High-pressure sulfur frameworks in Mg-S compounds: Tuning superconductivity through covalent connectivity

Understanding the behavior of compounds with rich covalent connectivity under high pressure is crucial for advancing superconductor design. In this study, we identify two superconducting S-rich Mg-S compounds, $P6$/mmm $\mathrm{Mg}{\mathrm{S}}_{2}$ and $I4/m \mathrm{Mg}{\mathrm{S}}_{4}$, using first-principles structure search calculations under high-pressure conditions. These compounds exhibit unique three-dimensional S covalent networks, featuring honeycomblike and dual-channel structures formed by fivefold-connected trigonal bipyramid and sixfold-connected quasitriangular prism units of S atoms, respectively. The partial occupation of antibonding states and the involvement of S $3d$ orbitals contribute to the formation of weak S-S covalent bonds, which support these high-connectivity motifs. Furthermore, the superconducting properties of these sulfides are largely contributed by their S frameworks, with $P6$/mmm $\mathrm{Mg}{\mathrm{S}}_{2}$ achieving a ${T}_{\mathrm{c}}$ of 26.1 K at 300 GPa. This structural arrangement enhances the electronic density of states at the Fermi surface and softens acoustic phonon modes, enabling strong electron-phonon coupling. This work offers valuable insights for designing novel superconductors through precise manipulation of covalent bonding patterns.