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-ErCo₃D_1.07−1.38 and γ-ErCo₃D_3.7−4.3. They were structurally characterized at the compositions β-ErCo₃D_1.37 and γ-ErCo₃D_3.7 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-ErCo₃D_1.37) and 4.4 D-atoms (γ-ErCo₃D_3.7) in disordered distorted octahedral configurations (point symmetry −3), in contrast to nickel that is surrounded by ∼3 (β₁- and β₂-RNi₃D_x, R=Er, Ho) and ∼4 (γ-ErNi₃D_3.7) 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.