A new isotropic negative thermal expansion material of CaSnF_(6)with facile and low-cost synthesis

Double ReO_(3)-type fluorides have a great interest in the field of negative thermal expansion(NTE)and luminescent materials.However,their application is limited by the scarcity of quantity,expensive raw materials,and harsh preparation conditions.In this work we have found a new NTE material,CaSnF_(6),by applying the concept of the average atomic volume.More importantly,different from the previous solid-phase sintering and direct fluorination methods,the nano CaSnF6 has been synthesized for the first time by solvothermal method.The results of X-ray diffraction(XRD)and Raman spectroscopy show that a phase transition occurs from rhombohedral(R3)to cubic(Fm3m)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.