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-NiUO, β-NiUO, and NiUO were synthesized, and their structures were refined-using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO containing corrugated two-dimensional (2D) layers of corner-sharing UO polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO rutile-like chains of NiO polyhedra in the [001] direction. β-NiUO is isostructural to MgUO and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO and NiO polyhedra in the [001] direction as in regular TiO rutile. NiUO forms a triclinic structure in space group P1̅ and is isostructural with CuUO, where it forms a three-dimensional (3D) framework structure built through a mixture of UO and UO polyhedra in which the NiO 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 NiUO 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 Ni and U cations, where the Ni 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 NiUO, which forms under ambient pressure conditions, where NiO polyhedra sit isolated within the framework of 3D connected UO/UO polyhedra. Synthesis conditions indicate that β-NiUO 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 AUO family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.