Colmenero Francisco, Cobos Joaquín, Timón Vicente
Instituto de Estructura de la Materia , Consejo Superior de Investigaciones Cientı́ficas (CSIC) , c/Serrano 113 , Madrid 28006 , Spain.
Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT) , Avda/Complutense 40 , Madrid 28040 , Spain.
Inorg Chem. 2018 Apr 16;57(8):4470-4481. doi: 10.1021/acs.inorgchem.8b00150. Epub 2018 Mar 30.
The structure and Raman spectrum of schoepite mineral, [(UO)O(OH)]·12HO, was studied by means of theoretical calculations. The computations were carried out by using density functional theory with plane waves and pseudopotentials. A norm-conserving pseudopotential specific for the U atom developed in a previous work was employed. Because it was not possible to locate H atoms directly from X-ray diffraction (XRD) data by structure refinement in previous experimental studies, all of the positions of the H atoms in the full unit cell were determined theoretically. The structural results, including the lattice parameters, bond lengths, bond angles, and powder XRD pattern, were found to be in good agreement with their experimental counterparts. However, the calculations performed using the unit cell designed by Ostanin and Zeller in 2007, involving half of the atoms of the full unit cell, led to significant errors in the computed powder XRD pattern. Furthermore, Ostanin and Zeller's unit cell contains hydronium ions, HO, which are incompatible with the experimental information. Therefore, while the use of this schoepite model may be a very useful approximation requiring a much smaller amount of computational effort, the full unit cell should be used to study this mineral accurately. The Raman spectrum was also computed by means of density functional perturbation theory and compared with the experimental spectrum. The results were also in agreement with the experimental data. A normal-mode analysis of the theoretical spectra was performed to assign the main bands of the Raman spectrum. This assignment significantly improved the current empirical assignment of the bands of the Raman spectrum of schoepite mineral. In addition, the equation of state and elastic properties of this mineral were determined. The crystal structure of schoepite was found to be stable mechanically and dynamically. Schoepite can be described as a brittle material exhibiting small anisotropy and large compressibility in the direction perpendicular to the layers, which characterize its structure. The calculated bulk modulus, B, was ∼35 GPa.
通过理论计算研究了水铀矿矿物[(UO)O(OH)]·12H₂O的结构和拉曼光谱。计算采用密度泛函理论结合平面波和赝势进行。使用了先前工作中开发的针对U原子的守恒规范赝势。由于在先前的实验研究中无法通过结构精修直接从X射线衍射（XRD）数据中确定H原子的位置，因此全晶胞中所有H原子的位置均通过理论确定。包括晶格参数、键长、键角和粉末XRD图谱在内的结构结果与实验结果吻合良好。然而，使用奥斯坦宁和泽勒在2007年设计的晶胞（包含全晶胞一半的原子）进行的计算，在计算得到的粉末XRD图谱中导致了显著误差。此外，奥斯坦宁和泽勒的晶胞包含水合氢离子H₃O⁺，这与实验信息不相符。因此，虽然使用这种水铀矿模型可能是一种非常有用的近似方法，所需的计算量要少得多，但应使用全晶胞来准确研究这种矿物。还通过密度泛函微扰理论计算了拉曼光谱，并与实验光谱进行了比较。结果也与实验数据相符。对理论光谱进行了简正模式分析，以确定拉曼光谱的主要谱带。这一归属显著改进了目前水铀矿矿物拉曼光谱谱带的经验归属。此外，还确定了该矿物的状态方程和弹性性质。发现水铀矿的晶体结构在力学和动力学上是稳定的。水铀矿可描述为一种脆性材料，在垂直于层的方向上表现出小的各向异性和大的压缩性，这是其结构的特征。计算得到的体模量B约为35 GPa。
