VANADIUM SITE STRUCTURE IN V2O5 GEL BY POLARIZED EXAFS AND XANES

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VANADIUM SITE STRUCTURE IN V2O5 GEL BY POLARIZED EXAFS AND XANES

S. Stizza, M. Benfatto, A. Bianconi, J. Garcia, G. Mancini, C. Natoli

To cite this version:

S. Stizza, M. Benfatto, A. Bianconi, J. Garcia, G. Mancini, et al.. VANADIUM SITE STRUCTURE

IN V2O5 GEL BY POLARIZED EXAFS AND XANES. Journal de Physique Colloques, 1986, 47

(C8), pp.C8-691-C8-696. �10.1051/jphyscol:19868130�. �jpa-00226030�

VANADIUM

SITE STRUCTURE

POLARIZED

S .

STIZZA", M. BENFATTO*',

B I A N C O N I * " ,

G A R C I A * * +

, ^G. MANCINI* and C.R. NATOLI'

" ~ i p a r t i m e n t o di Matematica e Fisica, ~ n i v e r s i t s di Camerino, 1-62032 Camerino, Italy

*

Dipartimento di Fisica, ~ n i v e r s i t s di Roma "La Sapienza", I-00185 Roma, Italy

'Laboratori Nazionali di Frascati, I-00044 Frascati, Italy

absfract:

The local structure at the

site is investigated by polarized X-ray absorption spectroscopy in vanadium pentoxide gel V2051 .6H20. Polarized absorption spectra allows to separate experimentally the contribution of neighbour atoms in different directions. EXAFS analysis were performed using spherical wave aproximation (SWA). When polarization vector E is in the direction of the axis of the pyramid, EXAFS analysis gives only the short double bond lenght at 1.58 A, but XANES indicates the aditionalpresence of the oxygen at 2.7A.

The vanadium pentoxide gels are an interesting class of xerogel materials because of their semiconducting properties and of their applications for switching devices and antistatic coatings.') We have investigated the vandium sites structure of V205 hydrated gel solution and of spontaneous dehydrated solid with approximate formula V205.1 .6 H20. The structure of this solid xerogel has been investigated by several techniquesB4) and it appears to be formed by layers of square pyramids with interlamellar spacing of 11.5 A. The interlamellar space is occupied by water molecules.

There is a large interest to solve the local stucture in amorphous and liquid vanadium pentoxide,5r6) in complex glasses7'9) and in catalysts1 O) containing V2O5. X-ray absorption spectroscopy provides a unique method for vanadium site structure determination but severe difficulties have to be faced in order to extract a detailed picture of vanadium site. In fact vanadium site structure in these materials is very asymmetric with large variations of the V-0 distances giving destructive interference effects in the EXAFS range. Moreover, the backscattering amplitude for oxygen atoms rapidly decreases with increasing the photoelectron wavector k which confines the main WAFS oscillations a1 low k values.

Permanent address : Department of Thermology. University of Zaragoza. Zaragoza. Spain

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868130

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For a correct interpretation of vanadium spectra in these systems it is necessary to extend the EXAFS analysis in the low k region where the spherical waves (SW) analysis is required because of the failure of plane wave approximation (PWA). Moreover, the XANES spectra have to be interpreted in order to get direct information on geometrical distortions. Finally, because of the large asymmetry, polarized absorption spectra is necessary to separate experimentally the contributions of neighbour atoms in different directions. In fact, using polarized spectra it is possible to detect angular resolved scattering pathways starting from the absorbing atom, both in the single scattering regime (EXAFS) and for multiple scattering (XANES).

Here we have investigated the angular resolved EXAFS and XANES of large oriented layers of xerogel. The site structure of

is formed by four oxygen atoms on the corners of the square basis on the layer plane, one oxygen with the shortest double bond along the normal n to the layer plane and a oxygen atom on the opposite site which belongs to a water molecule. When the polarization vector E is in the direction of the axis of the pyramids, EXAFS analysis gives only the short double bond lenght at 1.58 A but XANES indicates the additional presence of the water oxygen at

A. When the polarization vector is parallel to the layer plane, EXAFS analysis gives the distance for nearest neighbour oxygens and for the second shell of vanadium atoms. XANES analysis for the this last polarization indicates the distortion of the square pyramids.

EXAFS data have been analysed using the spherical wave approximation (SWA)~ which allows the analysis also in the k range 1-6 A-1 and XANES spectra were interpreted with the multipie scattering theory 2,

EXPERIMENTAL

Polarized XANES and EXAFS spectra have been measured at the Frascati synchrotron radiation facility using the storage ring Adone operated at 50 mA and 1.5 GeV. we have used a Si(l11) two reflection monochromator and a 0.5 mm entrance slit. A thin film of dehydrated xerogel, of

X 20 mm surface area, on a mylar window was obtained from spontaneous evaporation of the solution. The spectra were measured in transmission mode and the sample was mounted in such a way that the incidence angle

wuld be varied from o0 ^{to 70°.}

RESULTS AND DlSCUSSlON

The absorption spectra of oriented

1.6

sample obtained by changing the

angle

between the electric field E and the axis of the pyramids are reported in fig. 1. The spectra exhibit

large variations with the change of a. The intensity of the white line decreases toward zero for E l z

and it is maximum for Ellz.

