Investigation of urania within LDA+U method

Abstract

The local-spin-density approximation plus Hubbard U (commonly called LDA+U) is used in a comprehensive study of the structural, mechanical and optical properties of urania. It is shown that using the new LDA+U implementation in CASTEP calculations, the symmetry constraints, imposed in agreement with both the symmetry of the unit cell and the present magnetic interaction, can be used to predict the ground state with the occupancy of the orbitals (5f in urania) in agreement with Hund's rules. The inclusion of a Hubbard U correction to 5f electrons of uranium changes urania from a metal to an insulator and, therefore, has a dramatic effect on the localisation of the electron spin and charge density on uranium. The value of the adjustable parameter U, which predicts the optical band gap in agreement with the experiment, leads to a slightly larger lattice constant than experimentally observed. The real and imaginary dielectric constants, reflectivity and energy loss functions are calculated using independent particle excitation approximations. The 5f intraband transitions are described adequately by the LDA and the effective U parameter, which is equal to 3.5eV. However, additional corrections are needed to account for the 1-eV shift in the 2p–6d transitions.