Design of Ternary Ca-S-H System that are Superconducting Under Pressure

Hydrogen-rich compounds are promising candidates as high pressure superconductors under high compression. Among the most promising hydrides for high temperture superconductivity are those of sulphur and calcium. The high pressure phase transition of S-H system has resulted in the second highest T_c to date, 203 K at 160 GPa, while a Tc of 220-235K has been predicted for CaH₆ at 150 GPa. Here, we present a systematic investigation of Ca-S-H phases with various stoichiometries. We find that CaSH₂, CaSH₆ and CaSH₂₀ are thermodynamically stable at pressures accessible in diamond anvil cells. In these structures hydrogen still exists in its molecular form, and all of these phases are semiconductors with small band gaps. However, an interesting phase with the composition CaSH₃ has been found, which is metastable and metallic, and does not contain any H-H bonds. This unique structure has a T_c up to 46 K at 250 GPa. Analysis of its electronic structure, including the sulphur and hydrogen contributions, reveals a van Hove singularity in the density of states near the Fermi Energy. This feature may be important for the superconductivity. Our approach may pave the way for finding high-T_c superconductors in various ternary hydrides.