Low temperature tetragonal polymorph of CaZrF6

A new tetragonal polymorph of CaZrF6 can be prepared by high energy ball milling of a CaF2/ZrF4 mixture, followed by heat treatment at 325 °C. This polymorph is thermodynamically stable with respect to the well-known cubic form at low temperatures. However, it readily transforms to the cubic form on heating above ∼400 °C. The tetragonal (β) CaZrF6 is not isostructural with any previously known alkaline earth AZrF6 phase. Unlike the cubic form, which shows strong negative thermal expansion over a wide temperature range, the tetragonal form displays positive thermal expansion in all directions (100–400 K: αl ∼ +17 × 10−6 K−1 and +13 × 10−6 K−1 along the a- and c-axes, respectively).