Superconductivity in S-rich phases of lanthanum sulfide under high pressure

Pressure, a fundamental thermodynamic variable, enables phase transitions to exotic phases with unique physical properties, such as superconductivity. In this work we perform a complete study of crystal structures and relevant electronic properties of La--S crystalline systems in a pressure range of 0--200 GPa. A structural search based on first-principles swarm-intelligence identifies two hitherto unknown pressure-stabilized stoichiometries, namely, ${\mathrm{LaS}}_{3}$ and ${\mathrm{LaS}}_{5}$, in addition to the previously reported compounds. We find that the S-S bonding patterns in La--S compounds evolve in the following sequence with increasing S content and pressure: Atomic S, ${\mathrm{S}}_{2}$ dimers, one-dimensional linear S chains, and two-dimensional S ladders. Further electron-phonon calculations show that both ${\mathrm{LaS}}_{3}$ and ${\mathrm{LaS}}_{5}$ are superconductors with critical temperatures of 13.6 K at 100 GPa and 11 K at 120 GPa, respectively. The softened acoustic phonon branches are responsible for their superconductivity. Our current work is expected to guide future experimental studies investigating superconductivity and structural features of La--S system and more, in general, of other rare-earth chalcogenides.