Oxidation of reduced polycrystalline Nb-doped SrTiO3: Characterization of surface islands

Structural and chemical phenomena observed after high temperature oxidation (1200 °C for 30 h) of initially reduced Nb-doped (5 at.%) polycrystalline SrTiO3 are reported. The reoxidation procedure leads to the formation of Sr-rich islands on the surface. These islands exhibit characteristic areal densities, shapes, and sizes that depend on the surface orientation of the corresponding grain. A correlation between grain orientation (determined by orientation imaging microscopy) and tendency towards island formation was found and discussed in terms of surface energies. The structure and the composition of the islands were investigated by transmission electron microscopy and significant differences were observed for islands of different height located on differently oriented surfaces. While a small island stayed in the perovskite structure (despite a gradual and partly strong Ti depletion) a crystal structure change was found for a larger island containing Sr cations only.