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. Variable temperature and high pressure powder diffraction have been used to examine the behavior of CaZrF6. It remains cubic down to at least 10K and displays strong NTE from &lt; 10K to above 1000K with a room temperature linear CTE of ∼-11 ppm·K-1. Its NTE at room temperature and below is far stronger than that of ZrW2O8. On compression in silicone oil, long range order is lost at ~ 400 MPa. First principles calculations of phonons for CaZrF6 suggest a subtly different mechanism for the NTE in CaZrF6 when compared to that for ScF3. In contrast to CaZrF6, CoZrF6 undergoes a cubic-to-rhombohedral phase transitions near room temperature and display much weaker negative thermal expansion in the cubic phase (linear CTE∼-3 ppm·K-1 at 400 K). An understanding of the factors influencing the thermal expansion within this structure type may allow for the development of controlled, including zero, thermal expansion materials.

Negative thermal expansion and other anomalous properties in rock salt ordered mixed metal fluorides M(II)ZrF6 with a ReO3-type structure

249th National Meeting (2015)

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