Periodic density functional theory study of the Raman spectrum of the hydrated uranyl oxyhydroxide mineral becquerelite

Abstract

Raman spectroscopy is one of the main analytic techniques used to identify uranyl-containing minerals. However, the assignment of the Raman spectra of these minerals is usually performed by using empirical arguments leading to unreliable assignments. In this paper, the Raman spectrum of the hydrated uranyl oxyhydroxide mineral becquerelite, Ca(UO 2) 6 O 4 (OH) 6 • 8 H 2 O, was studied by means of rigorous theoretical solid-state calculations. The computations were carried out using Periodic Density Functional Theory with plane waves and pseudopotentials. The theoretical determination of this Raman spectrum was possible due to the previous development of a high-quality norm-conserving relativistic pseudopotential specific for the uranium atom and the recent optimization of the full crystal structure of this mineral, including the position of all hydrogen atoms in the corresponding unit cell. These two pieces of knowledge were formerly used in order to study the structural, mechanical, and thermodynamic properties of this mineral, but due to the very large size of the unit cell, the determination of the vibrational spectra was not possible. The corresponding results for the Raman spectrum, resulting from an intensive computational work, are reported here. The calculated Raman spectrum was compared with the experimental spectrum and the results were found to be in very good agreement. Therefore, a normal mode analysis of the theoretical spectra was performed to assign the main bands of the Raman spectrum. This assignment improved significantly the current empirical assignment of the bands of the Raman spectrum of becquerelite mineral.

Crystal structure, hydrogen bonding, mechanical properties and Raman spectrum of the lead uranyl silicate monohydrate mineral kasolite

“…35,36 This pseudopotential has been validated extensively in the research of uranyl containing materials. [29][30][31]33,[35][36][37][38][39][40][41][42][43][44][45][46][47] The optimization of the dimensions and shape as well as the atomic positions of the unit cell of kasolite was carried out by means of the Broyden-Fletcher-Goldfarb-Shanno (BFGS) technique. 55 The plane wave kinetic energy cut-off parameter employed in the calculations was very large, 3 ¼ 1000 eV, and the k-mesh 56 employed was 3 Â 3 Â 2.…”

“…The calculations have been carried out using rstprinciples theoretical solid-state methods based on density functional theory (DFT) using large plane wave basis sets and pseudopotentials for the description of internal atomic electrons. 34 At the basis of this work and a series of previous theoretical works concerning uranyl containing minerals, [29][30][31]33,[35][36][37][38][39][40][41][42][43][44][45][46][47] is the seminal generation of an accurate relativistic norm conserving pseudopotential specic for uranium atom. 35,36 This paper is organized as follows.…”

Structural, mechanical, spectroscopic and thermodynamic characterization of the copper-uranyl tetrahydroxide mineral vandenbrandeite

“…This pseudopotential has been validated widely for the research of the crystal structures, vibrational spectra and properties of uranium containing materials. [113][114][115][116][117][118][119][120][121][122][123][124][125][139][140][141][142][143][144] The unit-cell parameters of vandenbrandeite and the associated atomic positions were fully optimized by means of the Broyden-Fletcher-Goldfarb-Shanno (BFGS) technique. 145 A large plane wave kinetic energy cut-off parameter of 3 ¼ 1000 eV and a dense k-mesh 146 of 3 Â 5 Â 4 (30 k-points) were employed.…”

“…In fact, for a numerous set of materials in which uranium exists with VI oxidation state, the standard DFT description provides a reliable description of their structures and properties. [113][114][115][116][117][118][119][120][121][122][123][124][125][139][140][141][142][143][144][155][156][157][158][159][160][161][162] 2.2.2. Elastic tensor and equation of state.…”

“…These scaling procedures are frequently used to improve the computed Raman shis by correcting them empirically for the anharmonicity and remaining approximations used in the specic theoretical treatment used. 177 However, the unscaled Raman shis obtained using the DFPT method in previous works 113,[117][118][119][120]123,[139][140][141][142][143] have provided consistently accurate descriptions of the vibrational spectra of uranyl-containing materials. Therefore, we considered advisable to employ the DFTP method without applying any kind of scaling procedure.…”