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 M_xU_yO_z, where M represents a cation. The uranium atom in uranates(VI) has two short, co-linear, 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. "Ammonium diuranate" is an intermediate in the production of uranium oxide nuclear fuels.

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 are insoluble in water and other solvents, so pure samples can only be obtained by careful control of reaction conditions.

Naturally occurring uranates are rare and always occur as secondary minerals. Examples include curite, becquerelite and soddyite named after prominent radiochemists Pierre Curie and Marie Curie, Henri Becquerel and Frederick Soddy, respectively. 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.

All uranates(VI) are mixed oxides, that is, compounds made up of metal(s), uranium and oxygen atoms. No uranium oxyanion, such as [UO₄]²⁻ or [U₂O₇]²⁻, is known. Instead, all uranate structures are based on UO_n 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.

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