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

Abstract

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.