Electronic states of perovskite-type oxides and ferroelectricity

A relation between B-site transition metal and ferroelectricity is investigated in ABO(3) perovskite-type oxides. We focus on intensity of the covalent bonds between the B-site atom and the oxygen octahedron and evaluate Harrison's covalency parameter alpha. The density functional theory within the local density approximation is adopted to obtain the valence electron energy levels of free atoms from which the covalency alpha is calculated. Strong correlation between the covalency alpha and the Curie temperature (T-C) is found. As the covalency alpha increases, T-C of ferroelectricity arises. It is also noted that the hybridization between Pb 6p and O 2p is crucial for the high ferroelectricity of PbTiO3.