Colmenero Francisco, Plášil Jakub, Cobos Joaquín, Sejkora Jiří, Timón Vicente, Čejka Jiří, Bonales Laura J
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. 2019 May 16;9(27):15323-15334. doi: 10.1039/c9ra02931a. eCollection 2019 May 14.
The crystal structure, hydrogen bonding, mechanical properties and Raman spectrum of the lead uranyl silicate monohydrate mineral kasolite, Pb(UO)(SiO)·HO, are investigated by means of first-principles solid-state methods based on density functional theory using plane waves and pseudopotentials. The computed unit cell parameters, bond lengths and angles and X-ray powder pattern of kasolite are found to be in very good agreement with their experimental counterparts. The calculated hydrogen atom positions and associated hydrogen bond structure in the unit cell of kasolite confirmed the hydrogen bond scheme previously determined from X-ray diffraction data. The kasolite crystal structure is formed from uranyl silicate layers having the uranophane sheet anion-topology. The lead ions and water molecules are located in the interlayer space. Water molecules belong to the coordination structure of lead interlayer ions and reinforce the structure by hydrogen bonding between the uranyl silicate sheets. The hydrogen bonding in kasolite is strong and dual, that is, the water molecules are distributed in pairs, held together by two symmetrically related hydrogen bonds, one being directed from the first water molecule to the second one and the other from the second water molecule to the first one. As a result of the full structure determination of kasolite, the determination of its mechanical properties and Raman spectrum becomes possible using theoretical methods. The mechanical properties and mechanical stability of the structure of kasolite are studied using the finite deformation technique. The bulk modulus and its pressure derivatives, the Young and shear moduli, the Poisson ratio and the ductility, hardness and anisotropy indices are reported. Kasolite is a hard and brittle mineral possessing a large bulk modulus of the order of ∼ 71 GPa. The structure is mechanically stable and very isotropic. The large mechanical isotropy of the structure is unexpected since layered structures are commonly very anisotropic and results from the strong dual hydrogen bonding among the uranyl silicate sheets. The experimental Raman spectrum of kasolite is recorded from a natural mineral sample from the Jánská vein, Příbram base metal ore district, Czech Republic, and determined by using density functional perturbation theory. The agreement is excellent and, therefore, the theoretical calculations are employed to assign the experimental spectrum. Besides, the theoretical results are used to guide the resolution into single components of the bands from the experimental spectrum. A large number of kasolite Raman bands are reassigned. Three bands of the experimental spectrum located at the wavenumbers 1015, 977 and 813 cm, are identified as combination bands.
采用基于密度泛函理论的第一性原理固态方法，利用平面波和赝势，对一水合铅铀酰硅酸盐矿物硅钾铀矿（Pb(UO₂)(SiO₄)·H₂O）的晶体结构、氢键、力学性能和拉曼光谱进行了研究。计算得到的硅钾铀矿的晶胞参数、键长、键角和X射线粉末衍射图谱与实验结果非常吻合。计算得到的硅钾铀矿晶胞中的氢原子位置及相关氢键结构，证实了先前由X射线衍射数据确定的氢键模式。硅钾铀矿的晶体结构由具有硅钙铀云母片层阴离子拓扑结构的铀酰硅酸盐层构成。铅离子和水分子位于层间空间。水分子属于层间铅离子的配位结构，并通过铀酰硅酸盐片层之间的氢键增强结构稳定性。硅钾铀矿中的氢键很强且具有双重性，即水分子成对分布，由两个对称相关的氢键维系在一起，一个氢键从第一个水分子指向第二个水分子，另一个则从第二个水分子指向第一个水分子。由于对硅钾铀矿进行了完整的结构测定，因此可以使用理论方法确定其力学性能和拉曼光谱。利用有限变形技术研究了硅钾铀矿结构的力学性能和力学稳定性。报告了其体模量及其压力导数、杨氏模量和剪切模量、泊松比以及延展性、硬度和各向异性指数。硅钾铀矿是一种坚硬易碎的矿物，其体模量约为71 GPa。该结构在力学上是稳定的，且各向同性很强。该结构具有很大的力学各向同性，这出乎意料，因为层状结构通常各向异性很强，这是由铀酰硅酸盐片层之间强大的双重氢键所致。硅钾铀矿的实验拉曼光谱是从捷克共和国普日布拉姆贱金属矿区扬斯卡矿脉的天然矿物样品中记录得到的，并采用密度泛函微扰理论进行测定。二者吻合度极佳，因此利用理论计算对实验光谱进行归属。此外，理论结果还用于指导将实验光谱中的谱带解析为单个成分。大量硅钾铀矿的拉曼谱带被重新归属。实验光谱中位于波数1015、977和813 cm⁻¹处的三条谱带被确定为组合谱带。
