EXAMINATION OF Ti XANES SPECTRA OF MINERALS AND SOLIDS : EFFECTS OF SITE GEOMETRY ON SPECTRAL FEATURES

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EXAMINATION OF Ti XANES SPECTRA OF MINERALS AND SOLIDS : EFFECTS OF SITE

GEOMETRY ON SPECTRAL FEATURES

G. Waychunas

To cite this version:

G. Waychunas. EXAMINATION OF Ti XANES SPECTRA OF MINERALS AND SOLIDS : EF- FECTS OF SITE GEOMETRY ON SPECTRAL FEATURES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-841-C8-844. �10.1051/jphyscol:19868161�. �jpa-00226065�

JOURNAL DE PHYSIQUE

Colloque C8, supplbment au n o 12, Tome 47, dbcembre 1986

EXAMLNATION OF Ti XANES SPECTRA OF MINERALS AND SOLIDS _: EFFECTS OF SITE GEOMETRY ON SPECTRAL FEATURES

G

.

Center for Materials Research, 105 McCullough Building, Stanford University, Stanford, C A 94305-4045, U.S.A.

Abstract - The X-ray absorption near edge s t r u c t u r e of Ti in 16 oxides and s i 1 ic a t e s has been col lected and analyzed by computer f i t t i n a Gaussian 1 ines to the spectral envelopes. Comparison of feature i n t e n s i t i e s and energies with geometric s i t e parameters reveals t h a t the near edge changes system- a t i c a l l y with s i t e symmetry variation and mean Ti-0 bond length.

I INTRODUCTION

The X-ray absorption near edge s t r u c t u r e (XANES) of Ti i n insulating materials and minerals shows considerable variation. However, there has been l i t t l e e f f o r t made t o r e l a t e t h e changes observed t o v a r i a t i o n s i n the geometry of t h e Ti coordination polyhedron. This i s of p a r t i c u l a r importance i n the c l a r i f i c a t i o n of near edge absorption processes. Multiple s c a t t e r i n g resonances should present spectral signatures which d i f f e r from bound s t a t e or multi-electron t r a n s i t i o n s i n response t o varied s i t e symmetry or metal-ligand bond length. I t i s a l s o of i n t e r e s t i n geochemistry because Ti mineral s i t e partitioning could be d i r e c t l y determined from l e a s t squares f i t t i n g of the near edge envelope. Such partitioning information i s useful f o r complete characterization of the mineral s t r u c t u r e as well as the application of geochemical models.

In t h i s work the Ti near edge spectral features have been examined i n 16 d i f f e r e n t s t r u c t u r e s by computer f i t t i n g of Gaussian l i n e shapes t o the spectral envelopes.

The resulting feature energies and i n t e n s i t i e s have then been compared with s i t e fleometry parameters.

I1 Ti XANES General Features

The ~ iXANES of ~ + 9 representative spectra a r e shown i n f i g u r e 1. All of the Ti s i t e s are octahedral, but the s i t e symmetry varies between mmm and 1. The spectra have up t o 9 discernable features between 4960 and 5010 eV. These consist of three in the pre-edge region, best evident in the oxide spectra (ilmenite, anatase and r u t i l e ) , three on the f i r s t edge upslope and maximum which vary i n position and i n t e n s i t y , and three on the second edge maximum best seen in the benitoite and anatase spectra. In most spectra the features on the second main edge maximum a r e not resolved.

In benitoite the Ti s i t e has point symmetry 32 with s i x equal Ti-0 bond distances.

Only two pre-edge features a r e observed, both very weak. Also only two f i r s t edge maximum features a r e present. In ilmenite, the Ti s i t e i s more distorted than i n

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868161

C8-842 JOURNAL DE PHYSIQUE

b e n i t o i t e , b u t r e t a i n s t h e t h r e e f o l d symmetry a x i s and has o n l y two d i f f e r i n g bond distances. I t appears t h a t t h e main edge c r e s t has s p l i t i n t o two f e a t u r e s i n t h i s spectrum, and t h a t t h e t h i r d pre-edge feature has a r i s e n . I n t h e Ti-bearing garnets, t h e T i s i t e has 3 symmetry and t h e spectra a r e v e r y s i m i l a r t o t h a t o f i l m e n i t e . The garnet space group p e r m i t s o n l y one T i - 0 bond distance, b u t because a l l o f t h e garnets examined a r e a c t u a l l y s o l i d s o l u t i o n s i n several components, one expects some bond-distance v a r i a t i o n from s i t e t o s i t e i n t h e s t r u c t u r e . I n t h e s e r i e s o f spectra i l m e n i t e - Z r T i garnet-Magnet Cove schorlomite-San B e n i t o m e l a n i t e t h e r e l a t i v e i n t e n s i t y o f t h e f e a t u r e near 4988 eV increases. Also t h e i n t e n s i t y o f t h e f e a t u r e near 4977 eV and t h e pre-edge feature a t 4970 eV increases along t h i s s e r i e s .

