Temperature Dependent Micro-Structure of KAlF4 from Solid to Molten States

In situ high temperature X-ray diffraction and Raman spectroscopy were used to investigate the temperature dependent micro-structure of KAlF4. Density functional theory was applied to simulate the structure of crystalline KAlF4 while a quantum chemistry ab initio simulation was performed to explore the structure of molten KAlF4. Two crystal polymorphs demonstrated to be present in solid KAlF4. At the temperature below 673 K, it belongs to the tetragonal crystal system within the P4/mbm space group, while the high temperature phase is attributed to the monoclinic crystal system within the P21/m space group. Both polymorph KAlF4 phases are characterized by a layered structure consisting of K+ and [AlF6]3− octahedra, each of the [AlF6]3− octahedra equivalently shares four corners with other four [AlF6]3− octahedra along the layer. The layered structure became unstable at higher temperatures and crashed when the temperature exceeded the melting point. It demonstrated that the molten KAlF4 consisted of predominant [AlF4]− and a small amount of [AlF6]3−. The Raman spectrum of molten KAlF4 simulated by using a quantum chemistry ab initio method agreed well with the experimental Raman spectrum.