Indian Journal of ChemistryVol. 17A. May 1979. pp. 442-444

Studies on the Electrical Properties of Tungsten Doped V205O. G. PALANNA

Solid State Laboratory, St. Ph~omena College, Pnt tur 574215

Received 26 August 1977; revised and accepted 14 December 1978

. The electrical proper~ies of the monophasic W.,V.05 samples for ° < x ,;;;0·03 (iso-structuralWIth a-V.05) have been Investigated. The conduction mechanism of these non-stoichiometric~emiconductors involves hopping of electrons through equivalent vanadium lattice sites. ItIS found from experimental data that W6+ ions preferentially occupy interstitial ostti. fl . th I P SI onsIn uencmg e e ectrical properties of the host system.

PURE V205 is an electrical 'insulator. However,non-stoichiometric semiconductors of vanadiumpent oxide containing a non-integral number of

d-electrons per transition element lattice site areknown.

Dissolved impurities can be incorporated into thelattice of V20S either interstitially or substitutionallyor by both ways. Metal (Mn+) ions occupy inter-stitial sites similar to that in the case of thecorresponding vanadium bronzes, Mx V20S (ref. 1) fora limi.ted range of x. However, occupation of lattice(vanadium) sites by W6+ or M06+ ions is reportedin mixed oxide bronzes of vanadium=", a few dopedV20S lattices4•5 and other tungsten-vanadium oxidecompounds=",

The present investigation aims at examining theinfluence of the added impurity ions (W6+) uptox ~ 0·03, on the solid state properties of the hostV

20

5lattice under the conditions of their syn-

thesis.

Materials and MethodsPreparation and characterization of the samples-

Different samples of WXV205 Were obtained by thereactions carried out in the molten state at 1120 Kfor 48 hr between high purity tungsten metalpowder and V20S taken in the desired proportions(0<x~0·03, where x = W/V205). The molten massof the reaction Was cooled to room temperatureat a rate of lOo/mi.n, pressed into pellets of desiredthickness and sintered at 900 K for a few hoursto achieve the maximum packing density.

The compounds were characterized by X-raypowder diffraction technique using Cu Krx radiation{A= 1·517 A) and nickel filter.

Electrical conductivity measurements - The D.C.electrical conductivity (a) of the polycrystallinesamples Was measured between 300 and 875 Kthrough four probe technique. The samples werein the form of sintered compacts (~85 % packingdensity) of 18 mm diameter and 2 mm thick-ness.

The Seebeck coefficient (ex)Was determined between300 and 875 K by integral method maintaining atemperature gradient of approximately 10 K betweenthe two ends of the sample .

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Results and DiscussionThe X-r~y an<;tlys~sshows that the gross V205

structure IS maintained for x~ 0,03, called theex-phase9'~o; the W6+ ions are incorporated at well-defined SItes' of the lattice. Due to similar ionicsizes of W6+ (0·54 A) and V5+ (0·58 A), substitutionor t.he interstitial occupation does not change thelattice parameter to such an extent as to distortthe V205 structure.

The data on the electrical conductivities (a) of thesamples are presented in Fig. 1. The WXV205

co~p~unds, . isostr~ctural with ex-V205, are non-stoichiometric semiconductors, The electrical con-ductanc.e data presente?- are characteristic of hopping

. mechanism of conduction, The sign of the Seebeckcoeffi~ie~t (~) is negative and temperature-indepen-dent indicating a constant number of charge carriers(electrons). The elec~ron transfer in such compounds,takes place. by hopping of electrons on equivalentvanadium SItes of the lattice. Figs. 1 and 2 show~hat the e~ect~ical conductivities (a) of the samplesIncrease With In.crease in x at a given temperaturefor x~ 0·03, while the value of Seebeck coefficient(ex) decreases. However, it is observed that thenumber of charge carriers do not increase withincrease in temperature as to contribute to electricalconductivity. In such cases, the electrical con-duct~,:ity could be ascribed to thermally activatedmobility of the charge carriers. The temperature-dependence of a then follows the equationlogio aT = 10glOaoT -!l.G* j2·3KTwhere !l.G* is the free energyconduction (Table 1). This is

. ., (1)of activation forfurther evidenced

TABLE 1- ELECTRICAL CONDUCTANCE DATA FOR THESAMPLES

x=W!V.06 Seebeck Electrical Activationcoefficient conductivity, energy (eV)

iJ.Vo(n-type) cr (ohm-Icm-I)0 -200 1'5 X 10-5 0'20

0·005 -SO 2'86 X 10-' 0'240'01 -48 3,.13X 10-2 0·240·02 -40 3'63 X 10-2 0'230'03 -30 4'17xl0-2 0'22

