Evidence for the Small Polaron as the Charge Carrier in Glasses Containing Transition Metal Oxides

Low-temperature conductivity measurements on glasses containing transition-metal oxides TiO2 and V2O5 reveal a sharp break in the slope of the dependence of log conductivity upon reciprocal temperature. At low temperatures, the slope corresponds to a low-activation-energy process (0–0.2 eV) while at high temperatures the slope of the conductivity is that appropriate to a much higher activation energy (0.4–1 eV). Typical data obtained on some samples of V2O5–P2O5 glass are shown. Arguments for the small polaron as the charge carrier in such glasses are presented as well as a brief nonmathematical review of the theory of conduction by small polarons. A comparison with the previous Theory of Bridging and Nonbridging Oxygen Ratios is given. Finally, some difficulties with the suggested small polaron model are pointed out and some speculations which may avoid or explain these difficulties are made.