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 degrees 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 similar to 400 degrees C. The tetragonal (beta) CaZrF6 is not isostructural with any previously known alkaline earth AZrF(6) 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: alpha(l) similar to +17 x 10(-6) K-1 and +13 x 10(-6) K-1 along the a- and c-axes, respectively). (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).