Colmenero Francisco, Plášil Jakub, Timón Vicente, Čejka Jiří
Instituto de Estructura de la Materia (IEM-CSIC) C/ Serrano, 113 28006 Madrid Spain
Institute of Physics ASCR, v.v.i. Na Slovance 2 182 21 Praha 8 Czech Republic.
RSC Adv. 2020 Aug 27;10(53):31947-31960. doi: 10.1039/d0ra04596a. eCollection 2020 Aug 26.
The determination of the full crystal structure of the uranyl sulfate mineral uranopilite, (UO)(SO)O(OH)·14HO, including the positions of the hydrogen atoms within the corresponding unit cell, has not been feasible to date due to the poor quality of its X-ray diffraction pattern. In this paper, the complete crystal structure of uranopilite is established for the first time by means of first principles solid-state calculations based in density functional theory employing a large plane wave basis set and pseudopotential functions. The computed unit-cell parameters and structural data for the non-hydrogen atoms are in excellent agreement with the available experimental data. The computed X-ray diffraction pattern is also in satisfactory agreement with the experimental pattern. The infrared spectrum of uranopilite is collected from a natural crystal specimen originating in Jáchymov (Czech Republic) and computed employing density functional perturbation theory. The theoretical and experimental vibrational spectra are highly consistent. Therefore, a full assignment of the bands in the experimental infrared spectrum is performed using a normal mode analysis of the first principles vibrational results. One overtone and six combination bands are recognized in the infrared spectrum. The elasticity tensor and phonon spectra of uranopilite are computed from the optimized crystal structure and used to analyze its mechanical stability, to obtain a rich set of elastic properties and to derive its fundamental thermodynamic properties as a function of temperature. Uranopilite is shown to have a large mechanical anisotropy and to exhibit the negative Poisson's ratio and negative linear compressibility phenomena. The calculated specific heat and entropy at 298.15 K are 179.6 and 209.0 J K mol, respectively. The computed fundamental thermodynamic functions of uranopilite are employed to obtain its thermodynamic functions of formation in terms of the elements and the thermodynamic properties of a set of chemical reactions relating uranopilite with a representative group of secondary phases of spent nuclear fuel. From the reaction thermodynamic data, the relative stability of uranopilite with respect to these secondary phases is evaluated as a function of temperature and under different hydrogen peroxide concentrations. From the results, it follows that uranopilite has a very large thermodynamic stability in the presence of hydrogen peroxide. The high stability of uranopilite under this condition justify its early crystallization in the paragenetic sequence of secondary phases occurring when uranium dioxide is exposed to sulfur-rich solutions.
硫酸双氧铀矿物水铀矾（(UO)(SO)O(OH)·14HO）完整晶体结构的测定，包括相应晶胞内氢原子的位置，由于其X射线衍射图谱质量较差，至今仍无法实现。在本文中，首次通过基于密度泛函理论的第一性原理固态计算，采用大平面波基组和赝势函数，确定了水铀矾的完整晶体结构。计算得到的非氢原子晶胞参数和结构数据与现有的实验数据高度吻合。计算得到的X射线衍射图谱也与实验图谱令人满意地相符。水铀矾的红外光谱是从源自捷克亚希莫夫的天然晶体标本中采集的，并采用密度泛函微扰理论进行计算。理论和实验振动光谱高度一致。因此，利用第一性原理振动结果的简正模式分析对实验红外光谱中的谱带进行了全面归属。在红外光谱中识别出一个泛音带和六个组合带。根据优化后的晶体结构计算了水铀矾的弹性张量和声子谱，并用于分析其力学稳定性，获得丰富的弹性性质集，并推导其作为温度函数的基本热力学性质。结果表明，水铀矾具有较大的力学各向异性，并表现出负泊松比和负线性压缩性现象。在298. K时计算得到的比热容和熵分别为179.6和209.0 J K mol。利用计算得到的水铀矾基本热力学函数，根据元素得到其生成热力学函数，以及一组将水铀矾与乏核燃料代表性次生相组联系起来的化学反应的热力学性质。根据反应热力学数据，评估了水铀矾相对于这些次生相在不同温度和过氧化氢浓度下的相对稳定性。结果表明，在过氧化氢存在下，水铀矾具有非常大的热力学稳定性。在这种条件下水铀矾的高稳定性证明了它在二氧化铀暴露于富硫溶液时次生相共生序列中早期结晶的合理性。
