Solid State Communications, Vol. 80, No. 2, pp, 125-127, 1991. Printed in Great Britain.

0038-1098[91 $3.00 + .00 Pergamon Press plc

r ELECTRICAL CONDUCTION IN MOLYBDENUM TRIOXIDE SINGLE CRYSTAL

A.K. Pandit, M. Prasad, T.H. Ansari, R.A. Singh and B.M. Wanidyn*

Department of Physics, University of Gorakhpur, Gorakhpur, 273009, India

(Received 15 November 1990 by A. Pinczuk)

Electrical conductivity, thermoelectric power and dielectric constant of molybdenum trioxide single crystal have been studied in the tem- perature range 300-1000K. Intrinsic conduction occurs at T ~ 650K and activation energies below and above 650 K are 0.05 and 1.75 eV respectively. The compound exhibits n-type semiconducting behaviour below 600 K and above 600 K it becomes p-type.

1. INTRODUCTION

THE TRANSITION metal oxides form a group of predominantly ionic solids which exhibit a wide range of optical and electrical properties. Many of these oxides have considerable use in electronic and mag- netic devices, in heterogeneous catalysis [1] and in a number of other applications. Molybdenum trioxide (MOO3) is an orthorbombic lattice of space group

[2, 3]. Electrical conductivity and dielectric constant of

MoO~ single crystal have been studied [2, 4-7] in the temperature range 300-750K. The present paper reports our study on the a.c. and d.c. electrical con- ductivities, thermoelectric power and dielectric con- stant in the temperature range 300-1000 K.

2. EXPERIMENTAL DETAILS

Single crystals of MoO3 were grown by the flux method at the Clarendon Laboratory, Oxford, U.K. Details regarding crystal growth technique and identi- fication are given elsewhere [8]. The crystal on which the measurements were carded out has dimensions 5.5ram × 1.5mm × 1.0mm. The electrical conduc- tivity was measured by a digital multimeter 0PM 2522/ 90 Philips, India) with an accuracy better than +0.25%. The dielectric constant was determined by measuring the capacitance formed with the crystal as the dielectric medium. The capacitance of the capaci- tor was recorded at a fixed frequency of I kHz, using an autocomputing digital CR-~ meter (4910, Applied Electronics Ltd., Thane, India) with an accuracy better than :t:0.25%. For all the measurements, a two probe method was employed. Platinum foils and silver paint were treed as electrode materials. Temperature

* Department of Physics, University of Oxford, Or~ord, U.K.

was recorded with the help of chromel alumel thermo- couple wires attached to the platinum electrodes. The details regarding the sample holder assembly and measuring techniques are given elsewhere [9].

3. RESULTS AND DISCUSSION

The electrical conductivity (o) of single crystal M o O 3 has been measured in the temperature range 300-1000K perpendicular to C-axis. Variation of log o with reciprocal of absolute temperature (103 T- ' ) are shown in Fig. 1. The curve follows the well known exponential relation

o = 0o exp (-E, , /kT) , (1)

with two different slopes below and above 650 K. The data can be expressed by two straight lines one below 650 K and other above 650 K as follows:

( 005eV ol ffi 2.26 x 10 -s exp . ] o h m - l c m -I

300 K ~< T ~< 650 K, (2)

( 1.75eV~kT oil = 8.70 × 104exp ] o h m - l c m -~

650 K ~< T ~ 1000 K. (3)

The activation energies below and above T ~ 650 K are 0.05 and 1.75 eV respectively. In the lower tem- perature region (300-650K), the activation energy 0.05 eV is very small and cannot be assigned as the activation energy of the intrinsic conduction. Such a value of the activation energy is a characteristic of extrinsic conduction due to impurities, point defects or interstitial$, which are often present in the forbidden energy gap of the crystal. Such type of impurities present in the mixed oxides cannot be removed easily [10]. The contributions of defects or impurities towards conduction in a solid can be explained in

125

126 MOLYBDENUM TRIOXIDE SINGLE CRYSTAL

§

1.0 1.5 2.0 2.5

I031T (K "I)

Fig. 1. Variation of electrical conductivity as a func- tion of temperature in MoO3 single crystal: o: a.c. conductivity; A: d.c. conductivity.

terms of donors and acceptors and is represented by the expression [11]

od = A exp ( - Et/kT), (4)

where E~ is the ionization energy of donors or acceptors and usually E, ~ 0.1 eV for semiconducting materials. The activation energy calculated by us below 650K is 0.05eV and is approximately com-

parable to the ionization energy 0.1 eV. Therefore, it seems reasonable to conclude that electrical conduc- tion in MoO3 below 650 K is due to impurity charge carriers, point defects or interstitials present in the forbidden gap of the crystal. The activation energy above T ~ 650 K is 1.75 eV, which seems to be the activation energy of intrinsic conduction in molyb- denum trioxide. So the change in the nature of the log ~ versus 103T -~ curve at 650K is due to the change in the conduction mechanism i.e. transition from extrinsic to intrinsic conduction. The relevant bands [12] available for conduction in the crystal are filled O2-:2p and empty Mo 6+ :4d along with 5s empty band. There are three lower slightly split 4d levels and two more widely split upper levels due to the influence of crystal field splitting [13] and the differ- ence in the two energy level is usually given by the crystal field splitting parameter (A). Our estimated activation energy Eo, 1.75 eV represents an activation energy of band conduction. The energy gap of MoO3 has been estimated from the intrinsic region of the