FIG.l Vanadium polarized K-XANES of oriented V205 1.6 H20 for different angles u between the electric field E and the axis of the pyramids oriented along the z ^axis c.i=20~,30~,50~,70~and 90'.

In fig. 2 the polarized EXAFS signal for Ellz is reported. The conventional analysis by Fourier transform gives only a strong peak indicating a single coordination shell. The experimental spectrum has been fitted using a spherical wave aproximation (SWA) and theoretical phase shifts1 I ) . ~ h e best-fit obtained with the parameters R(V=O) 11.58 A, coordination number n=l and Debye-Waller factor n2=0.002 A*, is superimposed to the experimental spectrum. From this analysis there is no evidence for the expected oxygen long bond (R=2.7 A). The polarized E l z EXAFS spectrum is shown in figure

We observe in its Fourier transform the presence of two main peaks correspondig to two different coordination shells.

We have isolated the oscillation corresponding to the first coordination shell by back-Fourier transform of the first peak and by means of the fitting with theoretical curve wave approximation we have found that vanadium is coordinated by four oxygen atoms, .two at a distance of 1.87 A

(~2=0.002 A2) and other oxygen atoms at distances 1.78 and 2.02 A as in the crystalline V205.

The EXAFS signal due to the two oxygens at the shortest and longest distances vanish for k>6 A-l

because a destructive interference occurs in this region. The second peak located at about

A

indicates the presence of vanadium scatterers. The simulation obtained by using for the first

coordination shell the determined parameters and for the other shell with

vanadium atoms at a

distance of 3 A and 02-0.08 A2 agrees with the experimental data. The comparison between the

experimental spectra with this last fit and their Fourier transform are shown in figure 3.

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Fig.2

Polarized EXAFS spectrum for Ellz of xerogel and its fitt using the SWA with a single oxygen at 1.58 A . ~ h e Fourier of experiment (dotted) and of the fitt are r e p o r t e d o n top.

Fig.3

Polarized EXAFS spectrum

for Elz of xerogel and its fitt

using the SWA with four

oxygens at R=1.87 A and

three vanadium atoms at

R=3.0 A . ~ h e Fourier spectra

are shown on top.

z 0

i- n rc 0

V) m

<

9

$ model

a

z

model b

model c

5440.00 5467.50 5495.00 5522.50 5550.00 ENERGY (eV)

5440.00 5467.50 5495.00 5522.50 5550.00 ENERGY (eV)

Fig.4 Polarized experimental XANES spectra (ELz panel a and the Ellz in panel b) are reported on top of the figures. The multiple scattering calculations for differents geometries of the nearest neighbours .Model a) square pyramid symmetry C4,,. b) bipyramid with C2, symmetry b) bipyramid of C4, symmetry and four vanadium atoms in the second shell.

In order to obtain the geometrical information contained in experimental XANES spectra we have performed multiple scattering calculations for several geometries. The experimental polarized XANES spectra, E l z in fig. 4a and Ellz in fig. 4b, are reported on the top of the figures and they are compared with theoretical XANES simulations for differents local geometries.

Using the multiple scattering approach to X A N E S ~ ~ - ~ ~ ) W ~ have started from the simplest

model for the vanadium site structure in which only four oxygens atoms are in square planar

configuration with a V-0 distance of 1.87 A, an oxygen atom in the z axis at 1.58 A. The absorbing

V atom is at 0.25 A out of the plane (model a). Then, in order to see the effects of the distortions

of the square plane basis and of the long V-0 bond on the z axis, we have considered a bipyramid

with a rhomboedric basis with V-0 distances 2.10 and

A and the another oxygen on the z

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axis at 2.7 A (model b). Finally, in order to see the effect of the vanadium second shell we have considered a square bypyramid with a shell of four vanadium atoms at 3 A.

The features in the experimental XANES spectrum for Elz, in fig. 4a, are due to multiple scattering in the layer plane. A single shell of oxygen gives only the main qualitative prediction of the spectrum and the contribution of the vanadium second shell determines the characteristic features 1 a and 2a.

The presence of the 2.7 A oxygen bond on the z axis (not detected from EXAFS) is directly observed from the complex structure 1 b and 2b in the Ellz experimental spectrum in fig 4b. In fact this is predicted by the theoretical XANES for the model b but not for model a. Moreover, it is interesting to remark that the feature 3b, mainly due to the short double bond is splitted giving the peak3b* due to scattering by vanadium atoms in the second shell.

In conclusion we have reported a full interpretation of polarized EXAFS and XANES of vanadium bipyramid clusters using SWA and multiple scattering approaches. The results obtained are interesting for the interpretation of more complex systems like glasses and catalysts.

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