Delocalization and new phase in americium: Density-functional electronic structure calculations

Density-functional electronic structure calculations have been used to investigate the high pressure behavior of Am. At about 80 kbar (8 GPa) calculations reveal a monoclinic phase similar to the ground state structure of plutonium ({α}-Pu). The experimentally suggested {α}-U structure is found to be substantially higher in energy. The phase transition from fcc to the low symmetry structure is shown to originate from a drastic change in the nature of the electronic structure induced by the elevated pressure. A calculated volume collapse of about 25% is associated with the transition. For the low density phase, an orbital polarization correction to the local spin density theory was applied. Gradient terms of the electron density were included in the calculation of the exchange/correlation energy and potential, according to the generalized gradient approximation. The results are consistent with a Mott transition; the $5f$ electrons are delocalized and bonding on the high density side of the transition and chemically inert and nonbonding (localized) on the other.