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DETERMINATION OF STRUCTURE AND VALENCE IN VANADIUM BRONZE BY X-RAY
ABSORPTION SPECTROSCOPY
F. Lytle, R. Greegor, I. Raistrick
To cite this version:
F. Lytle, R. Greegor, I. Raistrick. DETERMINATION OF STRUCTURE AND VALENCE IN VANA- DIUM BRONZE BY X-RAY ABSORPTION SPECTROSCOPY. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-719-C8-723. �10.1051/jphyscol:19868135�. �jpa-00226035�
JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment au n o 12, Tome 47, d6cembre 1986
DETERMINATION OF STRUCTURE AND VALENCE IN VANADIUM BRONZE BY X-RAY ABSORPTION SPECTROSCOPY
F.W. LYTLE, R.B. GREEGOR and I.D. RAISTRICK'
The Boeing Co. , Seattle, W A 98124, U. S.A.
" L O S Alamos National Laboratory, Los Alamos, N M 87546, U.S.A.
ABSTRACT
We used XANES t o determine c o o r d i n a t i o n geometry and charge s t a t e i n a s e r i e s o f mixed valence vanadium bronzes, NaXV2O5 where x = 0.25, 0.40, and 0.75. Simple chemical arguments suggest t h a t each a d d i t i o n a l p o s i t i v e charge added t o t h e V205 l a t t i c e w i l l s w i t c h one v5+ t o v4+. Our r e s u l t s sugaest t h a t t h i s i s g e n e r a l l y t r u e . The energy p o s i t i o n and s p l i t t i n g o f t h e V K-edge orepeak, Is-to-3d, was a s e n s i t i v e i n d i c a t o r o f V valence(s). The i n t e n s i t y o f the orepeak decreased as t h e f i r s t neighbor bonds increased i n length.
18TRODUCTION
I n V205 the staggered sheets o f d i s t o r t e d oxynen octahedra o f f e r many p o s s i b i l i t i e s f o r preparing unique s u b s t i t u t i o n a l and i n t e r s t i t a l compounds. The vanadium bronzes, MxV205 o r MxTyV2-y05 where M i s a u n i v a l e n t metal and T i s Mo o r W have added i n t e r e s t due t o a mixed V valence, i . e . , i n order t o accommodate the charge o f the a d d i t i o n a l c a t i o n s v5+ apparently must change t o V 4+ . Many s t r u c t u r e s have been determined by x-ray d i f f r a c t i o n ( l , 2 ) ; however, the assignment o f valence has been p r i m a r i l y by chemical arguments(3). Extensive research i n t h i s area and a review have been g i v e n by Hagenmull e r (4).
For the present work we used x-ray a b s o r p t i o n spectroscopy (XAS) t o d i r e c t l y observe t h e valence s t a t e s i n a few examples o f these compounds. The XAS o f t h e compounds o f V i s p a r t i c u l a r l y i n t e r e s t i n q and v a r i a b l e because o f the wide range o f s t r u c t u r e and valence found i n i t s compounds. I n a p r e v i o u s p u b l i c a t i o n ( 5 ) a l a r g e number of V reference compounds were examined i n o r d e r t o understand and s y s t e n a t i z e the XAS o f V. Hsieh, e t a1.(6) and Tang, e t a1.(7) have used s i m i l a r arguments t o i d e n t i f y valence s t a t e s i n VOx and WxVl-x02, r e s p e c t i v e l y .
The s t r u c t u r e and i n t e r a t o m i c distances f o r Na.33V205-B were o r i g i n a l l y determined by Wadsley (1 ). He showed how oxygen-coordinated V i o n s w i t h d i s t o r t e d f i v e - a n d - s i x - f o l d polyhedra were l i n k e d i n t o stagflered sheets t o form the m a t r i x o f t h e s t r u c t u r e .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868135
C8-720 JOURNAL DE PHYSIQUE
There a r e three inequivalent V s i t e s . The Na adatoms were found in s p e c i f i c s i t e s in the tunnels between the sheets. From consideration of these s t r u c t u r e deter- minations Goodenough(3) nade plausible arguments f o r the various valence assignments and s i t e occupancies. A key arguaent was t h a t the p a r t i c u l a r charge of each atom was localized (a small polaron) and not contributing t o an overall i t i n e r a n t band structure.