ENERGY (eV)

Fig.l Normalized T i XANES spectra o f r e p r e s e n t a t i v e species.

BOND LENGTH VARIANCE (A2)

m 2 Octahedron bond l e n g t h variance i n 12 species p l o t t e d a g a i n s t pre-edge f e a t u r e i n t e n s i t y .

The b i o t i t e and k a e r s u t i t e T i s p e c t r a seem t o show a c o n t i n u i n g o f t h i s trend. The f e a t u r e a t 4988 eV s h i f t s t o lower energy and i n t e n s i f i e s f u r t h e r , becoming t h e edge maximum, w h i l e t h e f e a t u r e a t 4983 eV a l s o s h i f t s t o lower energy and weakens considerably. The pre-edge f e a t u r e a t 4970 eV increases i n i n t e n s i t y b u t s h i f t s h a r d l y a t a l l i n energy. The T i s i t e s i n these m i n e r a l s have symmetries 2 and 2/m ( b i o t i t e has 2 s i t e s , k a e r s u t i t e 3 ) and t h e s i t e s are r e l a t i v e l y more d i s t o r t e d than i n t h e o t h e r species studied. Anatase has ?2m T i s i t e symmetry and a much d i f f e r e n t near edge spectrum, somewhat l i k e b e n i t o i t e . I t ' s T i s i t e has two d i f f e r i n g bond distances.

I 1 1 Pre-edge Feature--Structure C o r r e l a t i o n s

I n f i g u r e s 2 and 3 t h e pre-edge f e a t u r e a t 4970 eV, which i s due t o a 1s-3d e l e c t r o n i c t r a n s i t i o n , i s p l o t t e d a g a i n s t bond l e n g t h v a r i a n c e and bond angle variance, r e s p e c t i v e l y , f o r most o f t h e species studied. There i s l i t t l e c o r r e l a t i o n between t h e pre-edge feature i n t e n s i t y and bond 1 ength v a r i a n c e no matter how t h e data p o i n t s a r e subdivided. On t h e o t h e r hand t h e r e i s good

0-Ti-0 BOND ANGLE (deg2) VARIANCE

30

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W

NEPTUNITE KAERSUTITE

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1.940 1960 1.980 2.000 2.020 2.040 2.060 2.080 MEAN TI-0 DISTANCE ( I )

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F i g . 3 1s-3d i n t e n s i t y versus octahedron Fig.4 1s-3d i n t e n s i t y versus mean T i - 0

bond angle variance. bond distance.

c o r r e l a t i o n o f t h e bond angle variance w i t h pre-edge f e a t u r e i n t e n s i t y among t h e s i l i c a t e s . Bond angle d e v i a t i o n from 90" i n an octahedron should r e s u l t i n increased 3d-4p m i x i n g i n t o t h e f i n a l s t a t e of t h e pre-edge e l e c t r o n i c t r a n s i t i o n , g i v i n g r i s e t o g r e a t e r a b s o r p t i o n p r o b a b i l i t y /I/. Bond l e n g t h v a r i a t i o n should have l i t t l e o r no e f f e c t s i n c e i t i s o n l y i n d i r e c t l y r e l a t e d t o o r b i t a l mixing.

I n f i g u r e 4 t h e 1s-3d f e a t u r e i n t e n s i t y i s p l o t t e d a g a i n s t t h e average T i - 0 bond distance. Again t h e r e i s c o r r e l a t i o n f o r t h e s i l i c a t e s . A s i m i l a r p l o t f o r t h e oxides shows l i t t l e c o r r e l a t i o n . A r e l a t i o n between s i t e s i z e and r e l a t i v e s i t e d i s t o r t i o n has been observed i n ~ e 2 + m i n e r a l s 1 2 1 such t h a t w i t h i n c r e a s i n g s i z e f o r a given c o o r d i n a t i o n number, t h e s i t e i s i n c r e a s i n g l y d i s t o r t e d . T h i s r e l a t i o n s h i p t i e s t o p e t h e r t h e trends seen i n f i g u r e s 3 and 4 f o r t h e s i l i c a t e T i s i t e s . The d i f f e r e n c e s between the s i l i c a t e s and oxides may be due i n p a r t t o t h e e f f e c t o f t h e s i l i c a t e bond on oxygen c o n t r i b u t i o n s t o t h e T i - 0 molecular o r b i t a l s . I V Near edge Feature--Structure C o r r e l a t i o n s

