High‐Temperature Thermoelectric Power Measurements in Wustite

Abstract

High‐temperature thermoelectric power was measured on single‐crystal samples of wustite in equilibrium with a carbon dioxide‐carbon monoxide atmosphere. These measurements were made using a “heat pulse” technique which allowed the thermoelectric power of a sample to be determined in seconds. The short time required for this measurement precluded any possibility of an ionic contribution to the measured thermoelectric power. The results of this study are similar to those of earlier workers in that the measured thermoelectric power changed sign from positive to negative with increasing defect concentration. A defect model for wustite is proposed to explain these results. This model assumes a defect structure dominated by clusters of four vacancies coordinated around a trivalent iron cation in a tetrahedral position. Due to the high negative charge of such a cluster, electron holes are trapped in octahedral sites adjacent the cluster vacancies. Conduction occurs through the thermally activated hopping of these trapped holes between cluster near‐neighbor sites. The high defect concentrations in wustite result in these near‐neighbor sites being shared between different clusters. It is therefore possible for a given electron hole to hop through the crystal on a continuous path of near‐neighbor sites. A modified Heikes‐type equation is used to show that such a model is consistent with the measured values of the thermoelectric power. The proposed model is qualitatively consistent with other studies using X‐ray and neutron diffraction, electrical conductivity, and diffusion.