Ab initiocomputational and experimental investigation of the electronic structure of actinide 218 materials

Abstract

We report a comprehensive investigation of the electronic structure and magnetic properties of actinide 218 compounds, which crystallize in the tetragonal ${\text{Ho}}_{2}{\text{CoGa}}_{8}$ crystal structure. Specifically, we study experimentally the group of plutonium-based compounds ${\text{Pu}}_{2}M{\text{Ga}}_{8}$ (with $M=\text{Rh}$, Co, and Fe), which are structurally related to the unconventional superconductors ${\text{PuCoGa}}_{5}$ and ${\text{PuRhGa}}_{5}$ and are measured to be nonmagnetic and nonsuperconducting down to 2 K, yet displaying relatively high linear specific-heat coefficients of 61 to $133\text{ }\text{mJ}/\text{mol}\text{ }{\text{K}}^{2}$. 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), $\text{LSDA}+U$, and the $5f$ open-core approach. For comparison to the above-mentioned compounds we also investigate computationally the plutonium compounds with $M=\text{Ir}$ and Pd, the uranium-based compounds ${\text{U}}_{2}M{\text{Ga}}_{8}$ (with $M=\text{Co}$, Fe, Rh, and Ru), as well as ${\text{Np}}_{2}{\text{CoGa}}_{8}$, and ${\text{Am}}_{2}{\text{CoGa}}_{8}$. On the basis of ab initio LSDA calculations we optimize the equilibrium lattice parameters and the internal fractional coordinates within the ${\text{Ho}}_{2}{\text{CoGa}}_{8}$ crystal structure. The obtained lattice parameters are in relatively good agreement with experimental values, when we assume delocalized $5f$ states for all compounds except ${\text{Am}}_{2}{\text{CoGa}}_{8}$. 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 $\text{LSDA}+U$ 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 ${\text{Am}}_{2}{\text{CoGa}}_{8}$ 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.