Born effective charges of Barium Titanate: Band by band decomposition and sensitivity to structural features

The Born effective charge tensors of barium titanate have been calculated for each of its four phases. Large effective charges of Ti and O, also predicted by shell-model calculations and made plausible by a simplified model, reflect the partly covalent character of the chemical bond. A band-by-band decomposition confirms that orbital hybridization is not restricted to Ti and O atoms but also involves Ba, which appears more covalent than generally assumed. Our calculations reveal a strong dependence of the effective charges on the atomic positions contrasting with a relative insensitivity on isotropic volume changes.