Phase Diagram of Ternary Carbon-Sulfur-Hydrogen System up to 300 GPa*

The recent discovery of a room-temperature superconductor[1] with T_c = 287.7±1.2 K at high pressures (267±10 GPa) in carbonaceous sulfur hydride has demonstrated the necessity of analyzing the phase diagrams of such ternary C-S-H systems. The stoichiometry and the electronic properties of this material are now an open question. Using evolutionary algorithms and density functional theory (DFT) calculations, we explore the phase diagram of this system. We report the candidate for the low-pressure phase III prior to metallization at ~60 GPa as C₂S₂H₁₀, using comparisons of the Raman modes from calculations vis-à-vis experiments. Due to the limit of chemical accuracy of DFT being ~50 meV, it is almost impossible to discern the candidate structures among a host of structures purely from free-energy comparisons. We present a phase diagram spanning from 30 to 300 GPa derived from pure thermodynamic considerations, inclusive of the zero-point effect which is significant because of the presence of hydrogen. Possible superconducting phase(s) will be discussed.


[1] E. Snider, N. Dasenbrock-Gammon, R. McBride et al., Nature 586, 373 (2020).