High-temperature powder synchrotron diffraction studies of synthetic cryolite Na3AlF6

A high-resolution synchrotron diffraction study of the structures of a synthetic sample of cryolite Na₃AlF₆ from room temperature to 800°C is reported. At room temperature Na₃AlF₆ is monoclinic and the structure is described in space group P2₁/n. Heating the sample to 560°C results in only minor changes to the structure. A first-order transition from this monoclinic structure to a high-temperature cubic structure is observed near 567°C. The cubic structure is characterized by disorder of the fluoride atoms.