Full-potential LAPW electronic structure study of δ-plutonium and the (001) surface -: art. no. 045115

The electronic and geometric properties of bulk fcc delta-plutonium and the quantum size effects in the surface energies and the work functions of the (001) ultrathin films (UTF) up to seven layers have been investigated with periodic density-functional theory calculations within the full-potential-linearized-augmented-plane-wave (FP-LAPW) approach as implemented in the WIEN2k package. The effects of several approximations have been examined: (i) nonspin polarization (NSP) versus spin polarization (SP); (ii) scalar-relativity [no spin-orbit coupling (NSO)] versus full-relativity [i.e., with spin-orbit (SO) coupling included]. Our calculations show that both spin-polarization and spin-orbit coupling play important roles in determining the equilibrium atomic volume and bulk modulus for delta-plutonium. Our calculated equilibrium atomic volume of 178.3 a.u.(3) and bulk modulus of 24.9 GPa at the fully relativistic level of theory, i.e., spin-polarization and spin-orbit coupling included, are in good agreement with the experimental values of 168.2 a.u.(3) and 25 GPa (593 K), respectively. In particular, the energy difference brought by spin-orbit coupling, similar to 7-8 eV, is dominant, but the energy difference brought by spin-polarization, from a few tenths to 2 eV, has a stronger dependence on the atomic volume. Features of the density of states show that 5f electrons are more itinerant when the volume of delta-plutonium is compressed and they are more localized when the volume is expanded, which provides evidence to explain the origin of the volume expansion between the alpha and delta phases. The calculated equilibrium lattice constants at different levels of approximation are used in the surface property calculations for the thin films. The surface energy is found to be rapidly converged at all four level approximations, NSP-NSO, NSP-SO, SP-NSO, and SP-SO. The semi-infinite surface energy is predicted to be 0.692 eV at the full relativistic level with spin-polarization and spin-orbit coupling. Quantum size effect for the work function is not pronounced for the (001) surface at the LAPW level of theory.