Nitrate-processing and characterization of a cobalt-doped barium tin oxide perovskite: Magnetic, transport and photoelectrochemical properties

The perovskite BaSn0.95Co0.05O3−δ, prepared by nitrate route, crystallizes in a cubic symmetry with a slight isotropic contraction of the host lattice. The oxygen crystal field splits the Co3+: 3d orbital by a value of 2.02 eV with a low spin configuration of Co3+ (t2g6eg0t2g6eg0). The average magnetic susceptibility (~2×10−5 emu cgs mol−1) is temperature independent, in conformity with itinerant electrons behavior. The sample is nominally non-stochiometric and the conductivity follows a thermally activated hopping of lattice polaron with an activation energy of 19 meV. The thermal variation of the thermo-power suggests a finite density of states at the Fermi level. The zero temperature conductivity is finite and the charge carriers in localized states are thermally excited into extended states above the mobility edge. A variable range hopping is predicted at low temperature from the non-linearity of ln(σ) versus T−1 plot while the hump observed near 25 K is ascribed to the phonon drag. The photo-electrochemical characterization in KOH medium indicates p type behavior with a flat band of −0.21VSCE and a donor density of 6.95×1020 cm−3. The electrochemical impedance spectroscopy is undertaken and the response is modeled by an equivalent circuit with the predominance of the bulk contribution. The depressed semicircle is attributed to the faradic charge transfer with a constant phase element (CPE) while the straight line at low frequencies is due to the Warburg diffusion.