Some materials of the type M(II)M'(IV)F6 have a cubic-ReO3 structure, which makes them 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 thermal expansion. There is little published thermal expansion data for this group of materials. However, it is known that many members of this family undergo a cubic-to-rhombohedral phase transition upon cooling, which would limit their potential as NTE materials. Variable temperature x-ray and neutron powder diffraction have been used to examine the thermal expansions and phase transitions in CaZrF6, CoZrF6, and ZnZrF6. CaZrF6 remains cubic down to at least 10K and displays strong NTE from < 10K to above 1000K with a room temperature CTE of ~-11 ppm�K-1. In contrast, CoZrF6 and ZnZrF6 undergo cubic-to-rhombohedral phase transitions near room temperature (272K for CoZrF6 and 310K for ZnZrF6), and display much weaker negative thermal expansion in the cubic phase (CTE~-2 ppm�K-1). An understanding of the factors that influence the thermal expansion within this specific structure type, such as cation size and covalency, and fluoride coordination environment, may allow for the development of controlled thermal expansion materials.

Isotropic negative thermal expansion in rock-salt ordered mixed metal fluorides M(II)ZrF6 (M(II)=Ca, Co, Zn) with a ReO?3?-type structure

247th National Meeting (2014)

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