Impurity effects in tungsten phosphate glasses. II. Electrical and optical properties

Abstract

For pt.I see ibid., vol.6, no.24, 3661 (1973). The electrical and optical properties of semiconducting glasses made from 67 MOL% WO₃:33 mol% P₂O₅ and 0.2-8.6 mol% V₂O₅, MoO₃ or CuO have been investigated. V₂O₅ and MoO₃ reduce the conductivity sharply while CuO and most other transition metal oxides have little effect. The dominant parameter is the activation energy for conduction which varies inversely as impurity spacing at low impurity concentrations. Isolated impurity ions induce localization energies of about 0.75 eV for V ions and 0.80 eV for Mo ions. In the case of vanadium, interaction effects reduce these energies eventually resulting in direct electron hopping between impurity sites. The results may be related to properties of V₂O₅:P₂O₅ glasses. Thermopower and optical absorption measurements on undoped glasses are consistent with a small-polaron hopping mechanism, and the correlation W_opt approximately 4 W_H, where W_opt is the energy of the absorption peak, is qualitatively observed even for doped glasses. However, the conduction process cannot be described in terms of a simple form of trap-controlled drift mobility, and a model involving electrostatic interactions between impurity sites appears to be necessary.