Colmenero Francisco, Fernández Ana María, Timón Vicente, Cobos Joaquin
Instituto de Estructura de la Materia (IEM-CSIC) C/Serrano, 123 28006 Madrid Spain
Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT) Avda/Complutense, 40 28040 Madrid Spain.
RSC Adv. 2018 Jul 10;8(43):24599-24616. doi: 10.1039/c8ra04678f. eCollection 2018 Jul 2.
The structure, thermodynamic and mechanical properties of becquerelite mineral, Ca(UO)O(OH)·8HO, were studied by means of theoretical solid-state calculations based on density functional theory using plane waves and pseudopotentials. The positions of the hydrogen atoms in the unit cell of becquerelite mineral were optimized theoretically since it was not possible to determine them from X-ray diffraction data by structure refinement. The structural results, including the lattice parameters, bond lengths and X-ray powder pattern, were found to be in excellent agreement with their experimental counterparts. The fundamental thermodynamic properties of becquerelite mineral, including specific heat, entropy, enthalpy and Gibbs free energy, were then computed by performing phonon calculations at the computed optimized structure. Since the experimental values of these properties are unknown, their values were predicted. The values obtained for the isobaric specific heat and entropy of becquerelite at the temperature of 298.15 K were 148.4 and 172.3 J K mol, respectively. The computed thermodynamic properties were combined with those of the corresponding elements in order to obtain the enthalpy and Gibbs free energy of formation as a function of temperature. The availability of these thermodynamic properties of formation allowed to determine the enthalpies and free energies and associated reaction constants of a series of reactions involving becquerelite and other uranyl containing materials. Futhermore, knowledge of these properties permitted the study of the thermodynamic stability of becquerelite with respect to a rich set of secondary phases of spent nuclear fuel, including dehydrated schoepite, schoepite, metaschoepite, studtite, metastudtite, rutherfordine and soddyite under different conditions of temperature. Becquerelite is shown to be highly stable in the presence of hydrogen peroxide. It is the second most stable phase under intermediate hydrogen peroxide concentrations (after schoepite), and the fourth most stable phase under high hydrogen peroxide concentrations (after studtite, schoepite and metaschoepite). Finally, the equation of state and elastic properties of this mineral, unknown to date, were determined. The crystal structure of becquerelite was found to be stable mechanically and dynamically. Becquerelite can be described as a brittle material exhibiting large anisotropy and large compressibility in the direction perpendicular to the sheets characterizing the structure of this layered uranyl containing material. The dependence of the elastic properties of becquerelite with respect to the strain orientation is shown to be analogous to that of schoepite mineral. The calculated bulk modulus is also very similar to that of schoepite, ∼ 31 GPa.
采用基于密度泛函理论的平面波赝势方法，通过理论固态计算研究了板铅铀矿矿物Ca(UO₂)₂O₂(OH)₂·8H₂O的结构、热力学和力学性质。由于无法通过结构精修从X射线衍射数据确定板铅铀矿矿物晶胞中氢原子的位置，因此对其位置进行了理论优化。发现包括晶格参数、键长和X射线粉末衍射图谱在内的结构结果与实验结果非常吻合。然后，通过在计算得到的优化结构上进行声子计算，计算了板铅铀矿矿物的基本热力学性质，包括比热、熵、焓和吉布斯自由能。由于这些性质的实验值未知，因此对其进行了预测。在298.15K温度下，板铅铀矿的等压比热和熵分别为148.4和172.3 J K⁻¹ mol⁻¹。将计算得到的热力学性质与相应元素的性质相结合，以获得形成焓和吉布斯自由能随温度的变化关系。这些形成热力学性质的可得性使得能够确定一系列涉及板铅铀矿和其他含铀酰材料的反应的焓、自由能和相关反应常数。此外，这些性质的知识使得能够研究板铅铀矿相对于乏核燃料的一系列次生相（包括脱水柱铀矿、柱铀矿、变柱铀矿、八氧化三铀、变八氧化三铀、碳酸铀酰和水硅铀矿）在不同温度条件下的热力学稳定性。结果表明，板铅铀矿在过氧化氢存在下高度稳定。在中等过氧化氢浓度下，它是第二稳定的相（仅次于柱铀矿），在高过氧化氢浓度下，它是第四稳定的相（仅次于八氧化三铀、柱铀矿和变柱铀矿）。最后，确定了该矿物迄今为止未知的状态方程和弹性性质。发现板铅铀矿的晶体结构在力学和动力学上都是稳定的。板铅铀矿可描述为一种脆性材料，在垂直于表征这种层状含铀酰材料结构的片层方向上表现出大的各向异性和大的压缩性。板铅铀矿弹性性质随应变取向的依赖性与柱铀矿矿物类似。计算得到的体积模量也与柱铀矿非常相似，约为31 GPa。
