Thermodynamics of Sulfur Vacancy Formation in the Chalcogenide Perovskite BaZrS₃

Chalcogenide perovskites such as BaZrS₃ hold potential as promising photovoltaic materials; however, their integration into solar energy devices is currently limited by the high-temperature processing requirements. To explore alternative low-temperature synthesis pathways, we performed an ab initio thermodynamic analysis, highlighting the critical role of sulfur vapor flux, mainly gaseous S₂ and S₈, in driving the synthesis. Our findings reveal that sulfur vapor precursors can provide a thermodynamic driving force 10-10² times stronger than that from traditional solid-state methods. Moreover, we find that sulfur gas composition significantly affects the concentration of sulfur vacancy defects in BaZrS₃. In particular, for low-temperature synthesis below 600 °C, gaseous S₂ emerges as the optimal precursor to produce high-quality BaZrS₃ with reduced defect concentrations. The thermodynamic trend of sulfur vacancy formation is governed by the evaporative nature of sulfur and is independent of specific synthesis reactions. This conclusion holds broader implications for generic chalcogenide synthesis where sulfur vacancy management is important.