Hydrogen atom distribution and hydrogen induced site depopulation for the La2-xMgxNi7-H system

La2-xMgxNi7 and its hydrides/deuterides were investigated by high resolution synchrotron powder X-ray and neutron diffraction. Upon deuteration the single phase sample of the intermetallic compound with the refined composition La1.63Mg0.37Ni7 (space group: P6(3)/mmc) expands isotropically, in contrast to the Mg free phase. The hydrogen uptake, similar to 9 D/f.u., is higher than in La2Ni7D6.5. The refined composition accounts for La1.63Mg0.37Ni7D8.8 (beta-phase). Rietveld refinements using the neutron and synchrotron diffraction data suggest that deuterium atoms occupy 5 different interstitial sites within both AB(2) and AB(5) slabs, either in an ordered or a disordered way. All determined D sites have an occupancy >50% and the shortest D-D contact is 1.96(3) angstrom. It is supposed that a competition between the tendency to form directional bonds and repulsive D-D (H-H) interactions is the most important factor that influences the distribution of deuterium atoms in this structure. A hitherto unknown second, alpha-phase with composition La1.63Mg0.37Ni7D0.56. crystallizing with the same hexagonal symmetry as La1.63Mg0.37Ni7D8.8, has been discovered. The unit cell parameters for this D-poor phase differ slightly from those of the intermetallic. Alpha-phase displays only one D site (4f, space group: P6(3)/mmc) occupied > 50%, which is not populated in the D-rich beta-phase. This hydrogen/deuterium induced site depopulation can be explained by repulsive D-D (H-H) interactions that are likely to influence non-occupancy of certain interstices in metal lattice when absorbing hydrogen. (C) 2011 Elsevier Inc. All rights reserved.