Ab initiocomputational 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 K². 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.