The incipient use of theoretical methods in the research of geomaterials reveals the great power of such methodology in the determination of the mineral properties. These methods provide a safe, accurate and cheap manner of obtaining these properties. Uranium-containing minerals are highly radiotoxic, and their experimental studies demand a careful handling of the samples used. However, theoretical methods are free of such inconveniences and may be used in the complete characterization of this type of minerals. Theoretical methods are not only a complement to the use of other experimental techniques but also a powerful predictive tool. The structural, mechanical and Raman spectroscopic properties of uranyl-containing materials, including rutherfordine soddyite, schoepite and uranophane-α, were studied by means of theoretical solid-state methods based on density functional theory using plane waves and pseudopotentials. A new norm-conserving relativistic pseudopotential for uranium atom developed in recent works was employed. These minerals are among the most important secondary phases arising from corrosion of spent nuclear fuel under the final geological disposal conditions. The computed crystal structures of these materials as well as the corresponding and X-ray powder patterns were found to be in excellent agreement with the experimental information. Therefore, the optimized structures of these minerals were employed to study the mechanical properties and stability of these minerals. These properties were obtained using the finite deformation technique. All these minerals were found to be mechanically stable since the corresponding Born stability conditions were satisfied. A large amount of relevant mechanical data were reported including bulk, Young and Shear moduli, Poisson ratios, ductility and hardness indices, anisotropy measures as well as longitudinal and transversal wave velocities. The large volume expansion and mechanical stress resulting from the corrosion of spent nuclear fuel during storage emphasize the great relevance of the mechanical information of the waste components. Finally, the computation of vibrational properties of these minerals is studied. The computed Raman spectra of these materials were found to be in good agreement with their experimental counterparts when they were available for comparison. These results demonstrate the power of the theoretical methods in the research of uranium-containing minerals.
Part of the book: Minerals
With the advent of increased computer capacities, improved computational resources, and easier access to large-scale computer facilities, the use of density functional theory methods has become nowadays a frequently used and highly successful approach for the research of solid-state materials. However, the study of solid materials containing heavy elements as lanthanide and actinide elements is very complex due to the large size of these atoms and the requirement of including relativistic effects. These features impose the availability of large computational resources and the use of high quality relativistic pseudopotentials for the description of the electrons localized in the inner shells of these atoms. The important case of the description of uranyl-containing materials and their properties has been faced recently. The study of these materials is very important in the energetic and environmental disciplines. Uranyl-containing materials are fundamental components of the paragenetic sequence of secondary phases that results from the weathering of uraninite ore deposits and are also prominent phases appearing from the alteration of the spent nuclear fuel. The development of a new norm-conserving relativistic pseudopotential for uranium, the use of energy density functionals specific for solids, and the inclusion of empirical dispersion corrections for describing the long-range interactions present in the structures of these materials have allowed the study of the properties of these materials with an unprecedented accuracy level. This feature is very relevant because these methods provide a safe, accurate, and cheap manner of obtaining these properties for uranium-containing materials which are highly radiotoxic, and their experimental studies demand a careful handling of the samples used. In this work, the results of recent applications of theoretical solid state methods based on density functional theory using plane waves and pseudopotentials to the determination of the thermodynamic properties and stability of uranyl-containing materials are reviewed. The knowledge of these thermodynamic properties is indispensable to model the dynamical behavior of nuclear materials under diverse geochemical conditions. The theoretical methods provide a profound understanding of the thermodynamic stability of these mineral phases and represent a powerful predictive tool to determine their thermodynamic properties.
Part of the book: Density Functional Theory