Revised model for thermopower and site inversion in Co3O4 spinel

Abstract

In recent years, the fundamental understanding of thermopower in strongly correlated systems as it relates to spin and orbital degeneracy has advanced, but the consequences for determining cation distribution have not been considered. In this work we compare measurements of the electrical conductivity and thermopower in ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ spinel with different models for the thermopower based on different assumptions of the cation distributions. Using thermopower measurements we calculate the in situ cation distribution according to three different models: case (i) the original Heikes equation, case (ii) Koshibae and co-workers' modified Heikes equation, and case (iii) a new model, using the modified Heikes equation but with contributions from both octahedral and tetrahedral sites. We find that only the modified Heikes equation that includes contributions from both octahedral and tetrahedral sites satisfies the constraints of stoichiometry in the spinel structure. The findings suggest either complete $(\ensuremath{\sim}100%)$ inversion of the spinel structure if no change in spin state, or a combination of a minimum 40% inversion with a change in spin state of the octahedral ${\mathrm{Co}}^{3+}$ cation.