ON THE BASIC STRUCTURAL UNIT OF AMORPHOUS V2O5 FROM XANES AND EXAFS

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ON THE BASIC STRUCTURAL UNIT OF

AMORPHOUS V2O5 FROM XANES AND EXAFS

S. Stizza, M. Benfatto, I. Davoli, G. Mancini, A. Marcelli, A. Bianconi, M.

Tomellini, J. Garcia

To cite this version:

ON T H E B A S I C STRUCTURAL U N I T OF AMORPHOUS V 2 0 5 FROM XANES AND EXAFS

S . S t i z z a , M. B e n f a t t o , I . Davoli, G . Mancini, A . M a r c e l l i , A . ~ i a n c o n i *

,

M. ~ o m e l l i n i * and J . ~ a r c i a *

Dipartimento d i Matematica e F i s i c a , U n i v e r s i t a d i Camerino, 1-62032 Camerino, I t a l y

*Pipartimento d i F i s i c a , U n i v e r s i t a "La Sapienza", 1-00185 Roma, I t a l y

RBsum6

-

Des B c h a n t i l l o n s de pentoxyde de vanadiumamorphe o n t B t B BtudiBs p a r XANES. Les s p e c t r e s o n t B t B a n a l y s e s B p a r t i r de modeles de d i f f u s i o n m u l t i p l e . Des s t r u c t u r e s l o c a l e s en forme de pyramides t e t r a g o n a l e s semblent e x i s t e r . La c r i s t a l l i s a t i o n p a r chauffage a a u s s i B t B Btudiee e t d e s d i s t o r s i o n s de l a c e l l u l e de base a u cours de l a t r a n s f o r m a t i o n o n t B t B mises en evidence.

Absfract

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XANES ( X rsy absorption near edge structure) spectra of rapid-quenched amorphous vanadium pentoxide have been measured using synchrotron radiation. The spectra have been analyzed with the multiple scattering theory.

Evidence for tetragonal pyramids has been found. The transition from glass to crystalline state with temperature has been Studied and the small distortions of the besic structural unit related with the transition hes been found.

Vanadium pentoxide i s a component of semiconducting oxide glasses which exhibit hopping conductivity. Amorphous V205 can be obtained by rapid-quenching techniques. The basic structural unit of this glass i s not well established(') i n fact i n the f i r s t X ray difraction work(2) the interatomic distance V-0 was found t o be 1.85

A,

on the contrary large angle X ray scattering ( 3 ) experiment found the V-0 distance 1.75

A

and on the basis of the pair correlation function the hypothesis of tetrahedra sharing corners as structural units of this glass has been advanced. A

similar controversy concerns the basic structural units of liquid vanadium p e n t o ~ i d e . ( ~ ~ ~ )

C8-256 JOURNAL DE PHYSIQUE

Here we report the study of the basic structural unit of V205 glass by XANES (x ray absorption near edge structure) and EXAFS. XANES spectroscop has been used to study local structure i n vanadium phosphate glasses (6-8Y and vanadium oxide crystalline compounds(g). XAND spectroscop probes the local geometry via the photoelectron multiple scattering ?lo) The total absorption cross section can be factorized in an atomic part

a,

and i n a part due t o the modulations of the multiple scattering processes w i t h neighbouring atoms. In principle a l l multiple scattering pathways which begin and end at the vanadium site ,classified according w i t h the number of scattering 'events from n=2 (EXAFS) to

n=3,4,5, .... contribute t o XANES i n the f u l l multiple scattering region. In the energy range where the expansion i s possible ( I I ) the total cross section i s given by

Theref ore XANES provides informat ion on higher correlation functions and joint w i t h EXAFS,which gives the pair correlation function are an ideal method to establish the local structure in glasses.

II

-

EXPERIMENTAL METHODS

EXAFS and XANES spectra were recorded at the Frascati synchrotron radiation f a c i l i t y uslng the storage ring Adone operated at 50 mA and 1.5

GeV. A Si(220) channel-cut crystal was used as monochromator w i t h a 0.5 mm exit s l i t and the resolution at V K-edge was about .8 eV. The samples were prepared at GdanSk Technical Untversity by rapid quenching technique. The experiment has been repeted several time and the crystallization of glasses has been realized by heating treatement.

In fig. 1 we report the XANES spectra of V205 glass of the same sample at different temperatures.Above ~ c = 2 2 0 O the amorphous sample crystallizes and the XANES spectrum becomes the characteristic spectrum of crystalline V205 (7'9). The main variation i n the spectra appears a t the f i r s t two features after the absorption jump edge. The EXAFS spectra are shown i n fig.2 where i n the upper (lower) panel the EXAFS of the crystalline (amorphous) spectrum i s reported. EXAFS analysis has been carried out and average V-0 distance has been found to remain at about

1.85

A

,the second shell a t about 2.8

A

shows a contract~on o f 0.02

fl

In

Fig. 1

-

XANES spectra of vanadium pentoxide glass at various temperatures. The intensity scale of the spectra above the crystallization temperature at 220°C has been changed.

amorphous V,O,

C8-258 JOURNAL DE PHYSIQUE

0 1 5 -

0.10

-

0 0 5 -

ENERGY (Ry) F=.2

Fig.3 Polarized theoretics1 multiple scattering calculations of vsnediurn XANES for a

tetrsgonal pyramidcluster. In the left (right) panel the spectrum with the polarization vector along the pyramid axis (basis) are reported.

The fact the EXAFS spectra are very similar i s an indication that the pair distribution function i s only l i t t l e affected by the transition from the crystal to the amorphous state and therefore the amorphous oxide i s formed by layers of tetragonal pyramids as in crystalline V205 . The differences observed in the XANES between the two forms i s therefore due to a small variation of the site structure geometry. In the crystal there i s short double bond of 1.58

A

and the atoms on the basis of the pyramid are at 1.78

A,

1.87A, 1.87

A,

2.02

A;

a sixth atom is at 2.8

A.

We have performed multiple scattering calculations of the XANES spectra of the structural unit of vanadium pentoxide. We have found a good f i t of the experimental XANES spectrum of the glass w i t h a cluster where all atoms

on

the pyramid basis are at the same distance. The f i r s t

peak

after the absorption jump has been found to be due to scattering in the plane of the pyramid base, and therefore the main variations between glass and crystal are due to a rearrangement of the atoms of the basis in a less distorted way i n the glass than in the crystal. The white line has been found to be due to the multiple scattering pathways along the axis of the pyramid and therefore depends on the positions of the atoms at 1.58

A,

Fig. 4 Total theoretical absorption spectrum for the tetragonal piremid cluster i n agreement with the experimental XANES spectrum of amorphous V205.

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