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 several M(II)M'(IV)F6 including CaZrF6, MgZrF6, CaNbF6 and MgNbF6. Of these compounds, CaZrF6 is best characterized. 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 ppmK-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. On compression in helium at room temperature, the material apparently starts to expand above ~ 300 MPa, indicating penetration by helium. This porosity is temperature dependent.

Negative thermal expansion and other anomalous properties in mixed metal fluorides with structures related to that of ReO3: A perovskite with helium on the A-site?

253rd National Meeting (2017)

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