Transport properties and photo electrochemical characterization of oxygen-deficient ASnO3−δ (A=Ca, Sr and Ba)

The deficient stannate BaSnO3−δ, prepared at 980 °C in sealed tube, crystallizes in an ideal perovskite structure. The oxide exhibits a blue color, the bandgap is found to be 2.65 eV and the transition is directly allowed. The non-stoichiometry could not be measured, indicating no appreciable oxygen deficiency. Nevertheless, the transport properties were found to change markedly with the heat treatment. BaSnO3−δ behaves as degenerate SC with a conductivity σ300 K (=1.63 Ω−1 cm−1) and where the level Ef drops above mobility edge to delocalized states. The electric balance leads to a simultaneous reduction of Sn4+ inducing n-type behavior. The thermal variation of the thermopower indicates an electron mobility μ300 K (=0.013 cm2 V−1 s−1) thermally activated. The metal-insulating transition is considered of Anderson type due to the random potential to oxygen vacancies. A predominant variable range hopping is predicted from the non-linearity of Ln σ versus T−1 plot. The capacitance measurement (C−2–V), measured in KOH media, shows a linear behavior from which a flat band potential Vfb (=−0.67VSCE) and a density ND (=7.59×1020 cm−3) are determined. The deficient stannates CaSnO3 and SrSnO3 are reported for a comparative purpose. The decrease of Eg is ascribed to the increase of the conduction band width when going from Ba to Ca.