Structure and thermodynamic stability of UTa₃O₁₀, a U(v)-bearing compound

Heating a mixture of uranyl(vi) nitrate and tantalum(v) oxide in the molar ratio of 2 : 3 to 1400 °C resulted in the formation of a new compound, UTa₃O₁₀. The honey colored to yellow brown crystals of UTa₃O₁₀ crystallize in an orthorhombic structure with the space group Fddd (no. 70), lattice parameters a = 7.3947(1), b = 12.7599(2), c = 15.8156(2) Å, and Z = 8. Vertex sharing [TaO₆]^7- octahedra of two crystallographically distinct Ta cations form a three dimensional tantalate framework. Within this framework, six membered rings of [TaO₆]^7- octahedra are formed within the (001) plane. The center of these rings is occupied by the uranyl cations [UO₂]⁺, with an oxidation state of +5 for uranium. The pentavalence of U and Ta was confirmed by X-ray photoelectron spectroscopy and X-ray adsorption spectroscopy. The enthalpy of formation of UTa₃O₁₀ from Ta₂O₅, β-U₃O₇, and U₃O₈ has been determined to be 13.1 ± 18.1 kJ mol^-1 using high temperature oxide melt solution calorimetry with sodium molybdate as the solvent at 700 °C. The close to zero enthalpy of formation of UTa₃O₁₀ can be explained by closely balanced structural stabilizing and destabilizing factors, which may also apply to other UM₃O₁₀ compounds.