\

1·2 1·4 1'6 1-8 2-0 2::6 24 2-6 2-8

103fT

Fig. 1 -Plots of loglo crT versus 103fT for WxV.Os samples [Curve 1, x=0'005; curve 2, x=O-Ol; curve 3, x=0-02;curve 4, x=0-03]

30

I-

bo

o 1·0o....J

- 400

-200

o••.•..>~'i0'00

+200

PALANNA: ELECTRICAL PROPERTIES OF TUNGSTEN nOPED V.Os

T:K834 714- 625 555 500 455 416 385 357 333 313

2-0

0·0

T,OK

83~ 714. 625 555 500 455 4.16 385 357 333 313

1-2 1-~ 1'6 1-8 2-0 2-2 2·4- 26 2-8 3'0 3'2

103, T

Fig. 2 - Plots of Seebeck coefficient (IX) versus 103fT for WxVaOa samples [Curve 1, x=0-005; curve 2, x=O-Ol;curve 3, x=0-02; curve 4, x=0-03]

443

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INDIAN J. CHEM., VOL. 17A, MAY 1979

by the linearity of loglo aT versus liT plots, andtemperat.ure~independence of IX.(Figs. 1 and 2).

Investigation of V205, weakly doped with WO~,s,and also of a few mixed oxide bronzes of the typeM%V2.yWyO~·3for x>0·33, indicated the occupationof lattice sites by the W6+ ions, in each case causinglocalization of the 3dl electron at one neighbouringvanadium site. This substitution of tungsten ionsfor vanadium would result in the formation ofW6+-0- V4+ pairs within the vanadium arrayof the host lattice. In this case the excess negativecharge of the V4+ is bound to the excess positivecharge of the W6+ and hence does not participatein the current transfer-t. In addition, it can beshown that with the increased number of W6+ ionsat the vanadium lattice sites, the V4+-0- VS+distance is enhanced for hopping of electron andthus, the conduction takes place through the latticewith higher activation and a would vary asav,o,>ax ~ aO'03' This behaviour, however, is notin agreement with the results observed in thepresent work.

On the contrary, the present investigation of' aweakly doped V20S sample with W6+ ions indicatesthat for O<x~ 0·03, the incorporated tungsten ionspreferentially occupy interstitial/tunnel sites of thehost lattice giving rise to an IX.-phase. This issupported by the following experimental data:

(i) The XRD data indicates that the samples aremonophasic and isostructural with. V20S lattice withno appreciable change in the lattice parametersof the unit cell. .

(ii) A characteristic IR bands of V20~2 is ob-served which shows no change in the local symmetryand nature of the bonds in the lattice.

(iii) An ESR signal at g = 1·96 is observed with anarroW line width. The narrowing line width at300 K 'indicates (a) localization of the d-electronsat the vanadium sites and (b) hopping mechanismof d-electronsl3.

(iv) The electrical conductance (a) and Seebeckcoefficient (IX.) of the samples change as a result ofthe incorporation of W6+ ions by a few orders ofmagnitude depending on the mode of addition andthe amount of impurity (W6+) present. Figs. 1

I

444

and 2 show that a values follow the order:av,o, < ax ~ aO'03' while the IX.value decreases du~ tothe increased number of electrons at a giventemperature.

The results of the present investigation indicatethat in the system WXV20S' a ~umber of chargecarriers (electrons) p~r formula unolt.occupy d-~ta~esof vanadium which IS empty (3d) in the stoichio-!Jletric V20S' A number of localised, yet mobile3dl centres are created within an array of vanadiumsites. When VV6+ ions occupy the interstitial sitesof the host lattice, a is expected to increaseenormously. The experimental data obtained in thepresent work show that av,o, <ax ~ aO'03 for thetemperature limit indicated. Accordingly, IX.valuesdecrease in the order IX.v,o,>IX.%~1X.0'03 due to anincreased number of electrons in the V20S lattice.

Acknowled~ementThe author thanks Prof. A. B. Biswas, I.I.T.,

Bombay for stimulating encouragement during thecourse of this investigation. The author also ac-knowledges the help of Dr M. D. Shastry, BARC,Bombay for the ESR spectra of the samples andUGC. New Delhi, for providing research grant.

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