X-ray photoelectron spectroscopy and electrical properties studies of La2O3-doped strontium titanate ceramics prepared by sol-precipitation method

X-ray photoelectron spectroscopy has been used to investigate the existence of Ti3+ on the surface of La2O3-doped strontium titanate and to determine its surface characteristics. The surfaces, having Sr/Ti ratios significantly varying from the stoichiometric ratio, revealed the presence of carbon and suggested the presence of hydroxyl groups on the surface, whose concentration largely decreased in the bulk. Ti3+ species existed as a function of the sintering conditions and were detected on the surface of (La, Sr)TiO3 sintered in air or in N2 by natural cooling. These samples had a lower electrical resistivity, especially when sintered in a N2 atmosphere. The surfaces of air oxidized SrTiO3 and quenched from high temperature contained no detectable amount of Ti3+, resulting in higher resistivity. However, the N2-sintered samples were dark blue in color and exhibited lower resistivity, semiconductivity, and lower valence oxidation state Ti existed when sintered above 1350°C.