Prediction of pressure-induced stabilization of noble-gas-atom compounds with alkali oxides and alkali sulfides

The cubic antifluorite structure comprises a face-centered cubic sublattice of anions with cations on the tetrahedral sites. The voids in the antifluorite structure that are crucial for superionicity in Li2O might also act as atomic traps. Trapping of guest atoms and small molecules within voids of a host structure leads to the formation of what are known as clathrate compounds. Here we investigate the possibility of trapping helium or larger neon guest atoms under pressure within alkali metal oxide and sulfide structures. We find stable helium and neon-bearing compounds at very low pressures. These structures are stabilized by a reduction in volume from incorporation of helium or neon atoms within the antifluorite structure. We predict that NeCs2S could be stable at ambient pressure. Our study suggests a novel class of alkali oxide and sulfide materials incorporating noble gas atoms that might potentially be useful for gas storage.