Vol. 80, No. 2

0.7

0.6

0.5

. . . 0.4 v

:~ 0.3

v) 0.2

0.1

0.0 o n O - -

-0.1 I I I I 1.0 1.5 2.0 2.5 3.0

103/T (K "I)

Fig. 2. Variation of thermoelectric power (S) as a function of temperature in MoO3 single crystal.

log Od~ VS 103T -t curve and it has been found to be 3.5eV (E, - 2Eo). However, the band gap has been reported to be 3.66eV by I)eb [2] from conductivity measurements while Dickens and* Nield [14] have reported a value of 2.96 eV based on spectral studies. A.c. electrical conductivity is almost coincident with' the d.c. electrical conductivity in the intrinsic region (T > 650K), which indicates that, in the intrinsic region electrical conductivity is entirely electronic and the ionic contribution to the electrical conductivity is almost negligible.

The thermoelectric power of molybdenum trioxide has been measured in the temperature range 300- 1000K using the formula

S = lim AE A~-0 ~-~' (5)

where AE is the e.m.f, produced across the crystal due to temperature ditference AT. Variation of thermo- electric power (S) with 103T -I is shown in Fig. 2. Thermoelectric power inereas~ with ~ tcm- lx~rature. Thermo ¢.m.f. has negative value upto 600 K and becomes positive at T > 600 K, which indicates that upto 600 K majority charge carrier are electrons and above 600 K, holes become the majority charge carriers. The transition from negative to positive value of thermoelectric power at 600 K may be due to the following reason: the thermoelectric power increases with. increasing temperature-in entire temperature range studied, which is due tothe increase in mobility of the charge carriers (holes). Here it appears that 4d=5~, 2p transitions are taking place, zweltia8 the~by that holes belong to relatively, wide 5s and ,2p bands rather than 4d bands in which the electron exist, due to which effective mass of the electrons would be larger than the effective mass of theholes. Therefore, holes

"w

MOLYBDENUM TRIOXIDE SINGLE CRYSTAL 127

3.9

3.6

3.3

3.0

2.7

2.4 300 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0

Vol. 80, No. 2

T (K)

The slow increase in e' of MoO3 may be due to space charge polarization caused by impurities or inter- stitials because the conduction in this material below 650 K is extrinsic type governed by impurities or inter- stitials. The large increase in 8' above 650 K is attri- buted to the exponential increase in the number of charge carders in the intrinsic region. In the narrow band semiconductors, the charge carriers are not able to move freely with the electric field. If these charges are trapped, a space charge polarization builds up and macroscopic field distortion results. Due to this phenomenon the dielectric constant of MoO3 increases very rapidly in the temperature range 650- 800 K and above 800 K 8' increases less rapidly.

Fig. 3. Variation of dielectric constant (e') as a func- tion of temperature in MoO3 single crystal.

will become more mobile than electrons and eventually the holes become the majority charge carriers.

The dielectric constant of MoO3 single crystal has been measured perpendicular to the C-axis in the temperature range 300-1000K. Due to small size of the crystal, measurement of dielectric constant paral- lel to the C-axis was not possible. The dielectric con- stant at various temperatures has been calculated using the formula [15]

e' = C l l .3t A ' (6)

where 8' is the dielectric constant, C is the capacitance of the crystal in picofarads, t is the thickness of the crystal in em and A is the surface area of the electrode in cm 2. Variation of dielectric constant 0og 8") vs temperature is shown in Fig. 3. The static dielectric constant (8') slowly increases with temperature upto 650 K and above 650 K it increases much rapidly with temperature upto 800 K. The slow variation of the dielectric constant with temperature is the usual trend in ionic solids [16, 17]. The temperature has a com- plicated influence on the dielectric constant. Gener- ally, increasing the temperature of a material decreases the electronic polarization. The increase of ionic distance due to the temperature, influences the ionic and electronic polarizations. The decrease in electronic dielectric constant is found to be less than 3% for a temperature change of about 400°C [18, 19]. Similarly, the changes in ionic polarization are not very large. Even assuming the presence of some dipoles and their contribution to the dielectric constant, we know from Debye's theory that the dielectric constant is inversely proportional to the temperature, therefore the dielectric constant of ionic solids should be changed considerably with increasing temperature.

Acknowledgement - T h e authors are thankful to the Council of Scientific and Industrial Research, New Delhi (India) for providing financial assistance.

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