Directional metal-hydrogen bonding in interstitial hydrides

Hydrogenation of ErNi3 (space group R3¯m) has been investigated by neutron powder diffraction on deuterided samples. At least three ErNi3Dx phases were found at deuterium compositions x∼ 1.3 (β1), x∼ 2 (β2) and x∼ 3.8 (γ), the latter being stable only under high deuterium pressure. Their structures show anisotropic lattice expansions and are non-centrosymmetric (space group R3m). In the β1-phase, deuterium occupies two interstitial sites in the AB2-type building block of which one provides three ligands of a Ni-centered NiD4 tetrahedron and the other bridges a Ni triangle. In the β2- and γ-phases, up to five new deuterium sites become partially occupied in both the AB2- and AB5-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.