Abstract
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: P63/mmc) expands isotropically, in contrast to the Mg free phase. The hydrogen uptake, ∼9D/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 AB2 and AB5 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)Å. 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: P63/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.
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