Prediction of high-T-c superconductivity in H6SX (X = Cl, Br) at pressures below one megabar

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 super-conductivity 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 H6SX (X = Cl and Br) from the theoretical viewpoint. The results show that H6SCl and H6SBr are potential superconductors with the transition temperatures of 155.4 K at 90 GPa and 136 K at 140 GPa, respectively. Remarkably, H6SCl can be stabilized at the pressure above 82.5 GPa but maintain the superconducting transition above 150 K. Compared with H3S, 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 H3S-like superconductors is substantially reduced below 100 GPa but the transition temperature can be maintained as high as 150 K.