Periodic DFT Study of the Structure, Raman Spectrum and Mechanical Properties of Schoepite Mineral Francisco Colmeneroa*, Joaquín Cobosb and Vicente Timóna aInstituto de Estructura de la Materia (IEM-CSIC). C/ Serrano, 113. 28006 – Madrid, Spain. bCentro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT). Avda/ Complutense, 40. 28040 – Madrid, Spain. Orcid Francisco Colmenero: https://orcid.org/0000-0003-3418-0735 Orcid Joaquín Cobos: https://orcid.org/0000-0003-0285-7617 Orcid Vicente Timón: https://orcid.org/0000-0002-7460-7572 *E-mail:
[email protected] 1 ABSTRACT The structure and Raman spectrum of schoepite mineral, [(UO2)8O2(OH)12] · 12 H2O, was studied by means of theoretical calculations. The computations were carried out by using Density Functional Theory with plane waves and pseudopotentials. A norm-conserving pseudopotential specific for the uranium atom developed in a previous work was employed. Since it was not possible to locate hydrogen atoms directly from X-ray diffraction data by structure refinement in the previous experimental studies, all the positions of the hydrogen atoms in the full unit cell were determined theoretically. The structural results, including the lattice parameters, bond lengths, bond angles and X-ray powder pattern were found in good agreement with their experimental counterparts. However, the calculations performed using the unit cell designed by Ostanin and Zeller in 2007, involving half of the atoms of the full unit cell, leads to significant errors in the computed X-ray powder pattern. Furthermore, Ostanin and Zeller’s unit cell contains hydronium ions, + H3O , that are incompatible with the experimental information. Therefore, while the use of this schoepite model may be a very useful approximation requiring a much smaller amount of computational effort, the full unit cell should be used to study this mineral accurately.