Novel Superconducting Electrides in Ca–S System under High Pressures

Due to their unique structure properties, most of the electrides that possess extra electrons locating in interstitial regions as anions are insulators. Metallic and superconducting electrides are very rare under ambient conditions. We systematically search possible compounds in Ca–S systems stabilized under various pressures up to 200 GPa, and investigate their crystal structures and properties using first-principles calculations. We predict a series of novel stoichiometries in Ca–S systems as potential superconductors, including P2_1/m Ca_3S, P4mbm Ca_3S, Pnma Ca_2S, Cmcm Ca_2S, Fddd CaS_2, Immm CaS_3 and C2/c CaS_4. The P4mbm Ca_3S phase exhibits a maximum T_\rm c value of \sim 20 K. It is interesting to notice that the P2_1/m Ca_3S and Pnma Ca_2S stabilized at 60 and 50 GPa behave as superconducting electrides with critical temperatures T_\rm c of 7.04 K and 0.26 K, respectively. More importantly, our results demonstrate that P2_1/m Ca_3S and Pnma Ca_2S are dynamically stable at 5 GPa and 0 GPa, respectively, indicating a high possibility to be quenched to ambient condition or synthesized using the large volume press.