Uranium trioxide in contact with water at about 180" gives an orthorhombic hydrate, U0,,0.8H2O, with the cell dimensions a = 10-23 & 0-01, b = 6.89 f 0.01, c = 4-28 & 0.01 A. These dimensions appear to vary slightly with deviation of the water content from 0-8 mole per uranium atom. Between 200" and 280' another phase appears, UO,,l*OH,O, also orthorhombic with cell dimensions a = 5.638 f 0.005, b = 6.273 f 0.005, c = 9-925 f 0.005 A, space-group Pbca. The hemihydrate, U0,,0.5Hz0,is stable in water above 250". A further phase, possibly due to a relatively high pH and probably triclinic, was observed when the uranium trioxide was heated in water at 230" in contact with glass.Some of the properties of these hydrates are described, together with observations on the preparation of pure anhydrous uranium trioxide from uranyl nitrate.PREVIOUS investigations on the hydrated oxides of uranium indicated that the trioxide is stable in water, up to high temperatures, and that over the range 70-300" (approx.) it forms a monohydrate.1 The system is quite complex, however, and Katz and Rabinowitz quoted the following four crystalline modifications observed by D. T. Vier : (i) a-UO,,H,O, large six-sided orthorhombic basal tablets ; (ii) B-UO,,H,O, small orthorhombic prismatic tablets with slightly larger unit cell than the a-form ; (iii) y-UO,,H,O, six-sided columnar crystals, with X-ray diffraction pattern similar to that of the a-and the @-form, but perhaps belonging to a different system : contains (0.9 mole of H20 per uranium atom; (iv) 8-UO3,H2O, triclinic, giving a complex X-ray diffraction pattern. The last form may be due to impurities.Very few details were given of the preparation and the relations between these various forms, and the present investigation was undertaken to obtain more extensive information in the range 180-320". The particular form of UO,,H,O obtained may be sensitive to the presence of traces of impurities, such as the soluble uranyl ion, and so some observations are made also on the preparation of uranium trioxide free from all contaminants down to a level of a few parts per million. EXPERIMENTALA sample (about 100 mg.) was suspended in a platinum boat on the end of a 35-cm. extension fibre from the lower end of the helix. The temperature of the sample was raised linearly at about 2"/min.

The tetrafluorides of uranium and plutonium form two hydrates, the higher one having 2-5 molecules of water per metal atom and the lower one 2.0 molecules of water or less. Methods for their formation have been investigated.The lower hydrate is pseudo-cubic, a = 5.69 kX for the uranium and 5.63 kX for the plutonium compound, and the proposed structure is similar to that of uranium dioxide, the water molecules occupying vacant uranium sites and being hydrogen-bonded to the fluorine. Very little change of cell dimension was observed over the range UF4,2H,0 to UF4,0.75H,0.The fluorination, at temperatures below loo", of the dioxides with gaseous hydrogen fluoride gives the lower fluoride hydrate and a second product which may be M(OH)F,,HF.

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