Uranate

A uranate is a ternary oxide involving the element uranium in one of the oxidation states 4, 5 or 6. A typical chemical formula is MxUyOz, where M represents a cation. The uranium atom in uranates(VI) has two short collinear U–O bonds and either four or six more next nearest oxygen atoms. The structures are infinite lattice structures with the uranium atoms linked by bridging oxygen atoms. β: hexagonalβ: Pnmaorthorhombicβ: orthorhombicβ: P2monoclinicβ: Pbcmorthorhombicβ: C2/mmonoclinic A uranate is a ternary oxide involving the element uranium in one of the oxidation states 4, 5 or 6. A typical chemical formula is MxUyOz, where M represents a cation. The uranium atom in uranates(VI) has two short collinear U–O bonds and either four or six more next nearest oxygen atoms. The structures are infinite lattice structures with the uranium atoms linked by bridging oxygen atoms. Uranium oxides are the foundation of the nuclear fuel cycle ('ammonium diuranate' and 'sodium diuranate' are intermediates in the production of uranium oxide nuclear fuels) and their long-term geological disposal requires a thorough understanding of their chemical reactivity, phase transitions, and physical and chemical properties. Such compounds indicates unusual oxygen transport behavior at higher temperatures. A method of general applicability involves combining two oxides in a high temperature reaction. For example, 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. 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. All uranates(VI) are mixed oxides, that is, compounds made up of metal(s), uranium and oxygen atoms. No uranium oxyanion, such as 2− or 2−, is known. Instead, all uranate structures are based on UOn polyhedra sharing oxygen atoms in an infinite lattice. The structures of uranates(VI) are unlike the structure of any mixed oxide of elements other than actinide elements. A particular feature is the presence of linear O-U-O moieties, which resemble the uranyl ion, UO22+. However, the U-O bond length varies from 167 pm, which is similar to the bond length of the uranyl ion, up to about 208 pm in the related compound α-UO3, so it is debatable as to whether these compounds all contain the uranyl ion. There are two principal types of uranate which are defined by the number of nearest-neighbour oxygen atoms in addition to the 'uranyl' oxygens. In one group, including M2UO4 (M=Li, Na, K) and MUO4 (M=Ca, Sr) there are six additional oxygen atoms. Taking calcium uranate, CaUO4, as an example, the six oxygen atoms are arranged as a flattened octahedron, 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. In the other group, exemplified by barium uranate, BaUO4, there are four additional oxygen atoms. These four oxygens lie in a plane and each is shared between two uranium atoms, which accounts for the stoichiometry, U 2×O 4×1/2 O = UO4. The structure may called a tetragonal layer structure.