Compositional dependence of the electrical conductivity of vanadium phosphate glass

The bulk resistivities of vanadium phosphate glasses in the compositional range from 65 to 85 mole % V₂O₅ were measured and fitted as a function of composition to the equation for conductivity as proposed by Mott and later modified by Sayer and Mansingh for n, the number of unoccupied mearest sites surrounding the final hopping site of the small polaron. The fit to the experimental data was found to be best if the number n is chosen to be proportional to the total number of V⁵⁺ sites in the glass. Also n is found to be non-integer and can be regarded as the effective number of unoccupied sites surrounding the final site in the sense of eqn (24) developed in this paper. Values for n ranged from 0.2 to 5 for different compositions of the glass. A model for the structure of vanadium phosphate glass is proposed to explain the data.