School of Chemistry , The University of Sydney , Sydney , NSW 2006 , Australia.
Australian Nuclear Science and Technology Organisation , Lucas Heights , NSW 2234 , Australia.
Inorg Chem. 2018 Nov 5;57(21):13847-13858. doi: 10.1021/acs.inorgchem.8b02355. Epub 2018 Oct 24.
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.
首次对 Ni-U-O 体系进行了全面的结构研究。合成了 α-NiUO、β-NiUO 和 NiUO 的单晶，并使用同步辐射单晶 X 射线衍射数据和 X 射线吸收光谱测量对其结构进行了精修。α-NiUO 采用四方晶系 Pbcn 空间群结构，与 CrUO 同构，后者包含波纹状二维（2D）层角共享 UO 多面体和沿 [001] 方向共享一维（1D）锯齿形α-PbO 金红石状链的 NiO 多面体。β-NiUO 与 MgUO 同构，具有正交晶系 Ibmm 空间群结构，其中沿[001]方向交替排列共享边的 UO 和 NiO 多面体一维链，类似于规则 TiO 金红石。NiUO 形成三方晶系 P1̅空间群结构，与 CuUO 同构，其中通过 UO 和 UO 多面体的混合物形成三维（3D）框架结构，NiO 多面体孤立地位于框架内。使用单晶 XANES 映射进行的 X 射线吸收近边结构（XANES）测量支持三种结构中均存在六价铀。发现 NiUO 的多晶型物仅在高压高温条件（≥4 GPa 和 700 °C）下形成。有人认为这是 Ni 和 U 阳离子相对大小差异的结果，其中 Ni 阳离子对于 Ibmm 结构太小，对于在环境压力条件下形成的 Pbcn 结构太大。对于在环境压力条件下形成的 NiUO 来说，这似乎不是一个问题，其中 NiO 多面体孤立地位于三维连接的 UO/UO 多面体的框架内。合成条件表明，β-NiUO 是高压下的首选相，当 P = 4 GPa 时，温度在 950 至 1000 °C 之间，该相不可逆地转变为该相。描述了合成氧化物的路线以及与之相关的结构和光谱结果，涉及到 Ni-U-O 体系的结构化学、更大的 AUO 氧化物家族（A = 二价或三价阳离子）以及它们与金红石相关氧化物家族的关系。
