Negative thermal expansion and other anomalous properties of metal fluorides with ReO3-related structures

Thermal expansion is a phenomenon that chemists do not often consider, and yet it is an important material’s characteristic for many applications. For example, Pyrex is used for laboratory glassware because it has a close to zero thermal expansion coefficient. This makes the glassware resistant to failure when subject to rapid temperature changes. While most materials expand on heating, there are materials that shrink when heated and this unusual property can have value.

Materials with a cubic-ReO3 structure have long been suggested as good candidates for negative thermal expansion (NTE). Additionally, the simplicity of the cubic-ReO3 structure is appealing for studies focused on understanding the factors governing a material’s properties. In 2010, the first ReO3-type material to display strong NTE over a broad temperature range, ScF3, was reported by us. Subsequently, we have prepared and examined by variable temperature and pressure diffraction experiments the effects of solid solution formation in Sc1-xMxF3 (M – trivalent cation), and the properties of the cation ordered   ReO3-type ABF6 (A – divalent ion, B tetravalent ion). This work has revealed an array of unusual properties including very strong negative thermal expansion, pronounced elastic stiffening on heating, strong softening on compression and temperature dependent porosity, which allows for the trapping and thermally induced release of small gas molecules.