Post density functional theoretical studies of highly polar semiconductive Pb(Ti_1-xNi_x)O_3-x solid solutions: Effects of cation arrangement on band gap

We use a combination of conventional density functional theory (DFT) and post-DFT methods, which include the local density approximation plus Hubbard U (LDA+U), PBE0, and self-consistent GW, to study the electronic properties of Ni-substituted PbTiO₃ (Ni-PTO) solid solutions. We find that LDA calculations yield unreasonable band structures, especially for Ni-PTO solid solutions that contain an uninterrupted NiO₂ layer. Accurate treatment of localized states in transition-metal oxides such as Ni-PTO requires post-DFT methods. B-site Ni/Ti cation ordering is also investigated. The B-site cation arrangement alters the bonding between Ni and O, and therefore strongly affects the band gap (E_g) of Ni-PTO. We predict that Ni-PTO solid solutions should have a direct band gap in the visible light energy range, with polarization similar to the parent PbTiO₃. This combination of properties makes Ni-PTO solid solutions promising candidate materials for solar energy conversion devices.