Crystal structure and hydrogen occupation of LaNi4.9Al0.1Dx (5.0 ≤ x ≤ 6.1) on the desorption isotherm studied by in situ neutron powder diffraction

Abstract

We performed in situ neutron powder diffraction measurements of LaNi4.9Al0.1Dx along the desorption P–C isotherm using 90° banks of a TOF diffractometer. Structure of the hydride phase for various hydrogen contents was determined by Rietveld refinement. Variation of hydrogen site occupation in the hydride phase was clearly observed. The full hydride LaNi4.9Al0.1D6.1 has a P6mm structure with four hydrogen sites; 3c sites (O[La2Ni4]) with occupancy of 0.95(2) and three T sites, 6e1 (T[La2Ni2]), 6e2 (T[La1Ni3]) and 2b (T[Ni4]), with occupancies of 0.324(6), 0.195(6) and 0.22(1), respectively. In the two-phase region, the hydride phase has the limiting composition LaNi4.9Al0.1D5.0. Decreased hydrogen content from x = 6.1 to 5.0 in the hydride phase involved decrease of occupation in all sites, but behavior of the decrease depended on the sites. The 3c and 6e1 sites are preferably occupied in the limiting composition, whereas extra occupation caused by hydrogen solution in the hydride single-phase region is most preferable in the 6e2 sites.