Stoichiometric change and solid decomposition in Ca–S compounds under high pressure

As an independent thermodynamic parameter, pressure significantly influences interatomic distances, leading to an increase in material density. In this work, we employ the CALYPSO structure search and density functional theory calculations to explore the structural phase transitions and electronic properties of calcium–sulfur compounds (Ca_xS_1−x, where x = 1/4, 1/3, 1/2, 2/3, 3/4, 4/5) under 0–1200 GPa. The calculated formation enthalpies suggest that Ca_xS_1−x compounds undergo multiple phase transitions and eventually decompose into elemental Ca and S, challenging the traditional view that pressure stabilizes and densifies compounds. The analysis of formation enthalpy indicates that an increase in pressure leads to a rise in internal energy and the PV term, resulting in thermodynamic instability. Bader charge analysis reveals that this phenomenon is attributed to a decrease in charge transfer under high pressure. The activation of Ca-3d orbitals is significantly enhanced under pressure, leading to competition with Ca-4s orbitals and S-3p orbitals. This may cause the formation enthalpy minimum on the convex hull to shift sequentially from CaS to CaS₃, then to Ca₃S and Ca₂S, and finally back to CaS. These findings provide critical insights into the behavior of alkaline-earth metal sulfides under high pressure, with implications for the synthesis and application of novel materials under extreme conditions and for understanding element distribution in planetary interiors.