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.