Anderson transition and intermediate polaron formation in WO_3-xTransport properties and optical absorption

Abstract

Abstract The temperature dependence of electrical conductivities In WO3-x (O ≤ x ≤ 0·28) has been investigated. These compounds show metallic behaviour for x < 0·10 and non-metallic behaviour for x < 0·10. An Anderson transition takes place for the composition WO2·90 with complete polaronic transport in the non-metallic phase. Optical and infrared absorption spectra were recorded for all compounds and they showed that polarons and quasi-free carriers are present simultaneously in the metallic phase. A tentative band model is derived from the experimental absorption spectra and the transport properties, and the influence of metallic dopants is discussed. It is found that the formation of intermediate polarons is independent of the polaron concentration, confirming theoretical predictions of Wegner and Entin-Wohlmann (1983).