Synthesis,structure,and thermal expansion in CaZrF_(6) with two polymorphs

Due to the high structural flexibility and controllable thermal expansion,cubic double ReO_(3)-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 CaZrF_(6),and it is expected to be generalized to the synthesis of other double ReO_(3)-fluorides.Intriguingly,a new orthorhombic phase CaZrF_(6) has been synthesized via this method in a lower temperature.Unlike the strong isotropic NTE of the cubic phase CaZrF_(6),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 CaZrF_(6) are strongly correlated with the transverse thermal vibration of F atoms and dominates the NTE of the material.