Effect of covalent bonding on the superconducting critical temperature of the H-S-Se system

Hydrogen-rich materials have attracted great interest since the recent discovery of superconductivity at 203 K in highly compressed hydrogen sulfide. To probe the role of covalent bonding in determining the ${T}_{\mathrm{c}}$ of hydrogen-related superconductors, we systematically studied the crystal structure and superconductivity of ${\mathrm{H}}_{6}\mathrm{SSe}$, a hypothetical compound derived from ${\mathrm{H}}_{3}\mathrm{S}$ with half its S atoms replaced by group neighbor Se. First-principles structure searches identify three dynamically stable structures for ${\mathrm{H}}_{6}\mathrm{SSe}$ at 200 GPa. Interestingly, all three structures keep the main feature of the cubic $Im\overline{3}m$ structure of ${\mathrm{H}}_{3}\mathrm{S}$, but with different Se substitution positions. Electron-phonon coupling calculations reveal the superconductive potential of the three phases of ${\mathrm{H}}_{6}\mathrm{SSe}$, with ${T}_{\mathrm{c}}$ decreasing (from 195 to 115 K) upon the declining strength of the weakest covalent H-S or H-Se bonds in each structure, thereby highlighting the key role of covalent bonding in determining ${T}_{\mathrm{c}}$. For comparison, O-substituted ${\mathrm{H}}_{6}\mathrm{SO}$ was predicted to assume a semiconducting phase with entirely different structural features from ${\mathrm{H}}_{6}\mathrm{SSe}$. We attribute this difference to the much stronger electronegativity of O (3.44) compared with S (2.58) or Se (2.55).