Directional metal–hydrogen bonding in interstitial hydrides. III. Structural study of ErCo 3D x (0⩽ x⩽4.3)

Abstract

The ErCo 3–D 2 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-ErCo 3D 1.07−1.38 and γ-ErCo 3D 3.7−4.3. They were structurally characterized at the compositions β-ErCo 3D 1.37 and γ-ErCo 3D 3.7 by high-resolution neutron and synchrotron powder diffraction. In contrast to the analogous nickel systems RNi 3–D 2 ( 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 (PuNi 3-type, space group R-3 m). Deuterium occupies mainly AB 2 building blocks in the β-phase, and AB 2 and AB 5 building blocks in the γ-phase. In the AB 2 building blocks cobalt is surrounded by an average of 3.8 ( β-ErCo 3D 1.37) and 4.4 D-atoms ( γ-ErCo 3D 3.7) in disordered distorted octahedral configurations (point symmetry −3), in contrast to nickel that is surrounded by ∼3 ( β 1- and β 2- RNi 3D x , R=Er, Ho) and ∼4 ( γ-ErNi 3D 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.