Crystal structure and hydrogen occupation of LaNi_4.9Al _0.1D_x (5.0 ≤ x ≤ 6.1) on the desorption isotherm studied by in situ neutron powder diffraction

We performed in situ neutron powder diffraction measurements of LaNi _4.9Al_0.1D_x 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 LaNi_4.9Al_0.1D_6.1, has a P6mm structure with four hydrogen sites; 3c sites (0[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 LaNi _4.9Al_0.1D_5.0. 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.