Electron-electron correlations and structural, spectral and polarization properties of tetragonal BaTiO₃

To analyze the role of electron-electron correlation effects in structural (local-geometry), spectral and polarization properties of tetragonal BaTiO₃we apply DFT +Uapproach. We demonstrate that the system properties drastically change when the value of the local Coulomb repulsionUcrosses the critical valueU_c≈ 7 eV. In particular, the correlation effects cause a change of the ratio of the in-plane and inter-plane Ti-O bond lengths, which results in a flip of the order of the Tid-bands and change of the polarizability of the system. Since the consensus value ofUin BaTiO₃is unknown, we discuss how the obtained results may be revealed in experimental data, especially in the optical response and ultrafast charge dynamics, where effectiveUis dynamically tuned.