Uranium-Induced Changes in Crystal-Field and Covalency Effects of Th4+ in Th1-xUxO2 Mixed Oxides Probed by High-Resolution X-ray Absorption Spectroscopy
Uranium-Induced Changes in Crystal-Field and Covalency Effects of Th4+ in Th1-xUxO2 Mixed Oxides Probed by High-Resolution X-ray Absorption Spectroscopy
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高分辨率 X 射线吸收光谱探测 Th1-xUxO2 混合氧化物中铀引起的晶体场变化和 Th4 共价效应
DOI：
10.1021/acs.inorgchem.8b01142
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发表时间：
2018
影响因子：
4.6
通讯作者：
Wang Jian Qjang
中科院分区：
文献类型：
--
作者：
Bao Hongliang;Duan Peiquan;Zhou Jing;Cao Hanjie;Li Jiong;Yu Haisheng;Jiang Zheng;Liu Hongtao;Zhang Linjuan;Lin Jian;Chen Ning;Lin Xiao;Liu Yancheng;Huang Yuying;Wang Jian Qjang
了解当地的Th结构是更好地了解Th基核燃料循环中所涉及的Th基混合氧化物(Th-MOX)的物理化学性质的先决条件。用β5发射线上的ThL3 EDGE高能分辨X射线吸收近边能谱研究了Th_(1-x)U_xO_2(x=0.25，0.5，0.75)固溶体中6d壳层的晶态电场分裂。在具有立方对称性的有序Th-O_8立方结构的ThO_2中，Th~(4+)离子的6deg2和t2格子的CEF分裂为∼3.5 eV。由于在HERFD-XANES谱中分辨了与未占据的6d态的晶场分裂相对应的白线分裂峰，结合第一性原理计算对这些分裂峰的分析表明，U含量的增加涉及Th1-xUxO2混合氧化物中Th-O8立方的扭曲。U的引入导致Th-O8立方的对称性降低，使Th~(4+)周围的局域晶场和Th_6T_2G-O_2p杂化降低，这是Th-O键共价性质的主要原因。这种现象在Th0.25U0.75O2中很明显，它的CEF分裂比纯ThO2减少了约10%，Th-6d-t2g和O-2p轨道之间的共价混合也大大减少。
Knowledge of the local Th structure is a prerequisite for a better understanding of the physicochemical properties of the thorium-based mixed oxides (Th-MOX) involved in the Th-based nuclear fuel cycle. The crystalline electric field (CEF) splitting of the 6d shell in Th1–xUxO2(x= 0.25, 0.5, 0.75) solid solution was probed by the ThL3edge high-energy-resolution fluorescence-detected (HERFD) X-ray absorption near-edge spectroscopy (XANES) collected at theLβ5emission line, which cannot be obtained by conventional X-ray absorption methods. The detected CEF split between the 6d egand t2gorbitals in ThO2consisting of ordered Th–O8cubes with cubic symmetry is ∼3.5 eV for the Th4+ion. Because the split peaks of the white line corresponding to the crystal-field splitting of the unoccupied 6d states were resolved in the HERFD-XANES spectra, the analysis of these split peaks combined with first-principles calculations revealed that an increase of the U content involves the distortion of the Th–O8cubes in the Th1–xUxO2mixed oxides. The lower symmetry of the Th–O8cube induced by the incorporated U tends to reduce the local crystal-field around Th4+as well as the hybridization of Th 6t2g–O 2p which is mainly responsible for the covalent property of the Th–O bond. The phenomenon is noticeable in Th0.25U0.75O2, whose CEF splitting is decreased by approximately 10%, and covalent mixing between Th 6d t2gand O 2p orbitals is substantially reduced compared to that of pure ThO2.