Cross Effect Between Ion and Electron Flows in Fe_3–δO₄

Abstract

The effective valence, ${\overline Z _{Fe}} - \alpha _{Fe}^*$, of mobile cations (Fe²⁺, Fe³⁺) in semiconducting Fe₃O₄ was determined at elevated temperatures via Tubandt-type electrotransport experiments in association with the literature data on the cation diffusivity and total electrical conductivity. It has been found that the value for ${\overline Z _{Fe}} - \alpha _{Fe}^*$ varies systematically from below 2 up to 3 with oxygen partial pressure at a fixed temperature. The effective valence is determined not only by the mobility difference of Fe²⁺ and Fe³⁺ ions $({\overline Z _{Fe}})$ but also by the cross effect between the cations and electrons upon their transfer $(\alpha _{Fe}^*)$. A value of ${\overline Z _{Fe}} - \alpha _{Fe}^*$ between 2 and 3 may be attributed to the mobility difference between Fe²⁺ and Fe³⁺ ions even in the absence of the cross effect, but the values of ${\overline Z _{Fe}} - \alpha _{Fe}^*$ < 2 clearly indicate that the cross effect is in play in Fe₃O₄.