Uranium at High Pressure from First Principles

TL;DR

This study uses first-principles calculations to explore the structural phases and stability of alpha-uranium under extremely high pressures up to 1.3 TPa, predicting phase transitions and elastic properties.

Contribution

It provides new insights into uranium's phase stability and structural behavior at high pressures using a simplified electronic structure model.

Findings

Alpha-uranium is stable up to ~285 GPa before transforming to bct phase.

Bcc phase remains energetically unfavorable up to 1.3 TPa.

The equation of state and elastic constants agree with experimental data up to 100 GPa.

Abstract

The equation of state, structural behavior and phase stability of {\alpha}-uranium have been investigated up to 1.3 TPa using density functional theory, adopting a simple description of electronic structure that neglects the spin-orbit coupling and strong electronic correlations. The comparison of the enthalpies of Cmcm (alpha-U), bcc, hcp, fcc, and bct predicts that the aplpha-U phase is stable up to a pressure of ~285 GPa, above which it transforms to a bct-U phase. The enthalpy differences between the bct and bcc phase decrease with pressure, but bcc is energetically unfavorable at least up to 1.3 TPa, the upper pressure limit of this study. The enthalpies of the close-packed hcp and fcc phases are 0.7 eV and 1.0 eV higher than that of the stable bct-U phase at a pressure of 1.3 TPa, supporting the wide stability field of the bcc phase. The equation of state, the lattice parameters and the anisotropic compression parameters are in good agreement with experiment up 100 GPa and previous theory. The elastic constants at the equilibrium volume of alpha-U confirm our bulk modulus. This suggests that our simplified description of electronic structure of uranium captures the relevant physics and may be used to describe bonding and other light actinides that show itinerant electronic behavior especially at high pressure.