Tuning the opto-electronic properties of BaTiO3 by S substitution towards energy harvesting applications: a DFT insight using the VASP code

Abstract

This study explored the structural, electrical, optical, mechanical, and thermal properties of sulfur-substituted barium titanate perovskites (BaTiO_3−xS_x) using first-principles density functional theory (DFT) calculations, implemented in the Vienna Ab initio Simulation Package (VASP). Sulfur (S) substitution at oxygen (O) sites led to the formation of BaTiO₂S, BaTiOS₂, and BaTiS₃. The formation energy, tolerance factors, and AIMD simulation ensured the structural stability of these phases. The band structure calculation revealed a reduction of band gap from 3.069 eV (BaTiO₃) to 1.55, 1.02, and 0.35 eV for BaTiO₂S, BaTiOS₂, and BaTiS₃, respectively. The optical properties calculations revealed that the S substitution enhances the absorption coefficient, improving the optical properties in the visible range. The reduction of the band gap, which enhances the optical properties, has been explained in terms of the partial density of states (PDOS). The mechanical stability of the BaTiO_3−xS_x compounds was confirmed by calculating the elastic constants. The changes in chemical bonding due to S substitution result in a variation in the elastic moduli. The effect of S substitution on the thermal properties was also studied, and a significant decrease in lattice thermal conductivity was explained in terms of phonon scattering. The results obtained for BaTiO₂S and BaTiOS₂ ordered cells were also cross-checked by performing calculations in supercell structures. The reduced band gap and high absorption coefficient of BaTiO₂S and BaTiOS₂ suggest a possible use in solar energy harvesting, whereas BaTiS₃ could be a potential thermoelectric material (with low lattice thermal conductivity).