Directional metal-hydrogen bonding in interstitial hydrides I.: Structural study of ErNi3Dx (0 ≤ x ≤ 3.75)

Hydrogenation of ErNi3 (space group R3m) has been investigated by neutron powder diffraction on deuterided samples. At least three ErNi3Dx phases were found at deuterium compositions x similar to 1.3 (beta(1)), x similar to 2 (beta(2)) and x similar to 3.8 (gamma), the latter being stable only under high deuterium pressure. Their structures show anisotropic lattice expansions and are non-centrosymmetric (space group R3m). In the beta(1)-phase, deuterium occupies two interstitial sites in the AB(2)-type building block of which one provides three ligands of a Ni-centered NiD4 tetrahedron and the other bridges a Ni triangle. In the beta(2)- and gamma-phases, up to five new deuterium sites become partially occupied in both the AB(2)- and AB(5)-type building blocks. One of them provides the fourth ligand to the NiD4 tetrahedron thus suggesting directional bonding effects similar to those observed in nickel-based complex metal hydrides, such as LaMg2NiH7 and Mg2NiH4. (c) 2005 Elsevier B.V. All rights reserved.