Perovskites are of great technological and geological importance, in large part, due to their considerable compositional and structural flexibility. However, the formation of perovskites with neutral species on their A-sites is very unusual. The formation, phase transitions, and properties of [He2][CaZr]F6, which is the first helium-containing perovskite to be made, are reported. It is likely that a large family of related materials can also be prepared. On compression in neon, the negative thermal expansion (NTE) material CaZrF6 amorphizes at ∼0.5 GPa. However, on compression in helium at room temperature, the gas is inserted into the structure to form a perovskite with helium on the A-site. This suppresses the amorphization until >3 GPa. The volume versus pressure and Raman measurements suggest that filling of the A-site, to give [He2][CaZr]F6, is complete at >1 GPa. The presence of helium on the A-site in this perovskite leads to a reduction in the magnitude of NTE when compared to the parent phase CaZrF6, likely due to steric impediment of the transverse vibrational motion of fluoride. Helium also leads to considerable stiffening of the structure. At room temperature and ∼2.5 GPa, the helium-containing hybrid perovskite has a bulk modulus of ∼47 GPa, whereas CaZrF6 has a bulk modulus of ∼40 GPa under ambient conditions. Cubic perovskite [He2][CaZr]F6 undergoes a structural phase transition at 15 K on compression, which may involve a cooperative tilting of framework octahedra to give a lower-symmetry phase, which is tentatively assigned as tetragonal.

Anthony J. Lloyd, Brett R. Hester, Samuel J. Baxter, Shangye Ma, Vitali B. Prakapenka, Sergey N. Tkachev, Changyong Park, and Angus P. Wilkinson, Hybrid Double Perovskite Containing Helium: [He2][CaZr]F6, Chemistry of Materials Article ASAP, DOI: 10.1021/acs.chemmater.0c04782 abstract