Electrons and phonons in uranium hydrides - effects of polar bonding

Abstract

GGA+U has been used as a framework for computational study of UH2, and α- and β-UH3, exploring specific features of the polar U-H bonding and its influence on magnetic and cohesion properties, including elastic parameters and vibrational properties. The description of the U-5f states with direct Coulomb U = 0.5 eV and equal exchange J = 0.5 eV not only reproduces equilibrium volumes but provides a realistic description of total magnetic moments, consisting of smaller spin and larger antiparallel orbital components. For UH2 and α-UH3, the spin-axis is aligned along the [111] direction. For β-UH3, there is a significant difference between both size and orientation U moments of atoms at the 2a and 6c Wyckoff positions. The former has U moments aligned along [111], while in the latter, not fixed to any specific direction by symmetry, they deviate by 10∘. The method corroborates previous bonding analyses, indicating a prominent hybridization and charge transfer, affecting the 6d and 7s states of U, being partly transferred to the H-1s states, as revealed by the Bader analysis. Analyzing individual effective inter-site magnetic coupling parameters it was possible to identify sources of relatively high Curie temperatures of 170 K for both UH3 variants and 120 K for UH2. Our results give predictions of elastic coefficients (consistent with the known bulk modulus in the case of β-UH3) and phonon densities of states, yielding expected infrared, Raman, and Hyper Raman active modes.