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

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, \({\text{Ca}}\left( {{\text{UO}}_{2} } \right)_{6} {\text{O}}_{4} \left( {\text{OH}} \right)_{6} \cdot 8{\text{H}}_{2} {\text{O}}\), 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.