Pressure-induced changes in the electronic structure of americium metal

Abstract

We have conducted electronic-structure calculations for Am metal under pressure to investigate the behavior of the 5$f$-electron states. Density-functional theory (DFT) does not reproduce the experimental photoemission spectra for the ground-state phase where the 5$f$ electrons are localized, but the theory is expected to be correct when 5$f$ delocalization occurs under pressure. The DFT prediction is that peak structures of the 5$f$ valence band will merge closer to the Fermi level during compression indicating the presence of itinerant 5$f$ electrons. Existence of such 5$f$ bands is argued to be a prerequisite for the phase transitions, particularly to the primitive orthorhombic AmIV phase, but does not agree with modern dynamical-mean-field theory (DMFT) results. Our DFT model further suggests insignificant changes of the 5$f$ valence under pressure in agreement with recent resonant x-ray emission spectroscopy, but in contradiction to the DMFT predictions. The influence of pressure on the 5$f$ valency in the actinides is discussed and is shown to depend in a nontrivial fashion on 5$f$-band position and occupation relative to the $spd$ valence bands.