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. 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, CaSnF6, 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 (R (3) over bar) to cubic (Fm (3) over barm) structure at about 200 K, and a strong isotropic NTE (alpha(nu) = -15.78 x 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.

Automated summary

In this study, a new negative thermal expansion (NTE) material, CaSnF6, was synthesized by solvothermal method for the first time. The XRD and Raman spectroscopy results showed a phase transition from rhombohedral to cubic structure at about 200 K, accompanied by a significant isotropic NTE effect in the cubic phase. Lattice dynamics calculations revealed that the NTE is attributed to the transverse vibration of fluorine atoms excited by low-frequency phonons. This work not only expands the NTE family of fluorides, but also provides a new facile and low-cost fabrication method for NTE fluorides.