Density functional theory study of hypothetical PbTiO₃ based oxysulfides

Using density functional theory (DFT) within the local density approximation (LDA), we calculate the physical and electronic properties of PbTiO₃ (PTO) and a series of hypothetical compounds PbTiO3−𝑥⁢Sₓ 𝑥=0.2, 0.25, 0.33, 0.5, 1, 2, and 3 arranged in the corner-sharing cubic perovskite structure. We determine that replacing the apical oxygen atom in the PTO tetragonal unit cell with a sulfur atom reduces the 𝑥=0 LDA calculated band gap of 1.47 eV to 0.43–0.67 eV for 𝑥=0.2–1 and increases the polarization. PBE0 and 𝐺⁡𝑊 methods predict that the compositions 𝑥=0.2–2 will have band gaps in the visible range. For all values of 𝑥 < 2, the oxysulfide perovskite retains the tetragonal phase of PbTiO3, and the 𝑎 lattice parameter remains within 2.5% of the oxide. Thermodynamic analysis indicates that chemical routes using high-temperature gas, such as H₂S and CS₂, can be used to substitute O for S in PTO for the compositions 𝑥=0.2–0.5.