Hybrid Double Perovskite Containing Helium: [He-2][Cazr]F-6

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 [He-2][CaZr]F-6, 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 similar to 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 [He-2][CaZr]F-6, 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 similar to 2.5 GPa, the helium-containing hybrid perovskite has a bulk modulus of similar to 47 GPa, whereas CaZrF6 has a bulk modulus of similar to 40 GPa under ambient conditions. Cubic perovskite [He-2][CaZr]F-6 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.