About a 6 eV s h i f t between iron K-edge in glasses containing e i t h e r purely divalent or purely t r i v a l e n t iron has already been described (1). In cases of mixing of both oxidation s t a t e s , the s h i f t of the main part of the edge follow t h e i r respective proportion ; taking i n t o account the influence of other parameters on the shape of the edge, i t i s nevertheless probably impossible to a t t a i n a precision b e t t e r than twenty p e r - c e n t .

1.2.- Site^sy_mmetry_ ; The effect of the d i s t o r t i o n of the s i t e is clearly shown on Fig. 1 by the s p l i t t i n g of the upper p a r t of the edge.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982909

C9-48 JOURNAL DE PHYSIQUE

Fig. 1 : Fe(I1) i n o c t a h e d r a l environment 1) Glass (pyroxene composition)

0 . 5 FeO, 1.5 MgO, 2Si02 2) Oxide Fe O1-x

q u a s i r e g u l a r octahedron 3) C r y s t a l l i n e pyroxene

0 . 5 FeO, 1.5 MgO, 2Si02 D i s t o r t e d s i t e (C

2v)

I n t h e case of a c r y s t a l l i n e pyroxene of m e t a s i l i c a t e composition, f e r r o u s i o n s a r e i n a s t r o n g l y d i s t o r t e d s i t e ; i n t h e c o n t r a r y t h e s i t e i s a n e a r l y r e g u l a r octahedron i n f e r r o u s oxide. The curve r e l a t i v e t o t h e g l a s s i s almost f e a t u r e l e s s i n d i c a t i n g an a p p a r e n t l y r e g u l a r s i t e . This f a c t i s observed i n most c a s e s and gives l i m i t a t i o n s on s t r u c t u r a l models t o be chosen f o r t h e s e i o n s i n g l a s s e s .

1.3.- E=3CgCalgpsy : I n oxide s y s tems t h e pre-edge region corresponds t o forbidden d i p o l a r t r a n s i t i o n s from 1s t o 3d p a r t i a l l y empty l e v e l s of t h e t r a n s i t i o n e l e m e n t ; i t s i n t e n s i t y i s t h e n u s u a l l y very weak and i s only enhanced by t h e 3d metal- 2p oxygen o r b i t a l s mixing (quadrupolar t r a n s i t i o n s g i v i n g s c a r c e l y s e v e r a l

of t h e t o t a l a b s o r p t i o n ) .

Three examples o f t h i s e f f e c t a r e p r e s e n t e d :

- f e r r i c ions i n g l a s s e s always e x h i b i t a s t r o n g l y marked p-re-edge peak c o n f o r t i n g t h e model of a & f o l d c o o r d i n a t i o n (Fig. 2 ) .

Fig. 2 : Enhancement of t h e pre-edge peak of F e ( I I 1 ) K-edge i n sodium d i s i l i c a t e g l a s s (2)Vz c r y s t a l l i n e f e r r i c oxide ( 1 ) .

I ^{PEAK :}

- i n well-known c o b a l t c o n t a i n i n g a l k a l i - b o r a t e g l a s s e s i t i s p o s s i b l e t o follow t h e c o o r d i n a t i o n change of c o b a l t a s a f u n c t i o n of t h e a l k a l i n i t y of t h e system by looking a t t h e pre-edge peak amplitude. The same e f f e c t , though l e s s pronounced, i s observed i n n i c k e l corresponding g l a s s e s .

- i n cases of an extremal o x i d a t i o n s t a t e , a s i n t h e molecular complex C r O 4 -- '

t h e pre-edge peak i s very i n t e n s e ; t h e t r a n s i t i o n i s favoured by t h e important

mixing of t h e chromium-3d o r b i t a l s with t h e oxygen - 2p o r b i t a l s having t h e T 2

symmetry ( 2 ) . The amplitude of t h i s pre-peak i n a g l a s s allows t o confirm

t h e presence of d i s c r e t e molecular complexes very s i m i l a r t o those observed i n

c r y s t a l s . The i d e n t i t y of t h e t h e two s p e c t r a (Fig. 3) c l e a r l y show t h a t i n t h i s

sodium b o r a t e g l a s s chromium i s mainly i n hexavalent s t a t e .

A

W

\ ~ i ~ . 3 : K-shell t h r e s h o l d of chromium i n :

2 Q ¹ .- chromic o x i d e (Cr203)

m

[r

2.- potassium chromate (K2Cr04)

8 3 . - sodium d i b o r a t e g l a s s ( 3 w t % Cr).

Q

2.- EXAFS.- From EXAFS o s c i l l a t i o n s w h i c h ENERGIE ,

5975ev extend up t o s e v e r a l hundred eV above t h e edge, i t i s p o s s i b l e t o e x t r a c t i n f o r m a t i o n about t h e s u c c e s s i v e atomic s h e l l s surrounding the observed element. I n t h e case of a t r a n s i t i o n element i n oxide g l a s s e s , i t seems only p o s s i b l e t o measure t h e M-0 d i s t a n c e (3) and t o g e t q u a l i t a t i v e i n d i c a t i o n s concerning i n t e r m e d i a t e range o r d e r . The development of t h e method has l e d t o a b e t t e r comprehension of i t s p o s s i b i l i t i e s but a l s o of i t s l i m i t s , i n p a r t i c u l a r concerning t h e d i r e c t determination of c o o r d i n a t i o n numbers u s i n g EXAFS amplitudes.

