A new isotropic negative thermal expansion material of CaSnF6 with facile and low-cost synthesis

Double ReO3-type fluorides have a great interest in the field of negative thermal expansion (NTE) and luminescent materials. \nHowever, their application is limited by the scarcity of quantity, expensive raw materials, and harsh preparation conditions. In this \nwork we have found a new NTE material, CaSnF6, by applying the concept of the average atomic volume. More importantly, \ndifferent from the previous solid-phase sintering and direct fluorination methods, the nano CaSnF6 has been synthesized for the \nfirst time by solvothermal method. The results of X-ray diffraction (XRD) and Raman spectroscopy show that a phase transition \noccurs from rhombohedral ( ) to cubic ( ) structure at about 200 K, and a strong isotropic NTE (αv = −15.78 × 10−6 K−1) appears in the cubic phase. Lattice dynamics calculations from first-principles illustrate that the NTE is due to the transverse vibration of fluorine atoms excited by low-frequency phonons. This work not only broadens the NTE family of fluorides, but also provides a new facile and low-cost fabricati on method for the preparation of NTE fluorides.