Theoretical investigation of pressure-induced structural transitions in americium usingGGA+Uand hybrid density functional theory methods

Abstract

First-principles calculations have been performed for americium (Am) metal using the generalized gradient approximation $+$ orbital-dependent onsite Coulomb repulsion via Hubbard interaction (GGA+$U$) and hybrid density functional theory (HYB-DFT) methods to investigate various ground state properties and pressure-induced structural transitions. Both methods yield equilibrium volume and bulk modulus in good agreement with the experimental results. The GGA+spin orbit coupling+$U$ method reproduced all structural transitions under pressure correctly, but the HYB-DFT method failed to reproduce the observed Am-I to Am-II transition. Good agreement was found between calculated and experimental equations of states for all phases, but the first three phases need larger $U$ (\ensuremath{\alpha}) parameters (where \ensuremath{\alpha} represents the fraction of Hartree-Fock exchange energy replacing the DFT exchange energy) than the fourth phase in order to match the experimental data. Thus, neither the GGA+$U$ nor the HYB-DFT methods are able to describe the energetics of Am metal properly in the entire pressure range from 0 GPa to 50 GPa with a single choice of their respective $U$ and \ensuremath{\alpha} parameters. Low binding-energy peaks in the experimental photoemission spectrum at ambient pressure relate, for some parameter choices, well to peak positions in the calculated density of states function of Am-I.