EXPERIMENT
Finely ground powders of the samples . 4 , . 75V205) and reference compounds were c a s t in Duco cement to thicknesses of approximately one absorption length.
Data were obtained on wiggler beam l i n e s a t Stanford Synchrotron Radiation Labora- tory during dedicated running conditions. A double crystal Si (220) monochromator was used detuned from the p a r a l l e l case as necessary t o redect higher harmonics.
The entrance s l i t was s e t a t 1 mm, which was a v e r t i c a l divergence of 5 x
radians. For a l l cases t h i s 1 imited the energy bandwidth A E of the monochromator t o 0.4 eV a t the V K-edge. Data of excellent quality was obtained i n the normal absorption mode by multiple pass averaging and with f i n e steps i n the neighborhood of the K-edge. In a1 1 cases the zero of energy was defined by the f i r s t i n f l e c t i o n point of the pure metal.
XANES ANALYSIS
I t was demonstrated i n the work of \dong, e t a7. (5) t h a t i n V compounds the prepeak i n t e n s i t y and position were predictive of the V coordination geometry and valence, respectively. Specifically, the energy position of the ore-edge peak s h i f t e d by approximately +1.1 eV when v4+ changed t o v5+. The i n t e n s i t y of the pre-peak i s an indication of the degree of asymmetry of the f i r s t coordination sphere. The V K- edge involves ionization of a I s electron, hence the normal dipole selection r u l e favors t r a n s i t i o n s to empty p-states or s t a t e s t h a t are similar t o p-states from the perspective of the V s i t e . For 3d elements w i t h oxygen coordination t h i s occurs by mixing of empty 3d o r b i t a l s with oxygen 2p o r b i t a l s ( 5 , 8 ) . Changes i n the V-0 bond distance change the degree of mixing, hence the i n t e n s i t y of the pre-edge peak(5).
McQuillan(9) has described elegantely the t r a n s i t i o n r u l e s f o r some V compounds in the language of group theory and given some V spectra. The strong dipole t r a n s i t i o n i s allowed only when the coordination of the absorbing aton lacks inversion symmetry as in t e t r a h e d r a l , pentagonal, or d i s t o r t e d octahedral coordination. These geo- metries a r e common f o r high valence V conpounds. In (5) a theory was deyeloped with reference to extensive experimental data t h a t showed the i n t e n s i t y of the prepeak was dependent upon the s i z e and d i s t o r t i o n of the 'molecular c a g e ' , a concept t h a t had been previously developed from a theoretical calculation by Kutzler, e t a1.(8).
Specifically, the experimental data showed t h a t the i n t e n s i t y of the V prepeak i s
I U. [0.74(1.39
R ~ ) '
exp (1R1 i s the s h o r t (vanadyl) bond and R2 i s t h e average o f the o t h e r bonds i n t h e f i r s t c o o r d i n a t i o n sphere. The numerical c o e f f i c i e n t s were obtained by f i t t i n g t o t h e data from the V reference compounds. Equation ( 1 ) shows t h a t the prepeak i n t e n s i t y increases i n v e r s e l y t o t h e l e n g t h o f the V-0 bonds and t h a t the e f f e c t o f a v e r y s h o r t bond i s d i s p r o p o r t i o n a t e t o a l l the others.
