Structural and Electronic Properties of the Hydrogenated ZrCr₂ Laves Phases

Abstract

A series of hydrogenated cubic C15 and hexagonal C14 ZrCr 2 Laves phases were studied by means of semiempirical extended Hu ¨ckel tight-binding, ab initio density functional theory methods and maximally localized Wannier functions, with a goal to find the most energetically favorable positions of interstitial H atoms in the host unit cells. We consider situations with one or two H atoms per primitive cell. Crystalorbital overlap population studies, performed for the C15 structure, show repulsion between two hydrogen atoms in close proximity. This is in accord with the ab initio calculations, performed for the hydrogenated C14 and C15 structures, which clearly favor two separated hydrogen atoms instead of the formation of moleculelike pairs in five-coordinated trigonal-bipyramidal environments.

Comparison of C14- and C15-Predomiated AB2 Metal Hydride Alloys for Electrochemical Applications

“…(a) (b) [13] Hydrogen occupation occurs first at the A2B2 site, next at the AB3 site, and finally at the B4 site [14][15][16][17]. Furthermore, the ratio between the tetrahedral sites is 12 A2B2, 4 AB3, and 1 B4 for both C14 and C15 structures.…”

“…In addition, the model for e/a at the C14/C15 threshold (C14:C15 = 1:1) was further improved to include the contribution from the A-site atoms by incorporating the average chemical potential of the A-site atoms [24]. [13] Hydrogen occupation occurs first at the A 2 B 2 site, next at the AB 3 site, and finally at the B 4 site [14][15][16][17]. Furthermore, the ratio between the tetrahedral sites is 12 A 2 B 2 , 4 AB 3 , and 1 B 4 for both C14 and C15 structures.…”

Hydrogen storage and compression

“…In the case of interstitial metal hydrides, the molecular hydrogen in the gas is split into atomic hydrogen on the surface of the material and then, it diffuses into the atomic structure of the host metal. In these hydrides, hydrogen acts as a metal and forms a metallic bond [29]. A large number of different metallic compounds exist that will absorb hydrogen in this manner.…”

In situ NMR study of hydrogenation/dehydrogenation of ZrCr2 and physisorbed hydrogen

“…According to Somenkov et al [16], occupation of g-and e-sites in Laves phase compounds is lattice size-dependent. For ZrCr 2 system, g-sites, with largest interstices, are preferentially occupied [15]. Eberle reported that for ZrCr 2 H x alloys with x < 2.5, only gsites are occupied [17].…”

“…Hydrogen in ZrCr 2 occupies tetrahedral interstitial sites. Three types of interstitial sites may be identified: those coordinated by 2Cr and 2Zr atoms (g-sites), 3Cr and 1Zr atoms (e-sites) and 4Cr atoms [15]. According to Somenkov et al [16], occupation of g-and e-sites in Laves phase compounds is lattice size-dependent.…”