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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Chemistry

Background:

• Potassium hexafluoroaluminate (KAlF₄) is a key material in various industrial applications.
• Understanding its structural behavior under varying temperatures is crucial for optimizing its use.
• Previous studies have explored its properties, but a comprehensive investigation of its high-temperature phases and molten state was lacking.

Purpose of the Study:

• To investigate the temperature-dependent microstructure of KAlF₄ using in situ experimental techniques.
• To elucidate the structural transformations of KAlF₄ from solid crystalline phases to the molten state.
• To compare experimental findings with theoretical simulations for validation.

Main Methods:

• In situ high-temperature X-ray diffraction and Raman spectroscopy were employed to study solid KAlF₄.
• Density functional theory (DFT) was used to simulate the crystalline structures.
• Quantum chemistry ab initio simulations were performed to explore the molten KAlF₄ structure.

Main Results:

• Two crystal polymorphs of KAlF₄ were identified: a tetragonal phase (P4/mbm) below 673 K and a monoclinic phase (P2₁/m) above 673 K.
• Both solid phases exhibit layered structures with K⁺ ions and [AlF₆]³⁻ octahedra sharing corners.
• Molten KAlF₄ primarily consists of [AlF₄]⁻ ions with a smaller fraction of [AlF₆]³⁻ ions.
• Simulated Raman spectra for molten KAlF₄ closely matched experimental results.

Conclusions:

• KAlF₄ undergoes a phase transition from tetragonal to monoclinic structure with increasing temperature.
• The layered structure of solid KAlF₄ becomes unstable above its melting point.
• The ionic composition of molten KAlF₄ is predominantly [AlF₄]⁻, supported by theoretical calculations.