Directional metal-hydrogen bonding in interstitial hydrides.: III.: Structural study of ErCo3Dx (0≤x≤4.3)

The ErCo3-D-2 system has been Studied by in situ neutron powder diffraction (NPD) at 60 degrees C and 0-16 bar deuterium pressure. Two deuteride phases were identified, beta-ErCo3D1.07-1.38 and gamma-ErCo3D3.7-4.3. They were structurally characterized at the compositions beta-ErCo(3)D1(.37) and gamma-ErCo3D37 by high-resolution neutron and synchrotron powder diffraction. In contrast to the analogous nickel systems RNi3-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 (PuNi3-type, space group R-31m). Deuterium occupies mainly AB(2) building blocks in the beta-phase, and AB(2) and AB(5) building blocks in the gamma-phase. In the AB(2) building blocks cobalt is surrounded by an average of 3.8 (beta-ErCo3D1.37) and 4.4 D-atoms (gamma-ErCo3D3.7) in disordered distorted octahedral configurations (point symmetry -3), in contrast to nickel that is surrounded by similar to 3 (beta(1-) and beta(2)-RNi3Dx, R = Er, Ho) and similar to 4 (gamma-ErNi3D3.7) D-atoms ill disordered trigonal (pyramidal) and tetrahedral configurations, respectively (point symmetry 3). These results indicate that the D-atom distributions ill this homlogous series depend on the nature of the transition clement rather than oil geometric factors, and that directional bonding effects similar to those in nn-metallic complex transition metal hydridcs also prevail in metallic interstitial metal hydrides. (c) 2006 Elsevier Inc. All rights reserved.