Uranyl Fluoride, UO₂F₂

Uranyl Fluoride, UO₂F₂, is formed with uranous oxyfluoride (see above) by the action of hydrofluoric acid on urano-uranic oxide:

U₃O₈ + 6HF = UOF₂ + 2UO₂F₂ + 3H₂O.

The green insoluble uranous oxyfluoride is removed by filtration, and the uranyl fluoride remains as a yellow mass after evaporation of the solution. It is also obtained as a yellow powder by repeated evaporation of uranyl acetate with hydrofluoric acid. A white crystalline form has been described by Smithells, who obtained it by carefully heating the tetrafluoride in air; it dissolved in water yielding a yellow solution.

Complex uranyl fluorides of the type UO₂F₂.3RF (R = K, NH₄) have been obtained by the addition of excess of the alkali fluoride to a solution of uranyl fluoride, both the yellow and the white variety acting in this way. A solution of uranyl acetate, or of potassium diuranate containing hydrofluoric acid, with excess of the alkali fluoride, also yields the same compounds. The potassium salt, UO₂F₂.3KF, separates as a heavy orange-yellow crystalline precipitate; it is dimorphous, and the crystals may be tetragonal or monoclinic according to the method of preparation. At 20° C. the crystals have density 4.263. The addition of excess of hydrogen peroxide to a solution of this salt gives a yellow solution from which a yellow peroxyfluoride, of composition 3(UO₄.KF).UO₃F₂.KF.4H₂O, separates on keeping. The ammonium salt, UO₂F₂.3NH₄F, only yields crystals of the tetragonal form, the density of which at 20° C. is 3.186.

If the potassium salt is dissolved in water containing not more than 13 per cent, of the acid fluoride of potassium, the solution on concentration yields triclinic crystals of the compound 2UO₂F₂.5KF. If excess of uranyl nitrate is present in the solution, monoclinic crystals of the compound 2UO₂F₂.3KF are obtained.

A soluble uranyl silicofluoride has been described.