F i g u r e 5 shows t h e energies of t h e t h r e e features on t h e f i r s t main edge absorption neak p l o t t e d versus t h e 1s-3d feature i n t e n s i t y . There i s a smooth progression f o r a l l o f t h e s i l i c a t e samples, suggesting t h a t t h e near edge spectrum i s comprised o f t h e same f e a t u r e s i n a l l cases, b u t s h i f t e d i n energy according t o t h e d i s t o r t i o n and concurrent bond d i tances. The s h i f t i n energy i s s i m i l a r i n magnitude t o t h a t observed f o r

el'

V ~ versus Ti3+ Spectra i ~ +

I n ~ s p e c t r a t h e 1s-3d f e a t u r e i s about 2.0 eV h i g h e r i n energy than i n i ~ + ~ i ~ + spectra. Comparisons between Ti02 and Ti203 (corundum s t r u c t u r e ) , and between

~ a ~ e ~ + S i ~ o ~ : T i 4 + and N ~ T ~ ~ + S ~ ~ O ~ a l s o suggest t h a t t h e r e i s a chemical s h i f t o f the bound s t a t e absorption between oxides and s i l i c a t e s , and comparisons among s i l i c a t e s suggest t h a t t h e r e may be a s h i f t depending on t h e type o f s i l i c a t e p o l y m e r i z a t i o n 131.

JOURNAL DE PHYSIQUE

TITANIUM FEATURE ENERGY (eV) C 25

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Fig.5 S h i f t o f ~ near edge f e a t u r e s i n s i l i c a t e s versus pre-edge i ~ +

f e a t u r e i n t e n s i t y . The two lower energy f e a t u r e s behave as expected f o r mu1 ti p l e s c a t t e r i n g resonances.

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V I CONCLUSIONS

4974 4976 4978 4980 4982 4984 4986 4988 4990

i ) The XANES spectra of ~ ii n mineral s i l i c a t e s and oxides c o n s i s t o f a w e l l ~ + d e f i n e d s e t o f f e a t u r e s which v a r y i n energy and i n t e n s i t y as a f u n c t i o n o f s i t e geometry. Lower symmetry r e s u l t s i n s p l i t t i n g o f one o f t h e edge f e a t u r e s i n t o two i n d i v i d u a l s , and lowered r e s o l u t i o n o f o t h e r f e a t u r e s normally v i s i b l e on t h e second edge maximum.

ii ) The T i near edge f e a t u r e s s h i f t t o lower energy w i t h i n c r e a s i n g bond distances and s i t e d i s t o r t i o n , these being s t r o n g l y c o r r e l a t e d i n s i l i c a t e s .

i i i ) The 1s-3d f e a t u r e i n t e n s i t y i s s t r o n g l y c o r r e l a t e d w i t h bond angle variance i n s i l i c a t e s t r u c t u r e s , w h i l e t h e r e appears t o be no s i m i l a r c o r r e l a t i o n f o r oxides. This i s probably due t o d i f f e r e n c e s i n t h e oxygen c o n t r i b u t i o n s t o t h e f i n a l s t a t e molecular o r b i t a l .

i v ) Th I s 3d f e a t u r e s h i f t s about 2.0 eV upward i n energy i n going from ~ t o i ~ + Tip+ spectra. Smaller s h i f t s a r e observed between s i l i c a t e s and oxides and

among v a r i o u s s i l i c a t e s o f d i f f e r i n g polymerization. These v a r i a t i o n s must a l s o be due t o t h e changing c o n t r i b u t i o n s t o t h e T i - 0 molecular o r b i t a l s from t h e 0 bonded t o d i f f e r i n g s i l i c a t e s t r u c t u r a l u n i t s .

ACKNOWLEDGEMENTS

Data c o l l e c t i o n a t SSRL was a s s i s t e d by M. Apted, G.E. Brown,Jr., D. McKeown, C. Ponader and J. Pressesky. The research was supported by t h e NSF-MRL program through t h e Center f o r M a t e r i a l s Research a t Stanford U n i v e r s i t y .

REFERENCES

/1/ Roe,A.L., Schneider,D.J., Mayer, R.J., Pyrz,J.W., Windom,J. and Que, L. ,Jr., J. Amer. Chem. Soc.

106

/2/ Goldman,D.S. and Rossman,G.R., Phys. Chem. Minerals, t o be published.

/3/ Waychunas ,G.A., Amer. M i n e r a l o g i s t , t o be pub1 ished.

/4/ Waychunas,G.A., Apted,M.J. and Brown, G.E. ,Jr., Phys. Chem. Minerals

10