Synthesis and Properties of the Helium Clathrate and Defect Perovskite [He □ ][CaNb]F.

The defect double perovskite [He □ ][CaNb]F, with helium on its A-site, can be prepared by the insertion of helium into ReO-type CaNbF at high pressure. Upon cooling from 300 to 100 K under 0.4 GPa helium, ∼60% of the A-sites become occupied.

Negative Thermal Expansion, Response to Pressure and Phase Transitions in CaTiF.

Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO-type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF, CaZrF, CaHfF, and CaNbF. CaTiF was examined in an effort to expand this small family of materials. However, it undergoes a cubic ( Fm3̅ m) to rhombohedral ( R3̅) transition on cooling to ∼120 K, with a minimum volume coefficient of thermal expansion (CTE) close to -42 ppm K at 180 K and a CTE of about -32 ppm K at room temperature.

Synthesis of Defect Perovskites (He□)(CaZr)F by Inserting Helium into the Negative Thermal Expansion Material CaZrF.

Defect perovskites (He□)(CaZr)F can be prepared by inserting helium into CaZrF at high pressure. They can be recovered to ambient pressure at low temperature. There are no prior examples of perovskites with noble gases on the A-sites.