Structural variations and cation distributions in Mn 3− xCo xO 4 (0 ≤ x ≤ 3) dense ceramics using neutron diffraction data

Abstract

Single phase ceramics of cobalt manganese oxide spinels Mn 3− x Co x O 4 were structurally characterized by neutron powder diffraction over the whole solid solution range. For x < 1.75, ceramics obtained at room temperature by conventional sintering techniques are tetragonal, while for x ≥ 1.75 ceramics sintered by Spark Plasma Sintering are of cubic symmetry. The unit cells, metal–metal and metal–oxygen average bonds decrease regularly with increasing cobalt content. Rietveld refinements using neutron data show that cobalt is first preferentially substituted on the tetrahedral site for x < 1, then on the octahedral site for increasing x values. Structural methods (bond valence sum computations and calculations based on Poix's work in oxide spinels) applied to our ceramics using element repartitions and [M–O] distances determined after neutron data refinements allowed us to specify the cation distributions in all phases. Mn 2+ and/or Co 2+ occupy the tetrahedral site while Mn 3+, Co 2+, Co III (cobalt in low-spin state) and Mn 4+ occupy the octahedral site. The electronic conduction mechanisms in our highly densified ceramics of pure cobalt and manganese oxide spinels are explained by the hopping of polarons between adjacent Mn 3+/Mn 4+ and Co 2+/Co III on the octahedral sites.