Exploring the effects of oxygen replacement by sulfur in SrTiO3: A comprehensive DFT investigation

This paper is based on the first-principles density functional (DFT) to investigate systematically the structure, mechanical, electronic, optical and thermal behavior of SrTiO 3- x S x . The structural optimizations of all the compounds indicated that they were dynamically and mechanically stable. Sulfur substitution at the oxygen site was modeled using a supercell approach. Substituting oxygen with sulfur causes lattice expansion, which weakens the bonding interactions and consequently leads to a reduction in both the elastic moduli and Debye temperature. HSE06 functional calculations demonstrated a positive change in value of the band gap as SrTiO 3 , SrTiO 2 S, SrTiOS 2 and SrTiS 3 were utilized with the following successive decrease of band gap of 2.737 eV, 1.584 eV, 0.784 eV, and 0.288 eV respectively. The band gap of SrTiO 2 S (1.584 eV) closely matches the Shockley–Queisser limit, highlighting its strong potential for efficient photovoltaic applications. Optical analysis revealed an increase in dielectric constant, refractive index and absorption at the visible light with increase in the amount of sulfur content. The quasi-harmonic Debye model predicts the thermal properties, revealing that sulfur substitution leads to an increase in both the Grüneisen parameter and the thermal expansion coefficient, accompanied by a significant decrease in the melting temperature. The lattice thermal conductivity ( K Ph ) drops sharply from 62.03 W/mK for SrTiO 3 to 5.53 W/mK SrTiS 3 . The minimum thermal conductivity values of S-rich compounds are low and it means that they can be used as thermal barrier coating materials. Overall, sulfur incorporation effectively tunes the mechanical stiffness, electronic band gap, and thermal transport properties, rendering SrTiO 2 S and SrTiS 3 promising candidates for efficient optoelectronic and high-temperature energy device applications.