Microstructure of single-phase cobalt and manganese oxide spinel Mn3−xCoxO4 ceramics

Microstructural studies of single-phase Mn3-xCoxO4 (x = 0.98-2.93) spinel ceramics, using TEM and EDX, are reported. The ceramics were obtained by conventional or spark plasma sintering (SPS) of powders prepared by thermal decomposition of coprecipitated oxalate precursors. For x < 1.78 or x at least 1.78, the monophasic ceramics had a quadratic (Q) or cubic (C) spinel structure. The ferroelastic character of the structural phase transition from C to Q was highlighted by specific microstructural features. The effect of chemical composition and heat treatment conditions on the microstructure and essentially on the presence and the characteristics of twins were investigated. The coherent twin interfaces were parallel to the (112) planes in the Q cell. Twins corresponded to: tweeds, single lamellae (widths: 5-306 nm) arranged parallel to each other, large lamellae (widths: 69-928 nm) internally twinned and sometimes arranged in cyclic forms (triangular shapes).