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首页> 外文期刊>RSC Advances >Full crystal structure, hydrogen bonding and spectroscopic, mechanical and thermodynamic properties of mineral uranopilite
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Full crystal structure, hydrogen bonding and spectroscopic, mechanical and thermodynamic properties of mineral uranopilite

机译：矿物质铀腈的全晶体结构，氢键和光谱，机械和热力学性质

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The determination of the full crystal structure of the uranyl sulfate mineral uranopilite, (UO _(2) ) _(6) (SO _(4) )O _(2) (OH) _(6) ·14H _(2) O, 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 ~(?1) mol ~(?1) , 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.
机译：硫酸铀酸盐矿物铀腈的全晶体结构的测定，（UO _（2））_（6）（SO _（4））O _（2）（OH）_（6）·14H _（2） o由于其X射线衍射图案的质量差，包括相应单元电池内的氢原子的位置迄今尚不可行。在本文中，借助于利用大平面波基基的密度功能理论的第一原理确定铀腈的完全晶体结构是第一次建立了采用大平面波基的密度函数理论和假势函数。非氢原子的计算单元 - 小区参数和结构数据与可用的实验数据很好。计算的X射线衍射图谱也与实验模式令人满意的协议。从源自Jáchymov（捷克共和国）的天然晶体标本中收集铀腈的红外光谱，并计算使用密度函数扰动理论。理论和实验振动光谱是高度一致的。因此，使用第一个原理振动结果的正常模式分析来执行实验红外光谱中的频带的完整分配。在红外光谱中识别出一个泛音和六个组合带。从优化的晶体结构计算铀腈的弹性张量和声子光谱，并用于分析其机械稳定性，获得浓度的弹性性能，并导致其基本的热力学性能作为温度的函数。铀素显示出具有大型机械各向异性，并表现出负泊松比和负线性可压缩现象。在298.15k的计算的特定热量和熵分别为179.6和209.0JK〜（？1）mol〜（？1）。使用铀素的计算基本热力学功能，以获得其在与所花费的核燃料的代表性次级的副阶段相关的一组化学反应的元素和热力学性能的形成的热力学功能。从反应热力学数据中，评价铀腈的相对稳定性被评价为温度和不同过氧化氢浓度的函数。从结果中，铀素在过氧化氢存在下具有非常大的热力学稳定性。在该条件下铀素的高稳定性证明了当二氧化铀暴露于富含硫的溶液时发生的继发性分段的次生序列的早期结晶。

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