High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni-U-O System

The first comprehensive structural study of the Ni-U-O system is reported. Single crystals of α-NiUO₄, β-NiUO₄, and NiU₃O₁₀ were synthesized, and their structures were refined-using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO₄ adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO₄ containing corrugated two-dimensional (2D) layers of corner-sharing UO₆ polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO₂ rutile-like chains of NiO₆ polyhedra in the [001] direction. β-NiUO₄ is isostructural to MgUO₄ and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO₆ and NiO₆ polyhedra in the [001] direction as in regular TiO₂ rutile. NiU₃O₁₀ forms a triclinic structure in space group P1̅ and is isostructural with CuU₃O₁₀, where it forms a three-dimensional (3D) framework structure built through a mixture of UO₆ and UO₇ polyhedra in which the NiO₆ polyhedra sit isolated within the framework. X-ray absorption near-edge structure (XANES) measurements, conducted using XANES mapping of single crystals, support the presence of hexavalent uranium in the three structures. The polymorphs of NiUO₄ were found to only form under high-pressure and high-temperature conditions (≥4 GPa and 700 °C). It is argued that this is a consequence of the relative size difference between the Ni²⁺ and U⁶⁺ cations, where the Ni²⁺ cation is effectively too small for the Ibmm structure and too large for the Pbcn structure to form under ambient pressure conditions. This does not appear to be an issue for NiU₃O₁₀, which forms under ambient pressure conditions, where NiO₆ polyhedra sit isolated within the framework of 3D connected UO₆/UO₇ polyhedra. Synthesis conditions indicate that β-NiUO₄ is the preferred higher-pressure phase and that the transformation to this occurs irreversibly at a temperature between 950 and 1000 °C, when P = 4 GPa. The routes toward the synthesis of the oxides and the associated structural and spectroscopic results are described with respect to the structural chemistry of the Ni-U-O system, the larger AUO₄ family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.