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_c of hydrogen-related superconductors, we systematically studied the crystal structure and superconductivity of H₆SSe , a hypothetical compound derived from H₃S with half its S atoms replaced by group neighbor Se. First-principles structure searches identify three dynamically stable structures for H₆SSe at 200 GPa. Interestingly, all three structures keep the main feature of the cubic I m 3 ¯m structure of H₃S , but with different Se substitution positions. Electron-phonon coupling calculations reveal the superconductive potential of the three phases of H₆SSe , with T_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_c. For comparison, O-substituted H₆SO was predicted to assume a semiconducting phase with entirely different structural features from H₆SSe . We attribute this difference to the much stronger electronegativity of O (3.44) compared with S (2.58) or Se (2.55).