After the discovery of near room-temperature superconductivity in superhydrides at extremely high pressure close to 300 GPa, there is increasing interest in finding superconducting systems to maintain the similar superconductivity but at pressures below megabar. To examine such a possibility in metal-free hydrides, we investigate the thermodynamical stability and dynamical stability, electronic structures, and electron-phonon interactions of ${\mathrm{H}}_6\mathrm{S}X$ ($X=$ Cl and Br) from the theoretical viewpoint. The results show that ${\mathrm{H}}_6\mathrm{SCl}$ and ${\mathrm{H}}_6\mathrm{SBr}$ are potential superconductors with the transition temperatures of 155.4 K at 90 GPa and 136 K at 140 GPa, respectively. Remarkably, ${\mathrm{H}}_6\mathrm{SCl}$ can be stabilized at the pressure above 82.5 GPa but maintain the superconducting transition above 150 K. Compared with ${\mathrm{H}}_3\mathrm{S}$, the substitution of S by Cl with lower electronic energy states leads to the enhancement of Cl-H covalent bonding. As a result, the stable pressure of ${\mathrm{H}}_3\mathrm{S}$-like superconductors is substantially reduced below 100 GPa but the transition temperature can be maintained as high as 150 K.

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• Electron-phonon coupling
• Electronic structure
• Impurities in superconductors
• Multiband superconductivity
• Phonons
• Pressure effects
• Superconducting phase transition
• High-temperature superconductors
• Density functional calculations
• Methods in superconductivity