A Density Functional Theory Study of the Charge State of Hydrogen in Metal Hydrides

Abstract

Density functional theory and Bader charge analyses were used to investigate the charge state of hydrogen in vanadium, niobium, tantalum, palladium, and niobium-palladium alloys. Over a range of concentrations and hydrogen-site configurations, it is found that hydrogen consistently acquires a net charge of between approximately -0.51e and -0.64e in the pure group 5 metals compared with a significantly smaller value of 0.3e in palladium. Although there is indirect evidence that the electronic charge plays a role in the solubility and diffusivity of hydrogen in the group 5 metals, this is the first work to quantify the value of the charge. Hydrogen tends to migrate to regions of the metal lattice that minimize its overall charge density, which generally corresponds to the T-site in the bcc metals. It is found that the charge of hydrogen at the O-site of vanadium can be slightly lower due to lattice deformation and may explain the occupation of both T-and O-sites in vanadium hydrides.

First-principles calculations of niobium hydride formation in superconducting radio-frequency cavities

“…The calculated absorption energy for hydrogen in tetrahedral sites is -0.28 eV relative to H 2 in the gas phase and pure b.c.c. niobium, which is in good agreement with previous DFT calculations [54,55] and the experimental heat of solution [56,57]. Hydrogen was found to be unstable in octahedral sites.…”

“…β. The importance of charge distribution for phase formation was suggested by Aboud and Wilcox [54], who studied the electronic charge state of hydrogen in niobium, vanadium, tantalum, palladium and palladium-niobium alloys utilizing DFT and Bader charge analysis. They found an increase in hydrogen's electronic charge, the host metal's lattice parameter, and the absorption energy for hydrogen into vanadium and tantalum for an increase in hydrogen concentration from 0.0625 to 0.125 H/host atomic ratio; however, the electronic charge on hydrogen decreased when the hydrogen concentration was further increased to 0.25 H/host ratio.…”

Changes in microstructures and hydrogen permeability of Nb30Hf35Co35 eutectic alloy membranes by annealing

“…These equations show clearly that a low F value is at all times obtained from the mixed type arrangement at a given volume fraction of the BCC-(Nb, Hf) phase, compared with that in the parallel type arrangement. Migration of H through the Nb-based alloys is via a series of jumps between interstitial sites [23]. Calculations based on theoretical models have shown that the diffusion of atomic H is usually governed by the electronic structure and the size of the interstitial sites [7,24].…”

Rational design of Nb-based alloys for hydrogen separation: A first principles study

“…8,9 However, the origin of H stabilization/destabilization mechanism in metals and alloys are yet to be revealed. Although Aboud and Wilcox found a correlation between the H solution energy and H charge state in pure Nb and Pd metals, 11 they are unable to quantitatively explain the interactions between H and metal atoms in alloys.…”

“…[7][8][9] It is reported that H resides at interstitial sites and interacts with the nearest neighbors of metal atoms, forming a hydrogeninduced state below the valence bands. [8][9][10][11][12] Computational studies of Ti-and Pd-alloys suggest that the interactions between H and metal alloying atoms may be elucidated by the atomic size effect and/or Miedema's "reverse stability" rule. 8,9 However, the origin of H stabilization/destabilization mechanism in metals and alloys are yet to be revealed.…”