Intercalating Helium into A-Site Vacant Perovskites

Evolutionary searches were employed to predict the most stable structures of perovskites with helium atoms on their A-sites up to a pressure of 10 GPa. The thermodynamics associated with helium intercalation into [CaZr]F₆ under pressure, and the mechanical properties of the parent perovskite and helium-bearing phase were studied via density functional theory (DFT) calculations. The pressure-temperature conditions where the formation of HeAlF₃, HeGaF₃, HeInF₃, HeScF₃, and HeReO₃ is favored from elemental helium and the vacant A-site perovskites were found. Our DFT calculations show that entropy can stabilize the helium-filled perovskites because the volume that the highly compressible noble gas atom occupies within the perovskite pores may be larger than the volume it adopts in its elemental form under pressure. We find that helium incorporation will increase the bulk modulus of AlF₃ from a value characteristic of tin (∼50 GPa) to one characteristic of stainless-steel (∼160 GPa) and hinders the pressure-induced rotation of its octahedra.