A First-Principles Thermodynamic Model for the Ba-Zr-S System in Equilibrium with Sulfur Vapor

The chalcogenide perovskite BaZrS₃ has strong visible light absorption and high chemical stability, is nontoxic, and is made from earth-abundant elements. As such, it is a promising candidate material for application in optoelectronic technologies. However, the synthesis of BaZrS₃ thin-films for characterization and device integration remains a challenge. Here, we use density functional theory and lattice dynamics to calculate the vibrational properties of elemental, binary, and ternary materials in the Ba-Zr-S system. This is used to build a thermodynamic model for the stability of BaZrS₃, BaS , and ZrS in equilibrium with sulfur gas across a range of temperatures and sulfur partial pressures. We highlight that reaction thermodynamics are highly sensitive to sulfur allotropes and the extent of allotrope mixing. We use our model to predict the synthesis conditions in which BaZrS₃ and the intermediate binary compounds can form. At a moderate temperature of 500 °C, we find that BaS₃, associated with fast reaction kinetics, is stable at pressures above 3 × 10⁵ Pa. We also find that BaZrS₃ is stable against decomposition into sulfur-rich binaries up to at least 1 × 10⁷ Pa. Our work provides insights into the chemistry of this promising material and suggests the experimental conditions required for the successful synthesis of BaZrS₃.