Electrical properties of copper–manganese spinel solutions and their cation valence and cation distribution

The cation valence and distribution in copper manganese spinels containing 1.0–1.6 mol copper per formula unit (CuxMn3−xO4) was resolved from their electrical conductivity and thermoelectric properties. The limit of thermal stability of the cubic spinel phase was also determined for each stoichiometry. A corrected phase diagram for the Cu–Mn–O system in air is proposed accordingly. The electronic defect structure could be described through a chemical approach, involving the competition between the redox of Cu+ and Mn4+ to Cu2+ and Mn3+ and the disproportionation of the Jahn–Teller ion Mn3+ into Mn2+ and Mn4+. Thermodynamic parameters for the redox reaction were determined from experimental data as well as calculated, confirming the validity of the modeled defect equilibria.