Mat. Res. Bull. Vol. 2, pp. 193-201, 1967. Pergamon P res s , Inc. Pr in ted in the United States.

PREPARATION AND PROPERTIES OF CRYSTALLINE

AND AMORPHOUS VANADIUM PENTOXIDE

T. N. Kennedy, R. Hakim and J. D. Mackenzie Rensselaer Polytechnic Institute

Troy, New York

(Received in final form December 19, 1966; Communicated by R. C. DeVries)

ABSTRACT A special technique has been developed for the prep- aration of single crystals of vanadium pentoxide having dimensions of 5 cm x 5 cm x 0.5 cm. Amorphous films of V205 have also been prepared by vapor depo- sition. The expansion coefficient, density and electrical resistivity of these phases have been measured.

Introduction

Many semiconducting oxide glasses have been prepared by the

simple fusion of one or more transition metal oxides with other

metal oxides (i). The most common transition metal oxide used for

the preparation of such electronically conducting glasses is

vanadium pentoxide. Glasses have been prepared with as much as

90 mole per cent vanadium oxide as one constituent (2). Since

conduction in these glasses is entirely attributed to the vana-

dium ions present, one question which immediately arises is

193

194 VANADIUM PENTOXIDE Vol. 2, No. 2

whether the behavior of such ions in a crystalline lattice is

similar to that in a non-crystalline matrix. A comparison of the

properties of single crystal V205 with those of glassy V205 is

therefore desirable. Single crystals of V205 of practicable

dimensions which will permit electrical measurements are not

commercially available. The preparation of glassy V205 contain-

ing no other oxide has not been reported. In this paper, the

successful preparation of large single crystals of V205 as well

as an amorphous V205 is described and some of the properties of

these phases reported.

Experimental

The V205 powder used for the preparation of single crystals

was obtained from the Fisher Chemical Company. The label analy-

sis showed 100.02% V205, 0.01% Fe and 0.01% Cl. All experiments

were carried out in platinum crucibles in air. Although the

molten V205 could be supercooled as much as 50°C easily, crystal-

lization was invariably rapid and only fine needles were obtained

Attempts to use the Czochralski method to pull large single

crystals we-re unsuccessful. However, by the careful manual

control of vertical and horizontal gas flames, large single

crystals were obtained. The heat envelope produced by the flames

around the crucible established a horizontal as well as a verti-

cal thermal gradient in the melt. Crystal growth was promoted by

allowing the central portion of the melt surface to drop to some

temperature just below the freezing point through radiation

Vol. 2, No. 2 VANADIUM PENTOXIDE 195

losses. By careful seeding and temperature control, large plate-

lets of V205 single crystals up to 5 cm in diameter and 5 mm thick

were prepared. Typical crystal products are shown in Figure i.

FIG. 1

Single crystals of vanadium pentoxide grown from the melt. (0.7x)

Two large single crystals inside the platinum crucibles are

surrounded by small needles which had developed rapidly when the

gas-flames were turned off. Crystallinity of the specimens was

confirmed by microscope examination and by x-ray diffraction.

After some hours at about 700°C, the melt was found to have

attacked the platinum crucible slightly. Analysis of the smaller

needle-shaped crystals and of a platelet single crystal revealed

the presence of 0.02 W/o of platinum in both samples.

Attempts to prepare a glassy V205 by casting the melt on to a

copper slab cooled to liquid nitrogen temperature were

196 VANADIUM PENTOXIDE Vol. 2, No. 2

unsuccessful. Thin films of a non-crystalline V205 were finally

prepared by condensing the vapor on cold substrates in vacuum.

The absence of crystallinity was confirmed by microscopic exami-

nation in polarized light and by electron diffraction. Density

measurements were made on stripped films by dropping them into a

density gradient column, and also from the observed weight/volume

ratios. Electrical resistivity for the non-crystalline V205 films

was measured by a two point probe technique and air-dried silver

paste electrodes. Resistivity was constant with time up to three

hours.

The expansion coefficient of the polycrystalline oxide from

room temperature to 600oC was determined with an as-cast rod of

length 6.5 cm using a Chevenard expansion apparatus. Linear

expansion along the three separate directions of the crystal was

determined from room temperature to 620°C by observing the changes

in lattice spacings for the (600), (020) and (002) reflections

with a General Electric x-ray diffractometer with a high tempera-

ture furnace attachment. A Gouy-type apparatus consisting of a

Varian V4004 magnet and Mettler H-16 analytical balance was used

to measure the magnetic susceptibility of powdered samples. D.C.

electrical resistivity measurements for the crystals were m&de on

gold-plated samples with two and four point probe techniques.

Identical results were Dbtained from the two methods. Resistivity

was not dependent on time up to three hours.

