Bismuth Trifluoride, BiF₃

Bismuth Trifluoride, BiF₃, is obtained by the addition of a concentrated solution of potassium fluoride to a neutral solution of bismuth nitrate; or by the action of hydrofluoric acid upon bismuth trioxide. In the latter case the mixture is warmed, and hydrofluoric acid is added as evaporation takes place. When the action ceases, the liquid is decanted and evaporated; the residue is then heated until no more fumes of hydrofluoric acid are evolved.

Bismuth trifluoride is a heavy, greyish-white, crystalline powder. The crystal has a face-centred cubic structure with four molecules in the unit cell:

a = 5.853 ± 0.004 A.

Its density is 5.32 at 20° C. It is the most stable halide of bismuth. It melts without decomposition and is only slightly volatile even when strongly heated. It is almost insoluble in both water and alcohol, and is not hydrolysed by cold or boiling water. It is decomposed by, and dissolves in, hot mineral acids. It does not react with sulphur, or with the oxides of nitrogen. It dissolves readily on boiling in a concentrated solution of potassium fluoride, but it has not been possible to isolate a complex salt from this solution.

If, when preparing bismuth trifluoride by the action of hydrofluoric acid upon bismuth hydroxide, the residue is only gently heated, a substance is produced which appears to be a complex hydrofluobismuthic acid, H₃BiF₆ or BiF₃.3HF. It is a greyish-white, crystalline, deliquescent substance; on heating it loses hydrofluoric acid, and on heating with water it is decomposed with the formation of bismuthyl fluoride, BiOF, an intermediate compound BiOF.2HF being formed, which is decomposed on washing with water. No salts of this acid are known.

If a boiling concentrated solution of ammonium fluoride is saturated with precipitated bismuth hydroxide, and the solution allowed to stand for a long time, on cooling, small, transparent, rhombic or monoclinic crystals of ammonium fluobismuthate or bismuth ammonium fluoride, (NH₄)BiF₄, separate out. The crystals are easily decomposed by water, yielding bismuthyl fluoride; and they dissolve in hot moderately dilute acids.