Examples o f V XANES a r e g i v e n i n f i g u r e 1. The apparent change i n i n t e n s i t y and s p l i t t i n g o f t h e prepeak i n the Na,V205 compounds becomes e v i d e n t i n t h e expanded scale o f f i g u r e l a . The e f f e c t s a r e due t o the lengthening o f V-0 bond distances and the mixed valence o f V, r e s p e c t i v e l y . We devised an a n a l y s i s schene t o separate the valence components o f the prepeak. A s i n p l e s u b t r a c t i o n produced t h e r e s u l t shown i n f i g u r e 2. A t t h e t o p i s t h e spectrum from Na 4Vp05,shifted downwards f o r c l a r i t y i s t h a t o f V205, both normalized t o u n i t edqe jump. The d i f f e r e n c e spectra as i n c r e a s i n g f r a c t i o n s o f V205 were s u b t r a c t e d a r e shown below. As the s u b t r a c t i o n was c a r r i e d t o t h e expected s t o i c h i o m e t r y t h e l a r g e s t remaining f e a t u r e was a t t h e c o r r e c t energy t o be t h a t o f v4+. By p r o j e c t i n g t h e energy p o s i t i o n o f t h e V205 prepeak, t h e dashed l i n e , t o t h e d i f f e r e n c e spectra i t was observed t h a t t h e r e - maining h i g h e r energy prepeak f e a t u r e was due t o a r e s i d u e o f t h e v5+ prepeak which was n o t p r e c i s e l y a l i g n e d w i t h t h a t o f V205. This f e a t u r e was removed simply by s h i f t i n g t h e V205 spectrum by +0.2 eV b e f o r e t h e s u b t r a c t i o n . I n t h i s way w i t h 1 ) s l i g h t v a r i a t i o n i n the energy p o s i t i o n and 2) w i t h s u b t r a c t i o n o f V205 f r a c t i o n s i n t h e neighborhood o f t h e expected stoichometry, the v4+ spectrum from each sample was i s o l a t e d . The c r i t e r i o n f o r an optimum s u b t r a c t i o n was t h a t t h e v5: prepeak be removed b u t n o t go negative. For these spectra values near b u t s l i g h t l y l e s s than t h e expected stoichometry produced c l e a n v4+ spectra i n d i c a t i n g t h a t the simple chemical argument (3) f o r change i n valence i s v a l i d .
We r a t i o n a l i z e t h i s simple a n a l y s i s scheme by nature o f the comparable V s i t e s i n V205 and Na,V205 and by v i r t u e o f o u r previous a n a l y s i s o f the V prepeak(5). The
~ 5 + s i t e i s s i m i l a r i n a l l t h e compounds and, moreover, smeared by the i n e q u i v a l e n t V s i t e s . Therefore we expect the spectra t o be s i m i l a r . The v a r i a t i o n o f the pre- peak i n t e n s i t y can be estimated f r o m t h e known s t r u c t u r e s o f V205 and Na.33V205 u s i n g Eq. (1). From Eq. (1) the p r e d i c t e d i n t e n s i t y o f t h e prepeak i n V205 and the Yl, V2 and V3 s i t e s i n Na.33V205 i s 1.0: .88: .82: .53 , r e s p e c t i v e l y , o r 1.0: .84 on average. Note t h a t t h e prepeak f o r Na.25V205 i s approximately 80% o f t h a t of '2'5-
I n conclusion we have observed two e f f e c t s of the a d d i t i o n o f ~ a + i o n s t o V205: 1 ) s p l i t t i n g o f the 1s-to-3d prepeak due t o v4+ and v5+ and 2) an o v e r a l l decrease i n i n t e n s i t y o f the prepeak due t o lengthening of t h e f i r s t neighbor bonds.
ACKNOWLEDGMENT
The research o f L y t l e and Greegor was supported by ONR. Lie are g r a t e f u l f o r beam time a t SSRL.
JOURNAL DE PHYSIQUE
Figure 1. XANES of V and Na,V205 Compounds. l a ) Expanded View of P r e ~ e a k .
Figure 2. Subtraction of V205 XANES From Na. 4V205. Difference Spectra Shown a t t h e Bottom.
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