Uranate

A method of general applicability involves combining two oxides in a high temperature reaction. For example, :Na2O + UO3 → Na2UO4 Another method is the thermal decomposition of a complex, such as an acetate complex. For example, microcrystalline barium diuranate, BaU2O7, was made by thermal decomposition of barium uranyl acetate at 900 °C. :Ba[UO2(ac)3]2 → BaU2O7 ... (ac=CH3CO2−) Uranates can be prepared by adding alkali to an aqueous solution of a uranyl salt. However, the composition of the precipitate that forms is variable and depends on the chemical and physical conditions used. Uranates are insoluble in water and other solvents, so pure samples can only be obtained by careful control of reaction conditions. ==Uranium(VI)==

Structures , flattened along the 3-fold symmetry axis of the octahedron which also runs through the O-U-O axis (local point group D3d at the uranium atom). Each of these oxygen atoms is shared between three uranium atoms, which accounts for the stoichiometry, U 2×O 6×1/3 O = UO4. The structure has been described as a hexagonal layer structure. It can also viewed as a distorted fluorite structure in which two U-O distances have decreased and the other six have increased.Magnesium uranate, MgUO4, has a quite different structure. Distorted UO6 octahedra are linked into infinite chains; the "uranyl" U-O bond length is 192 pm, not much shorter than the other U-O bond length of 218 pm. The uranate structures in these compounds differ in the way the UOx structural units are linked together. Properties and uses 2(NH2O)2·4H2O structure. When the alkali used is ammonia, so-called ammonium diuranate, known in the industry as ADU, is the main constituent of yellowcake. The exact composition of the precipitate depends to some extent on the conditions and anions that are present and the formula (NH4)2U2O7, is only an approximation. The precipitates obtained on addition of ammonia to uranyl nitrate solution under different conditions of temperature and final pH, when dried, were considered as loosely bound compounds with an ammonia/uranium ratio of 0.37 containing varying amounts of water and ammonium nitrate. In other studies it was found to approximate to the gross formulas 3UO3·NH3·5H2O, 2(NH2O)2·3H2O or UO2(NH2O)2·4H2O. --> The asymmetric stretching frequency of the uranyl ion was found to decrease with increasing NH4+ content. This decrease is continuous and no band splitting was observed, indicating that the ammonium uranate system is homogeneous and continuous. ADU is an intermediate in the production of uranium oxides to be used as nuclear fuel; it is converted directly into an oxide by heating. β-UO3 is produced at about 350 °C and U3O8 is obtained at higher temperatures. When the alkali used is sodium hydroxide, so-called sodium diuranate, SDU, is produced. This can also be converted into an oxide. Another choice of alkali is magnesium oxide, making magnesium diuranate, known as MDU. Oxides and uranates of uranium(VI) have been used in the past as yellow ceramic glazes as in Fiesta and to make yellow-green uranium glass. Uranates are also important in radioactive waste management. ==Uranium(V)==

Several series of uranates(V) have been characterized. Compounds with the formula MIUO3 have a perovskite structure. Compounds MI3UO4 have a defect rock-salt structure. MI7UO6 structures are based on a hexagonally close-packed array of oxygen atoms. In all cases the uranium is at the centre of an octahedron of oxygen atoms. Also MIIIUO4 have been recently synthesized and characterized (MIII=Bi, Fe, Cr etc.). Few other compounds of uranium(V) are stable. 3UO4, Li7UO6 --> ==Uranium(IV)==

Barium uranate, BaUO3, is made from barium oxide and uranium dioxide in an atmosphere that contains absolutely no oxygen. It has a cubic crystal structure (space group Pmm). ==References==