Intercalating Helium into A-Site Vacant Perovskites

Evolutionary searches were employed to predict the moststablestructures of perovskites with helium atoms on their A-sites up toa pressure of 10 GPa. The thermodynamics associated with helium intercalationinto [CaZr]-F-6 under pressure, and the mechanical propertiesof the parent perovskite and helium-bearing phase were studied viadensity functional theory (DFT) calculations. The pressure-temperatureconditions where the formation of HeAlF3, HeGaF3, HeInF3, HeScF3, and HeReO3 isfavored from elemental helium and the vacant A-site perovskites werefound. Our DFT calculations show that entropy can stabilize the helium-filledperovskites because the volume that the highly compressible noblegas atom occupies within the perovskite pores may be larger than thevolume it adopts in its elemental form under pressure. We find thathelium incorporation will increase the bulk modulus of AlF3 from a value characteristic of tin (similar to 50 GPa) to one characteristicof stainless-steel (similar to 160 GPa) and hinders the pressure-inducedrotation of its octahedra.