Abstract:
We present a first-principles study of the low-temperature rhombohedral phase of BaTiO3 using Hubbard-corrected density-functional theory. By employing density-functional perturbation theory, we compute the onsite
HubbardU for Ti(3d) states and the intersite HubbardV between Ti(3d) and O(2p) states. We show that applying
the onsite Hubbard U correction alone to Ti(3d) states proves detrimental, as it suppresses the Ti(3d)-O(2p)
hybridization and drives the system towards a cubic phase. Conversely, when both onsite U and intersite V are
considered, the localized character of the Ti(3d) states is maintained while also preserving the Ti(3d)-O(2p)
hybridization, restoring the rhombohedral phase of BaTiO3. The generalized PBEsol+U+V functional yields
good agreement with experimental results for the band gap and dielectric constant, while the optimized geometry
is slightly less accurate compared to PBEsol. Zone-center phonon frequencies and Raman spectra are found to be
significantly influenced by the underlying geometry. PBEsol and PBEsol+U+V provide satisfactory agreement
with the experimental Raman spectrum when the PBEsol geometry is used, while PBEsol+U Raman spectrum
diverges strongly from experimental data, highlighting the adverse impact of the U correction alone in BaTiO3.
Our findings underscore the promise of the extended Hubbard PBEsol+U+V functional with first-principles U
and V for the investigation of other ferroelectric perovskites with mixed ionic-covalent interactions.
