Recently, the superconducting transition temperature $T_c=287\mathrm{K}$ has been experimentally obtained in a material composed of carbon, sulfur, and hydrogen under a high pressure of 267 GPa. The material structure is currently unknown, where carbon and sulfur were added at a molar ratio of $1:1$. Here, fixing the molar ratio of $\text{C}:\text{S}=1:1$, we studied several possible C-S-H structures, and found a stable structure ${\mathrm{C}}_2{\mathrm{S}}_2{\mathrm{H}}_4$ using first-principles calculations. ${\mathrm{C}}_2{\mathrm{S}}_2{\mathrm{H}}_4$ shows an insulator-to-metal transition and a superconducting ground state at a pressure of 64 GPa, and its $T_c$ can reach 16.5 K at 300 GPa. In addition, we found another stable structure of ${\mathrm{C}}_2{\mathrm{S}}_3{\mathrm{H}}_4$, whose $T_c$ is 47.4 K at 300 GPa. The calculations show that the added S atom in ${\mathrm{C}}_2{\mathrm{S}}_3{\mathrm{H}}_4$ breaks part of the C-H bonds in ${\mathrm{C}}_2{\mathrm{S}}_2{\mathrm{H}}_4$, makes the vibration of H atom occur at a lower frequency, and thus enhances the electron-phonon coupling and $T_c$. Our results suggest that a molar ratio of $\text{C}:\text{S}$ lower than $1:1$ in C-S-H systems may be favorable to enhance $T_c$. This can be useful to figure out the structure of a C-S-H material with room-temperature $T_c$ in a recent experiment.

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• Superconductivity
• Superconductors
• BCS theory
• Density functional theory