Electron transport in Nb xTi 1− xO 2 with 0.04 < x < 1

Abstract

Electrical conductivity and thermopower have been determined in the Nb x Ti 1− x O 2 solid solutions for 0.04 < x < 1 and between 300 and 1300 K. Experimental results are fitted with a small polaron model of electron transport. Thermopower results are consistent with localized particle motion with a binding energy increasing with Nb content up to 0.1 eV. Nevertheless, it seems that this localized particle is a single small polaron only at low temperatures because of the high calculated energy at high temperatures for the associated phonon according to the single small polaron theory. A tentative interpretation for the high-temperature range is proposed: the existence of a bipolaron for 0.04 < x < 0.2 (as in Ti 4O 7) and a tripolaron for 0.2 < x < 0.5. This assumption accounts correctly for the hightemperature thermopower and the variation of the activation energy of mobility with temperature. Nevertheless, new experiments such as magnetic susceptibility determination are required to prove this idea.