Title: The Control of Thermal Expansion and the Synthesis of Helium Hybrid Perovskites using ReO3-type Fluorides

Abstract

Cubic ReO3-type fluorides, such as ScF3 and CaZrF6, display strong negative thermal expansion (NTE) over a wide temperature range. They combine NTE with low absorption from the mid-IR out into the UV, and they can be processed as ceramics, suggesting application in controlled thermal expansion optics. However, they undergo undesirable phase transitions at low pressures. Solid solution formation by cation substitution typically makes this problem worse. The introduction of defects in the form of interstitial fluoride, by moving away from the ideal stoichiometry, enables their thermal expansion to be tuned while also improving their resistance to both pressure and temperature induced structural phase transitions. This will be demonstrated using Mg1-xZr1+xF6+2x as an example.

The introduction of helium onto the vacant A-sites of materials such as CaZrF6, by exposure to high pressure gas, leads to the formation of defect perovskites, such as [He2-x][CaZr]F6. Similar “syntheses” can be performed for other hosts. The resulting materials contain very high volumetric densities of stored gas, and gas incorporation changes the physical properties of the material.