Redox behavior and transport properties of brownmillerite Ca2(Fe,M)2O5 ± δ (M = Mn, Co)

Abstract

Thermogravimetric and Mössbauer spectroscopy studies of brownmillerite-type Ca2Fe1.6M0.4O5±δ (M = Mn, Fe and Co) showed that the dominant oxidation states of transition metal cations at low temperatures are Mn4+, Fe3+ and Co3+. Doping with cobalt was found to increase the total conductivity at temperatures above 600K, to decrease average thermal expansion coefficients from (11–13)×10−6K−1 down to (7–11)×10−6K−1 at 300–1270K, and to promote reductive decomposition at moderate oxygen pressures. Although the effects of Mn substitution on the conductivity and thermal expansion of calcium ferrite are opposite, this donor-type doping also leads to lower phase stability. The oxygen partial pressure dependencies of the total conductivity and Seebeck coefficient of Ca2Fe1.6M0.4O5±δ under oxidizing conditions indicate that the electronic transport is governed by p-type charge carriers irrespective of the oxygen nonstoichiometry level, suggesting important roles of iron disproportionation and localization of Mn4+ and Fe2+ states. On reduction, undoped Ca2Fe2O5−δ exhibits a transition to the n-type electronic conduction and minor segregation of CaO. The corresponding variations of the oxygen deficiency, measured by coulometric titration in combination with thermogravimetry, can be described by defect models assuming that the anion vacancy formation and Fe2+ localization occur in the perovskite-like octahedral and tetrahedral layers of the brownmillerite structure, respectively.