Post density functional theoretical studies of highly polar semiconductive Pb(Ti ₁₋ₓ Ni 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 (𝐸𝑔) 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.