Lattice dynamics, dielectric properties and structural instabilities of SrTiO3 and BaTiO3

We extend our linear response description of materials with a strong ionic component of the electronic structure, previously applied successfully to the lattice dynamics, electron-phonon interaction and dielectric properties of the high-temperature superconductors (HTSC), to SrTiO3 and BaTiO3 which crystallize in the (perovskite) parent structure of the HTSC. Besides phonon-induced localized polarization processes at the ions in the form of dipole fluctuations, calculated with a modified Sternheimer method, which are typical for the ionic component of binding, on-site and off-site charge fluctuations modelling a charge flow along the Ti-O axis prove to be important. The latter delocalized polarization processes related to the covalent component of binding are introduced in our modelling to simulate changes of the hybridization between O 2p and Ti 3d orbitals under atomic displacements. Altogether, we obtain a reasonable description of the phonon dispersion and the dielectric properties of both materials, including the anomalous Born effective charges and the macroscopic dielectric constant. Moreover, comparative investigations within our microscopic model provide physical insight into the ferroelectric (FE) phase transition of BaTiO3 and the antiferrodistortive (AFD) transition of SrTiO3