The Electronic and Lattice Dynamics Related Properties of Uranium and Thorium Based Systems

There are number of 1:1 properties that could be directly compared between quantum mechanical calculations (using density functional theory) based on lattice dynamics and the experimental measurements. One of the examples is lattice heat capacity, thermal expansion, and thermal conductivity. The latter two are the key parameters of the potential nuclear fuels, the former one can shed much light on the magnetic systems of actinide hydrides. Actinides and especially their carbides as prospective nuclear fuel materials for the generation IV reactors were investigated using the density functional theory. We demonstrate that their electronic, magnetic, elastic, and thermal properties can be at present well described if the spin-orbit interaction and partial delocalization 5f electrons is properly included in the computational approaches. One can well reproduce not only basic electronic structure but also elastic constants, phonon dispersions, and their density of states, provided by XPS, UPS, BIS, and inelastic neutron scattering data [1-4]. Often, the localization of the 5f electrons could be captured using a moderate value of the on-site Coulomb interaction parameter. The case studies include a realistic description of the ground-state properties of elemental metals as Th, U and their monocarbides ThC and UC. In this study, published in Ref. 2 and 4, the realistic description of the electronic structure and lattice dynamics (phonons) explains why there is much higher thermal expansion in pure actinides (as Th) comparing with respective actinide monocarbides. The modeling also gives an insight up to which temperature the heat transport depends on lattice vibrations and where the electron transport starts to dominate. We analyzed the force constants of defected systems in order to reveal the effect of the oxygen impurity and vacancy at carbon site on the thermal expansion, summarized in Ref. 4. Additionally<br/>investigated thermodynamic properties, such as for instance heat capacity, were compared to the experimental data in the large temperature scan showing very excellent agreement up to 2000K and explained some additional features of phonon DOS not presented before.<br/>In the second part, we present the calculations of the stability, mechanical, and magnetic properties of the uranium hydrides including 3 different cubic compounds, α- and β-UH₃ and UH₂, all undergoing ferromagnetic ordering. Our first-principles calculations revealed a complex (non-collinear) magnetic order in β--UH₃. Unlike the other uranium hydrides, α--UH₃ and UH₂, b-UH₃ with two different U sites exhibits a site-dependent size and direction of U magnetic moments. While the U moments at the 2a sites are locked in the body diagonal direction, the moments at the 6c sites are inclined by approx. 15 degrees. The difference stems from 5f orbital moments. Comparison of results for all 3 species reveals that the U-U spacing is not the primary parameter to control the magnetism in uranium hydrides. Further insight is provided by evaluating individual exchange interactions between different neighbours, yielding the transition temperatures in a reasonable agreement with the experiment-[5-7].<br/>[1] U. D. Wdowik, P. Piekarz, D. Legut, and G. Jaglo, Phys. Rev. B 94, 054303 (2016).<br/>[2] L. Kyvala and D. Legut, Phys. Rev. B 101, 075117 (2020).<br/>[3] Y. Yun, D. Legut and P. M. Oppeneer, J. Nucl. Mat. 426, 109 (2012).<br/>[4] U. D. Wdowik, V. Buturlim, L. Havela, and D. Legut, J. Nucl. Mat. 545, 152547 (2021).<br/>[5] L Havela et al. Inorg. Chem. 57, 14727 (2018).<br/>[6] J. Prchal et al. J. Magn. Mag. Mater. 497, 165993 (2020).<br/>[7] L. Havela, M. Paukov, et al. J. Elect. Spectr. and Rel. Phenom. 239, 146904 (2020).<br/>[8] L. Kývala, L. Havela, A. P. Kadzielawa, and D. Legut, (submitted to J. Nucl. Mater. 2021).