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 5f -electron states. Density-functional theory (DFT) does not reproduce the experimental photoemission spectra for the ground-state phase where the 5f electrons are localized, but the theory is expected to be correct when 5f delocalization occurs under pressure. The DFT prediction is that peak structures of the 5f valence band will merge closer to the Fermi level during compression indicating presence of itinerant 5f electrons. Existence of such 5f 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 5f 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 5f valency in the actinides is discussed and is shown to depend in a non-trivial fashion on 5f band position and occupation relative to the spd valence bands.

First-principles phase stability, bonding, and electronic structure of actinide metals

“…This is evident from the calculated and measured electron spectra for americium, see Figure 5. Models that include stronger electron correlations, such as "DFT+U" and dynamical mean-field theory (DMFT) [35,36] reproduce experimental spectra [37] far better [38]. Energetically, however, the DFT-GGA approach is still viable because the delocalized 5f states can spin polarize which in terms of energetics and bonding captures much of the localization process [39].…”

Structural investigation of californium under pressure

“…The formation of the Am IV structure (space group, Pnma), its stability at very high pressures and its subsequently discovered or theorized 7,8 presence in neighboring actinides has established it as an important actinide high-pressure structure supported by theoretical calculations on the structural behaviour of the higher actinides 9--13 . However recent resonant x-ray emission spectroscopy and x-ray absorption near-edge structure experiments 14 and theory 15 on Am have not substantiated the hypothesis of strong mixing of 5f states with the valence band and show that the general behavior of actinide 5f electrons under compression is not yet fully understood.…”

Delocalization and occupancy effects of 5f orbitals in plutonium intermetallics using L3-edge resonant X-ray emission spectroscopy

“…Several excellent reviews describing the RXES technique are available in the literature [17,18,19] Since different oxidation states in otherwise similar bonding environments should be separated by about 5 eV [10], the broadening expected from a 3d core hole (about 4 eV) should be sufficiently narrow to see features associated with different oxidation states. This fact has been successfully exploited in RXES studies of An coordination compounds [20,21], and a recent null result in a search for different oxidation state components in Am metal under applied pressure [22] has helped limit theoretical models [23] of the observed structural transitions in that element [24].…”