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.