NaAlF4: Preparation, Crystal Structure and Thermal Stability.

Abstract The compound NaAlF 4 has been obtained in the form of thin fibrous crystals or fine colorless powder by condensation at 18 °C of vapors arising over chiolite Na 5 Al 3 F 14 or NaCaAlF 6 , 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, NaAlF 4 is stable up to 390–400 °C, then there is an exothermal solid state decay into Na 5 Al 3 F 14 (s) and AlF 3 (s). At higher temperature Na 5 Al 3 F 14 (s) decays into Na 3 AlF 6 (s) and NaAlF 4 (g). The crystal structure (space group Cmcm , a =3.6124(1) A, b =14.9469(7) A, c =5.2617(3) A, V =284.10 A 3 ) has been determined by X-ray powder diffraction method. In the crystal structure of NaAlF 4 the octahedrons [AlF 6 ] 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 A, and those for Na…F are in the range 2.297–2.439 A. In spite of limited thermal stability of the crystal form, the compound NaAlF 4 is the main component of the gas mixture over solid and molten salts in the ternary system NaF–AlF 3 –CaF 2 and participates in chemical transformations between the phases at high temperature.