NaAlF4: Preparation, crystal structure and thermal stability

The compound NaAlF4 has been obtained in the form of thin fibrous crystals or fine colorless powder by condensation at 18 °C of vapors arising over chiolite Na5Al3F14 or NaCaAlF6, heated up to 800 °C. Thermal stability has been investigated by the methods of thermal analysis and high temperature X-ray diffraction. When heated in air, NaAlF4 is stable up to 390–400 °C, then there is an exothermal solid state decay into Na5Al3F14(s) and AlF3(s). At higher temperature Na5Al3F14(s) decays into Na3AlF6(s) and NaAlF4(g). The crystal structure (space group Cmcm, a=3.6124(1) Å, b=14.9469(7) Å, c=5.2617(3) Å, V=284.10 Å3) has been determined by X-ray powder diffraction method. In the crystal structure of NaAlF4 the octahedrons [AlF6] are joined through vertices and form corrugated layers, sodium ion layers being located between them. The distances between the atoms of Al–F are in the range 1.791–1.814 Å, and those for Na…F are in the range 2.297–2.439 Å. In spite of limited thermal stability of the crystal form, the compound NaAlF4 is the main component of the gas mixture over solid and molten salts in the ternary system NaF–AlF3–CaF2 and participates in chemical transformations between the phases at high temperature.

Graphical abstractThe compound NaAlF4 was for the first time described 55 years ago, but until now it was not properly studied. Meantime the phase is responsible for the most low-melting part of the NaF–AlF3 system, which is the great importance for the aluminum production. The lack of information about NaAlF4 is due to narrow interval of stability which is close to liquid part of the system.Figure optionsDownload full-size imageDownload as PowerPoint slide