Synthesis, structure, and thermal expansion in CaZrF₆ with two polymorphs

Due to the high structural flexibility and controllable thermal expansion, cubic double ReO₃-type negative thermal expansion (NTE) fluorides provide a solution for solving the prominent phenomenon of thermal expansion mismatch between materials. However, the expensive raw materials and complex synthesis steps limit its practical application. In this work, we have designed a more advantageous method for the synthesis of NTE material CaZrF₆, and it is expected to be generalized to the synthesis of other double ReO₃-fluorides. Intriguingly, a new orthorhombic phase CaZrF₆ has been synthesized via this method in a lower temperature. Unlike the strong isotropic NTE of the cubic phase CaZrF₆, the orthorhombic phase shows the strong anisotropic positive thermal expansion (PTE). The combined analysis of temperature-dependent X-ray diffraction (XRD), Raman spectra, and first-principles calculations shows that the low frequency phonon vibration mode with negative Grüneisen parameter in cubic CaZrF₆ are strongly correlated with the transverse thermal vibration of F atoms and dominates the NTE of the material.