Tantalum Oxyfluorides

Vanadium yields more or less stable oxyfluorides, and an oxyfluoride of pentavalent niobium, NbOF₃, has been prepared in the dry way, but in the case of tantalum, free oxyfluorides are unknown. Tantalum oxytrifluoride, TaOF₃, does occur, however, in combination with ammonium and potassium fluorides in some double salts. The latter are not so numerous as the analogous niobium compounds, and differ from them also in method of preparation, since they are not formed in the presence of hydrofluoric acid, which gives rise to the double fluorides described above. On being boiled, the double oxyfluorides of tantalum undergo ready hydrolysis, and precipitate either tantalic acid or an oxyfluoride in which the proportion of tantalic acid is greater than in the original salt.

Ammonium Tantalum Oxyfluorides or ammonium fluoroxytantalates. - A substance which has the composition 3NH₄F.TaOF₃ or has been obtained by dissolving tantalic acid in a hot, concentrated solution of ammonium fluoride and cooling, whereupon large transparent octahedra are thrown down. This fluoride is isomorphous both with the corresponding niobium salt, 3NH₄F.NbOF₃, and with the double fluorides of some metals in which the oxygen atom is substituted by another fluoride atom, for example ammonium titanium fluoride, SNH₄F.TiF₄. Evaporation of ammonia from the mother-liquors and cooling yields crystals of the compound 3NH₄.HF.TaOF₃.

Potassium Tantalum Oxyfluoride, 4KF.2TaF₅.Ta₂O₅ or Ta₂O₅. 2(2KF.TaF₅). - Two other potassium tantalum oxyfluorides have been reported, 2KF.TaOF₃ and 3KF.TaOF₃.