Structural variations and cation distributions in Mn3-xCoxO4 (0 ≤ x ≤ 3) dense ceramics using neutron diffraction data

Single phase ceramics of cobalt manganese oxide spinels Mn3-xCoxO4 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. Mn2+ and/or Co2+ occupy the tetrahedral site while Mn3+, Co2+, Co-III (cobalt in low-spin state) and Mn4+ 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 Mn3+/Mn4+ and Co2+/Co-III on the octahedral sites. (C) 2009 Elsevier Masson SAS. All rights reserved.