Electronic behavior and cationic defects in cubic transition metal oxides

A comparison study has been made between the cubic monoxides and oxide spinels of iron, manganese, and cobalt in terms of cation defect structures and electrical properties. Although all six oxides share close-packed oxygen sublattices and have similar point defect structures, there are considerable differences between conduction processes. Whether or not an octahedral/tetrahedral cation exchange reaction occurs may play a central role in governing conduction. In the iron oxides, n-type small polaron conduction takes place with small hopping energies (0.11-0.16 eV) between octahedral Fe²⁺ and Fe³⁺, with the cation intersite reaction playing a dominant role. Very different behaviors are observed in the spinels and monoxides of manganese and cobalt. Intersite exchange is highly unlikely in these materials. Instead, disproportionation on the octahedral sites results in p-type small polaron conduction with larger hopping energies (0.25, 0.79 eV) between octahedral M⁴⁺ and M³⁺ in the spinels of cobalt and manganese. At higher temperatures, yet another small polaron mechanism prevails in cobalt spinel. In contrast, no hopping energy is apparent for either MnO or CoO.