Aluminium trifluoride, AlF₃

Aluminium trifluoride, AlF₃, is not found in nature in the anhydrous state, but the hydrated fluoride occurs as the mineral fluellite, AlF₃.H₂O. It crystallises in the rhombic system (bipyramidal; a:b:c = 0.770:1:1.874); its density is 2.17.

Aluminium is completely converted into the fluoride when heated in fluorine (Moissan). The fluoride is also formed when aluminium or alumina is heated to redness in hydrogen fluoride. Deville, who made an exhaustive study of the fluoride, obtained it first by heating aluminium in silicon fluoride, and afterwards prepared it by treating alumina with hydrofluoric acid and subliming the product in hydrogen at a white heat. He also obtained the fluoride by heating a mixture of fluorspar and alumina in a stream of hydrogen fluoride, and by fusing cryolite with anhydrous aluminium sulphate. The impure fluoride is best purified by sublimation at 1100°.

Anhydrous aluminium fluoride forms colourless, transparent crystals which are described as rhombohedra, but, according to Poulenc, are probably triclinic. It is exceedingly refractory, being insoluble in water and unattacked by alkalies and acids, including concentrated sulphuric acid. It may be decomposed by prolonged fusion with an alkali carbonate.

When 42 grams of basic aluminium acetate, 54 grams of alumina, 100 grams of water, and 66 grams of 40 per cent, hydrofluoric acid are mixed and warmed, complete solution results, and a gelatinous, hydrated aluminium fluoride separates on standing. If the solution be somewhat diluted, crystals of the trihydrate, AlF₃.3H₂O, slowly separate; if only 75 grams of water are employed in the preparation, crystals of another hydrate, 2AlF₃.17H₂O, separate. The latter hydrate effloresces in air, passing into the former. The monohydrate, AlF₃.H₂O, can also be obtained in sparingly soluble, silky needles, and likewise the hydrate, 2AlF₃.7H₂O, which slowly changes to the hexahydrate, AlF₃.6H₂O, on standing in water.

The hydrate 2AlF₃.7H₂O has been obtained by Baud in two forms. The first, practically insoluble in water, is obtained by dissolving aluminium hydroxide in aqueous hydrofluoric acid and evaporating the solution at 100°; the second, easily soluble in water, by concentrating a solution of aluminium hydroxide in hydrofluoric acid and adding twice its volume of alcohol. An aqueous solution of the latter form is acid to litmus. The hydrate 2AlF₃.7H₂O loses water at 140°, leaving the hemihydrate 2AlF₃.H₂O, which decomposes at a bright red heat.

According to Deville, the compounds AlF₃.3HF, 3AlF₃.2HF.5H₂O, and 2AlF₃.HF.5H₂O may be prepared. Aluminium fluoride enters into the composition of a large number of double compounds. With each of the alkali fluorides it forms a compound of the type AlF₃.3XF stable at its melting-point. The melting-points are given by Puschin and Baskov as follows: -

Alkali Metal	Li	Na	K	Rb	Cs
Melting-point of Salt, ° C	800°	1020°	1035°	985°	823°

According to the same authors, compounds of the type 2AlF₃.3XF are also formed when X = Na, K, or Rb; but this conclusion is not justified from their experiments and is probably erroneous. In the case of the sodium aluminium fluorides, the second compound has the formula 3AlF₃.5NaF, and has no melting-point, but dissociates at 723° into AlF₃.3NaF and aluminium fluoride.

The equilibrium diagram, as far as it has been worked out, is shown in fig. Calcium and aluminium fluorides form no compound; the eutectic point is 815° to 820.°

Hydrated double fluorides of the formulse 2AlF₃.6KF.7H₂O, 2AlF₃.6NaF. 7H₂O, and 2AlF₃.4NH₄F.3H₂O are obtained as gelatinous precipitates when the requisite alkali fluoride solutions are added to an aqueous solution of Baud's soluble hydrate of aluminium fluoride. They are slightly soluble in water, 100 parts of which dissolve 0.385 of the potassium, 0.352 of the sodium, and 1.0 of the ammonium compound at 16° (Baud, loc. cit.). Berzelius has described an insoluble double fluoride AlF₃.3NH₄F, and Petersen and Helmolt have prepared a soluble double fluoride of the same composition.

The double fluoride 3AlF₃.5NaF occurs in nature as the tetragonal mineral chiolite; the compound AlF₃.3NaF is found as the mineral cryolite. Cryolite (i.e. ice-stone) occurs at Ivigtut, an Esquimaux hamlet on the southwest coast of Greenland, in one huge deposit, contaminated with siderite, zincblende, galena, etc. Density, 2.96; hardness, 2.5; melting-point, c. 1000°. It forms monoclinic prisms (Holohedral; a:b:c = 0.9662:1:1.3882, β = 90°11') and at c. 565° is converted into a cubic modification. At 15°, 100 parts of water dissolve 0.034 of cryolite (Baud), but the latter is appreciably soluble in aqueous solutions of aluminium salts. It is decomposed by sulphuric acid. In the purification of crude cryolite, advantage is taken of the superior density of its impurities to effect a first purification, and other impurities are then removed by means of an electromagnet. Cryolite is used as a solvent for alumina in the process of manufacturing aluminium, in the preparation of opaque white glass, and of an enamel for steel. It was formerly used as raw material for the Danish alkali industry, and ha been powdered and used by the Esquimaux as snuff.

The following double salts may be prepared by dissolving the requisite hydroxides in hydrofluoric acid and evaporating the solution: -

AlF₃.ZnF₂.7H₂O; 2AlF₃.3CuF₂.18H₂O; AlF₃.2CuF₂.11H₂O; AlF₃.CuF₂.HF.8H₂O.

Aluminium subchloride has been said to be produced by heating aluminium trichloride with aluminium in a sealed tube, but the statement is in all probability erroneous.