On the magnetic and thermodynamic properties of Americium-II: A hybrid density functional theoretic study

Abstract

The ground states of the actinides have been matters of considerable controversies, theory often contradicting experiment specifically in regards to the magnetic nature. To resolve this discrepancy, we present here hybrid density functional theory (HYB-DFT) based studies of the structural, magnetic, electronic, and thermodynamic properties of Americium-II. Three configurations of non-magnetic (NM), anti-ferromagnetic (AFM), and ferromagnetic (FM) with and without spin–orbit coupling (SOC) have been considered to determine the ground state of Am-II. We find that the experimental NM ground state configuration is indeed obtained for Am-II at a level of 40% Hartree–Fock (HF) exchange with SOC and the computed structural properties and the electronic density of states are in good agreement with experimental observations. We also find that HBY-DFT with NSOC fails to predict the correct magnetic and electronic structures for Am-II, indicating the importance of the inclusion of SOC for studies of strongly correlated materials. The phonon related thermodynamic properties of Am-II are presented for the NM ground state configuration and the computed heat capacity and entropy are found to be in good agreement with the experimental measurements. The lattice constant, bulk modulus, heat capacity, and entropy of AM-II are predicted to be 9.44 a.u., 21.7 GPa, 24.3 J K − 1 mol − 1 , and 55.7 J K − 1 mol − 1 , respectively.