Electron correlations and spin-orbit coupling for lattice dynamics of UC, Ac and Th

Abstract: Uranium monocarbide, a potential fuel material for the generation IV reactors, is investigated within density functional theory. Its electronic, magnetic, elastic, and phonon properties are analyzed and discussed in terms of spin-orbit interaction and localized versus itinerant behavior of the 5f electrons. The localization of the 5f states is tuned by varying the local Coulomb repulsion interaction parameter. We demonstrate that the theoretical electronic structure, elastic constants, phonon dispersions, and their densities of states can reproduce accurately the results of x-ray photoemission and bremsstrahlung isochromat measurements as well as inelastic neutron scattering experiments only when the 5f states experience the spin-orbit interaction and simultaneously remain partially localized [1]. The partial localization of the 5f electrons could be represented by a moderate value of the on-site Coulomb interaction parameter of about 2 eV. The results of the present studies indicate that both strong electron correlations and spin-orbit effects are crucial for realistic theoretical description of the ground-state properties of uranium carbide. Similarly in case of pure Ac, Th, where we aim to determine phase stability and the thermal conductivity contributions from electronic subsystem [2] as well as from lattice vibrations (phonons). Here the effect of the of spin-orbit coupling and electron correlations on the force constants (lattice vibrations) were investigated. Various phases of Ac (hcp, bcc, fcc, and sc) were calculated within the approach of quasi-harmonic theory [3] to determine the phase transition in Ac as a function of pressure and temperature. Results of these calculations lead to somewhat different conclusions than those reported by Rubio-Ponce et al. [4].

References:
[1] U. D. Wdowik, P. Piekarz, D. Legut, and G. Jaglo, Phys. Rev. B 94, 054303 (2016).
[2] G. K.H.Madsen and D. J.Singh, Comp. Phys. Comm. 175, 67 (2006).K
[3] Parlinski, Z.-Q. Li, and Y. Kawazoe, Phys. Rev. Lett. 78, 4063 (1997), A. Togo and I. Tanaka, Scr. Mater., 108, 1-5 (2015).
[4] A. Rubio-Ponce, J. Rivera, and D. Olguın, Phys. Status Solidi B 252, 695 (2015).

(*) In collaboration with L. Kývala (Technical University of Ostrava), U. D. Wdowik. and G. Jaglo, Institute of Technology, Cracow, Poland, P. Piekarz, Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego 152, 31-342 Cracow, Poland