Precipitate concentration of Co2SnO4 in CoO-doped SnO2 ceramics at different oxygen chemical potentials

The precipitate concentration of Co2SnO4 phase in CoO-doped SnO2 ceramics was investigated as a function of oxygen chemical potential during heat treatment. When the SnO2 ceramic doped with 2 mol% CoO was sintered at 1400°C for 2 h in oxygen (O2) or nitrogen (N2) atmosphere, a small amount of precipitated Co2SnO4 phase was observed in the SnO2 samples regardless of sintering atmosphere. When the sample sintered in N2 was afterwards heat-treated at 1300°C for 20 min in O2, the amount of the Co2SnO4 phase on the surface of the sample was largely increased. On the other hand, when the sample sintered in O2 was afterwards heat-treated in N2, the Co2SnO4 phase on the sample surface disappeared. It is suggested that outward or inward diffusion of oxygen vacancies (VO) in the sample under different oxygen chemical potentials is coupled to outward or inward diffusion of Co-ions due to a defect association of oxygen vacancies with CoSn″, which causes modifications of the Co concentration at the surface and controls the precipitation fraction of Co2SnO4 phase at the sample surface.