Electronic structures of Ba1-xCaxTiO3 studied by x-ray absorption spectroscopy and theoretical calculation

We report O and Ca K-edges x-ray absorption near edge structure (XANES) spectra of Ba1-xCaxTiO3 (x=0.01 and 0.08), BaTiO3 and CaTiO3 and the electronic structure of Ba0.875Ca0.125TiO3 obtained by first-principles calculation. The characteristic features in the O K-edge XANES spectra of these ferroelectric perovskites are influenced by the Ca concentration. They differ substantially from those of the reference TiO2. The O K-edge spectra suggest that the combination of the alkaline-earth-metal oxides, CaO and/or BaO, with TiO2 enhance the effective charge of the O ions. Thus, a large dipole moment may result from the displacement of the Ti ion from the centre of the TiO6 octahedron leading to collective displacement of Ti ions through attractive dipole-dipole couplings and may give rise to ferroelectricity. In the Ca K-edge XANES spectra there is a pre-edge feature similar to those found in other 3d transition-metal perovskites, which may provide information about hole doping. (author)