Correlation between the Electrostatic Potential and the Occupation of Interstitial Sites by Hydrogen in Metallic Systems

Abstract

The electrostatic crystal potentials V(r) of the cubic metals Pd, V, Nb, Ta, and the intermetallic phases FeTi and XEr, X = Cu, Ag, Au, have been determined by the full potential augmented‐plane‐wave method (FP‐APW). Furthermore, to study the electronic states in dilute metal‐hydrogen‐systems the partial densities of states have been calculated by the supercell procedure using the augmented‐spherical‐wave method (ASW) for PdH1/32. The results are compared with the values for PdH1/4, PdH, and Pd. – A relation is found between the depth of the host lattice electrostatic potential and the occupation of interstitial lattice sites by hydrogen in the metal‐hydrogen α‐phases. For bcc V, Nb, and Ta the crystal potential “seen” by a proton has its deepest minimum at the tetrahedral site, for fcc Pd the minimum occurs at the octahedral site and in FeTi (B2‐type) the minimum of Vproton(r) is found for the octahedral site surrounded by 2 Fe atoms and 4 Ti atoms. In all these cases the calculated site of the minimum is in accordance with the location of hydrogen in the respective α‐phases found experimentally by neutron scattering. For XEr. X = Cu, Ag, Au, a preferred occupation by hydrogen is predicted.

The Electronic State of Hydrogen in the α Phase of the Hydrogen‐Storage Material PdH(D)x: Does a Chemical Bond Between Palladium and Hydrogen Exist?

“…Much research on the Pd–H system has been carried out, mostly focused on the β phase (Pd hydride) so far . In the β phase, the electronic structure is quite different from that of the parent Pd because Pd−H bonding and antibonding states are formed in the Pd hydride.…”

“…In the β phase, the electronic structure is quite different from that of the parent Pd because Pd−H bonding and antibonding states are formed in the Pd hydride. Such a change in the electronic structure by the hydride formation strongly influences the physical properties, which depend on the concentration of absorbed hydrogen, and has been well studied from both theoretical and experimental aspects . In contrast to the β phase, there has been no systematic study on the α phase, limited only to some fragmented researches, owing to the extremely low hydrogen concentration in the α phase compared with that in the β phase.…”

“…Pure Pd exhibits strong paramagnetism derived from the large DOS at the Fermi energy located in the 4d band . In the β phase, formation of Pd‐H(D) σ‐bonding and σ‐antibonding bands causes a decrease in the number of d‐band holes, which corresponds to the shift of the Fermi level from the 4d to 5s band . This results in a large decrease of the DOS at the Fermi level and thus decrease of magnetic susceptibility.…”

Diffusion of interstitial hydrogen into and through bcc Fe from first principles

“…18 Of the two previous studies, X␣ in general could be considered superior to EH (which is basically tight-binding). Philipp et al 13 found a correlation between the crystal electrostatic potential and the occupation of interstitial sites by H in bulk metals. They found that the t-site preference of H in bcc V, Nb, and Ta, and the o-site preference of H in fcc Pd could be correlated qualitatively with pure bulk metal's protonic electrostatic potential (i.e., the potential seen by a proton), which had its lowest value at the respective sites.…”

“…10 However, conflicting theoretical predictions of both site preferences and the direction of charge transfer have been reported. Geometrical factors, 11 lattice relaxations, 12 and electronic arguments 13 have been invoked to explain the site preference of H in bulk transition metals, though no universal theory can explain all cases.…”