Spinel precipitation in Sn4+-doped Co1−xO polycrystals with dispersed Co2+xSn1−xO4 particles

Co1−xO–SnO2 powders in molar ratio of 92:8 were reactively sintered at 1400 °C to form Co1−xO–Co2+xSn1−xO4 composite and then cooled in furnace or air quenched for secondary Co2+xSn1−xO4 spinel precipitation from the Sn4+ doped Co1−xO grains. Electron microscope observations indicated the secondary spinel to precipitate at grain boundaries when slowly cooled, but as parallel-epitaxial platelets within the Sn4+ doped Co1−xO grains with a precipitate free zone near the grain boundary when air quenched. A process of thermal-mismatch induced {1 1 0} cleaving, taking advantage of cobalt vacancies, and spontaneous healing by oxidation precipitation accounts for the platy spinel precipitation within the grains. The precipitate free zone can be attributed to cobalt vacancy depletion, i.e. site saturation, near the grain boundary during rapid cooling in air. The spinel nanocrystals nucleated from cobalt vacancies in association with Sn4+ dopant have well-developed {1 1 1} habit plane in order to minimize the coherency strain energy.