Directional metal–hydrogen bonding in interstitial hydrides. III. Structural study of ErCo3Dx (0⩽x⩽4.3)

The ErCo3–D2 system has been studied by in situ neutron powder diffraction (NPD) at 60 °C and 0–16 bar deuterium pressure. Two deuteride phases were identified, β-ErCo3D1.07−1.38 and γ-ErCo3D3.7−4.3. They were structurally characterized at the compositions β-ErCo3D1.37 and γ-ErCo3D3.7 by high-resolution neutron and synchrotron powder diffraction. In contrast to the analogous nickel systems RNi3–D2 (R=Er, Ho; see part I, J. Alloys and Compds. 404–406 (2005) 89–94, and part II, J. Alloys and Compds. 2005, in press), their structures preserve the symmetry of the parent alloy (PuNi3-type, space group R-3m). Deuterium occupies mainly AB2 building blocks in the β-phase, and AB2 and AB5 building blocks in the γ-phase. In the AB2 building blocks cobalt is surrounded by an average of 3.8 (β-ErCo3D1.37) and 4.4 D-atoms (γ-ErCo3D3.7) in disordered distorted octahedral configurations (point symmetry −3), in contrast to nickel that is surrounded by ∼3 (β1- and β2-RNi3Dx, R=Er, Ho) and ∼4 (γ-ErNi3D3.7) D-atoms in disordered trigonal (pyramidal) and tetrahedral configurations, respectively (point symmetry 3). These results indicate that the D-atom distributions in this homologous series depend on the nature of the transition element rather than on geometric factors, and that directional bonding effects similar to those in non-metallic complex transition metal hydrides also prevail in metallic interstitial metal hydrides.