Ab initio computational and experimental investigation of the electronic structure of actinide 218 materials

We report a comprehensive investigation of the electronic structure and magnetic properties of actinide 218 compounds, which crystallize in the tetragonal Ho2CoGa8 crystal structure. Specifically, we study experimentally the group of plutonium-based compounds Pu2MGa8 (with M=Rh, Co, and Fe), which are structurally related to the unconventional superconductors PuCoGa5 and PuRhGa5 and are measured to be nonmagnetic and nonsuperconducting down to 2 K, yet displaying relatively high linear specific-heat coefficients of 61 to 133 mJ/mol K-2. We perform density-functional theory based calculations, in which we apply three different approaches to access the tendency of 5f electron localization, the local spin-density approximation (LSDA) LSDA+U, and the 5f open-core approach. For comparison to the above-mentioned compounds we also investigate computationally the plutonium compounds with M=Ir and Pd, the uranium-based compounds U2MGa8 (with M=Co, Fe, Rh, and Ru), as well as Np2CoGa8, and Am2CoGa8. On the basis of ab initio LSDA calculations we optimize the equilibrium lattice parameters and the internal fractional coordinates within the Ho2CoGa8 crystal structure. The obtained lattice parameters are in relatively good agreement with experimental values, when we assume delocalized 5f states for all compounds except Am2CoGa8. We discuss the computed electronic structures and the theoretical Fermi surfaces. For the Pu-218 compounds we find that LSDA calculations, in which the 5f's are treated as delocalized, predict a magnetically ordered ground state, whereas LSDA+U calculations predict a nonmagnetic ground state in accordance with experiment. For the U-218 compounds the LSDA itinerant 5f approach predicts a nonmagnetic ground state, in accordance with available experimental data. For Am2CoGa8 our calculations are consistent with the scenario of localized 5f electrons. We find that, on account of the elongated tetragonal structure, most of the theoretical Fermi surfaces are quasi-two-dimensional.