The compound NaAlF 4 has been obtained in the form of thin fibrous crystals or fine colorless powder by condensation at 18 1C of vapors arising over chiolite Na 5 Al 3 F 14 or NaCaAlF 6 , heated up to 800 1C. 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 1C, 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) ร
, b =14.9469(7) ร
, c =5.2617(3) ร
, V= 284.10 ร
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 ร
, and those for NayF are in the range 2.297-2.439 ร
. 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.