Metal-oxygen d i s t a n c e s a r e c a l c u l a t e d by F o u r i e r f i l t e r i n g on both g l a s s e s and r e f e r e n c e c r y s t a l l i n e compounds, using t h e o r e t i c a l p h a s e - s h i f t s ( 4 ) . T y p i c a l F o u r i e r transforms a r e shown on F i g . 4 f o r some Co-containing g l a s s e s . The main

c o n t r i b u t i o n i s given by t h e metal-oxygen peak, which i s t h e only s i g n i f i c a t i v e f e a t u r e i n many oxide g l a s s e s , e s p e c i a l l y i n i r o n case ( 5 ) . O n Fig. 4, f u r t h e r peaks can be seen a t l a r g e r d i s t a n c e s , i n d i c a t i n g a small i n t e r m e d i a t e range o r d e r .

I n s i l i c a t e g l a s s e s , t h i s peak corresponds t o c h a r a c t e r i s t i c c o b a l t - c a t i o n distances observed i n c r y s t a l l i n e s i l i c a t e s . I n b o r a t e g l a s s e s , t h e i n t e r p r e t a t i o n of t h e broad peaks a t l a r g e d i s t a n c e (4-5 A) i s not c l e a r ; t h i s f e a t u r e i s a l s o p r e s e n t i n Ni-borate g l a s s e s and remains s t a b l e when changing F o u r i e r t r a n s f o r m c o n d i t i o n s .

o

- . . .

/ i '..

' J

7 @

(k)

?'

Fig. 4 : Radial d i s t r i b u t i o n around c o b a l t i n t h r e e g l a s s e s :

1 ) CaO, MgO, 2Si02

2) 27 w t % Na20, B203 4 w t % Co 3) 7 w t % NagO, B 0 2 3

The measured d i s t a n c e s can b e used t o d i s c u s s t h e s t r u c t u r a l models of t r a n s i t i o n metals ions i n oxide g l a s s e s .

I f compared t o o c t a h e d r a l case i n c r y s t a l s , we observed always a s h o r t e n i n g

of t h e metal-oxygen d i s t a n c e (Table 1 ) . I n t h e case of f e r r i c i o n , i n sodium

s i l i c a t e g l a s s e s , i t corresponds t o t h e predominance of t e t r a h e d r a l s i t e which i s

coherent w i t h t h e pre-edge peak amplitude. I n c o b a l t - and n i ckel-containing b o r a t e

g l a s s e s t h e s h o r t e n i n g of the d i s t a n c e w i t h t h e i n c r e a s i n g of a l k l i n i t y confirms

C9-50 JOURNAL DE PHYSIQUE

t h e well-known c o o r d i n a t i o n change. The d i s t a n c e measured i n s i l i c a t e g l a s s e s i n d i c a t e a r a t h e r t e t r a h e d r a l c o o r d i n a t i o n , b e i n g s i m i l a r t o t h o s e o b s e r v e d i n Na-rich b o r a t e g l a s s e s .

TABLE 1

o c t a h e d r a l s i t e

o c t a h e d r a l s i t e

However t h e measurement, by EXAFS, of a s h o r t M-0 d i s t a n c e i s n o t , by i t s e l f , a d i r e c t p r o o f o f t h e c o o r d i n a t i o n change ; r a d i a l d i s t r i b u t i o n e f f e c t may l e d t o t h e same r e s u l t s i n c e i t i s now known t h a t EXAFS g i v e p r e f e r e n t i a l i n f o r m a t i o n on s h o r - t e s t d i s t a n c e s . T h i s e f f e c t was v e r i f i e d i n t h e c a s e o f F e ( I 1 ) i n t h e s t r o n g l y d i s t o r t e d s i t e s of t h e pyroxenes, b u t i n t h i s c a s e t h e d i s t a n c e d i s t r i b u t i o n i s a l s o p u t i n e v i d e n c e i n t h e edge s t r u c t u r e , a s opposed t o t h e edge s h a p e s i n g l a s s e s p r e s e n t e d above.

R e f e r e n c e s

1) J . PETIAU, G. CALAS, P. BONDOT, C . LAPEYRE, P. LEVITZ and G. LOUPIAS.

Daresbury Meeting on "EXAFS i n i n o r g a n i c systems1' 127 (1981).

2) F.W. KUTZLER, C.R. NATOLI, D.K. MISEMER, S. DONIACH and

HODGSON.

J. Chem. Phys. 2, (7) 1 (1980).

3 ) D . R . SANDSTROM, F.W. LYTLE, P.S.P. W E I , R.B. GREEGOR, J . WONG and P . SCHULTS.

J. Non C r y s t . S o l i d s 5, 201 (1980).

( 4 ) B.K. TEO and P.A. LEE. - J . Am. Chem. Soc. 101 2815 (1979).

( 5 ) G. CALAS, P. LEVITZ, J . PETIAU, P. BONDOT and G. LOUPIAS. - Rev. Phys. Appl. IS,

1161 (1980).

(6) P . EISENBERGER and G.S. BROWN - S o l i d S t a t e Comm. 481 ( 1 9 7 9 ) .