Vol. 2, No. 2 VANADIUM PENTOXIDE 197

Results and Discussion

Vanadium pentoxide is an oxygen-deficient semiconductor. At

high temperatures, liquid V205 is known to dissociate (3)

according to the equation:

V205 ~ V205_ x + x/2 02 (i)

For each atom of oxygen gas evolved, two unpaired electrons are

left behind. Their presence are detectable by magnetic suscepti-

bility measurements and their concentrations are indicative of the

degree of non-stoichiometry of the vanadium pentoxide. These

electrons may be considered as (a) trapped at an oxygen ion

vacancy, (b) interacted with two V 5+ ions giving rise to two V 4+

ions, and (c) interacted with one V 5+ ion giving rise to one V 3+

ion. Optical measurements to distinguish between these possi-

bilities were not carried out in the present study. However,

recent spin resonance studies indicated that V 4+ ions are the most

probable species present (4,5). On this assumption, the magnetic

susceptibility ~ , of the Fisher certified powder of 0.32 x 10 -6

cgs units at room temperature would correspond to V4+/V 5+= 0.024.

There was a slight increase in the susceptibility of crystalline

V205 after three hrs. at 700°C when ~ = 0.39 x 10 -6 cgs units and

V4+/V 5+ = 0.030. No further change was observed up to 7 hrs. at

700°C.

Vanadium pentoxide has a corrugated sheet-type structure (6).

Along the plane of such sheets, bonding is due to strong V-0 bonds

whereas adjacent sheets are held together by weak van der Waals'

198 VANADIUM PENTOXIDE Vol. 2, No. 2

forces. Ease of cleavage along the (010) planes in V205 is well

known. The observed expansion results in Table 1 for the three

axes of the crystal are compatible with this structure.

TABLE 1

Linear Expansion and Electrical Resistivity of Vanadium Pentoxide Along Three Crystal Axes Compared with the

Polycrystalline and Amorphous Phases.

Crystal Axes

Avg.Coef.Linear Expansion, Electrical Resistivity per °C 25-600°C, x 106 ohm-cm at 25oC, x 10 -2

B 55.4 47.0

C 8.0 6.7

A 2.0 1.7

Polycrystalline 13.0 16.0

Amorphous - ii000

For some transition metal oxides involving low mobility

carriers, the mechanism of conduction has been depicted as one

where an electron or polaron can migrate from an ion of lower

valence to one of higher valence (7). For vanadium oxide this

may be formally represented by:

v 5+ - 0 - v 4+ 4 v 4+ - 0 - v 5+ (2)

Since V205 has a sheet-type structure in which the sheets are

perpendicular to the B axis, the electrical resistivity within

the sheets will be less than that between sheets. As expected,

the resistivities in the A and C directions shown in Table i, are

much less than that in the B direction.

The average thermal expansion coefficient for the three

Vol. 2, No. 2 VANADIUM PENTOXIDE 199

crystal directions from single crystal data is 21.8 x 10 -6 while

the polycrystalline sample had a measured thermal expansion coef-

ficient of 13.0 x 10 -6 . This discrepancy is due to the preferred

orientation of the needle-like crystals of the polycrystalline

sample during freezing. Freezing point determinations were made

with a Pt/10 Rh thermocouple immersed in the melt and by observing

the thermal arrest on a chart recorder during cooling and heating.

The melting point (8,9) of V205 reported in the literature ranges

from 658 ° to 690°C, and is probably due to variations of oxygen

content. The present sample with V4+/V 5+ = 0.024 was found to

melt at 668 + 0.5°C.

The apparent density of the amorphous V205 films was found to

be dependent on rates of vaporization and condensation as well as

on thickness. The thickness of the samples varied from 0.5~ to

5~ and observed bulk density ranged from 2.420 g/cc to 2.691 g/cc,

which is 80% or less of the single crystal density, 3.357 gm/cc at

25°C. These values are not unusually low when compared with other

glass forming systems such as B203, Ge02 and Si02. The amorphous

films are stable at least up to 150°C. Films kept at this tem-

perature for 24 hours were still non-crystalline by electron

diffraction. The apparent electrical resistivity of I.I x 106

ohm cm is surprisingly high. Electrical resistivity values are

available on glasses with 6~ to 9~ V205 in P205 (2). Extrapo-

lation of these data to i0~ V205 gives a resistivity of 6 x 103

ohm cm. It is unlikely that the presence of micropores in the

200 VANADIUM PENTOXIDE Vol. 2, No. 2

amorphous film would alone result in this large discrepancy

between i.i x 106 and 6 x 103 ohm cm. Adsorbed water would give

rise to ionic contribution to conductivity. However, the con-

stancy of conductivity with time showed that this is unlikely. A

probable cause is the composition of the film itself since elec-

tronic conduction is directly dependent on the value of x in

V205_ x, which was not determined because of the limited amount of

amorphous V205 presently available.

Since the V 4+ content of the crystalline and amorphous V205

was two orders of magnitude higher than the Pt impurity present,

the latter is considered to have no appreciable effect on conduc-

tivity. Trace impurities of Fe and Cl in the Fisher reagent V205

would also have a negligible contribution to conductivity.

Acknowledgements

The authors are grateful to Mr. T. Allersma for the magnetic

susceptibility measurements. This research was supported by the

Office of Naval Research under Contract Nonr 591(21).

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Vol. 2, No. 2 VANADIUM PENTOXIDE 201

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