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 (V_O^··) 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 Co″_Sn, which causes modifications of the Co concentration at the surface and controls the precipitation fraction of Co₂SnO₄ phase at the sample surface.