Correlated 5f electronic states and phase stability in americium under high pressure: Insights from DFT+DMFT calculations

TL;DR

This study uses advanced computational methods to explore how americium's 5f electrons behave under high pressure, revealing partial delocalization, phase stability mechanisms, and electronic configuration changes across multiple phases.

Contribution

It provides new insights into americium's electronic structure and phase stability under pressure using DFT+DMFT, highlighting the role of 5f electron hybridization and Peierls-like distortions.

Findings

Localized 5f peak in Am-I matches experimental data

Increased hybridization in Am-III and Am-IV under pressure

Phase stability linked to Peierls-like lattice distortions

Abstract

We investigate the electronic structure of americium (Am) across its four experimentally confirmed high-pressure phases Am-I (P63/mmc), Am-II (Fm-3m), Am-III (Fddd), and Am-IV (Pnma) up to 100 GPa, using density functional theory combined with embedded dynamical mean-field theory. Our results successfully reproduce the prominent localized 5f peak observed in ultraviolet photoelectron spectroscopy around -2.8 eV below the Fermi level in the Am-I phase. While 5f electrons in Am-I and Am-II remain strongly localized, those in Am-III and Am-IV manifest discernible signatures of increased hybridization: a noticeable shift of spectral weight toward the Fermi level, enhanced hybridization strength, and the emergence of distinct multi-peak structures. These changes indicate that 5f electrons begin to participate in bonding and undergo partial delocalization under pressure. Nevertheless, the spectral weight of 5f electrons near the Fermi level in Am-IV remains relatively low, indicating that, compared to U and Pu, Am retains stronger localized 5f electrons even under high pressure. Analysis of the electronic configurations reveals pressure-enhanced valence state fluctuation, characterized by the mixing of 5f5, 5f6, and 5f7 electronic configurations. The X-ray absorption branching ratio further shows that the angular-momentum coupling scheme approaches the jj limit. Additionally, we demonstrate that the stability of the low-symmetry high-pressure phases (Am-III and Am-IV) is governed by a Peierls-like distortion mechanism, which reduces the total energy through symmetry-lowering lattice distortions accompanied by electronic reconstruction. This study offers a new microscopic perspective on high-pressure phase transitions and emergent quantum phenomena in actinide materials.