Crystal structure of the deuteride LaCu5D3.2 studied by neutron powder diffraction

The crystal structure of the deuteride LaCu5D3.2 is investigated by neutron powder diffraction. Analysis of the diffraction pattern shows that the initial CaCu5-type cell adopted by LaCu5 is not preserved upon deuteration. The pattern of the deuteride can be indexed in a new ortho-hexagonal cell (Pmma space group; a′ = 2a = 10.7647 Å, b′ = c = 4.1493 Å, c′ = 2a + b = 9.4445 Å) leading to a cell volume four times larger than that of LaCu5. All possible hydrogen sites derived from the original CaCu5-type cell have been tested. Eight different sites have been found occupied by deuterium in 2c, 2e, two 4i and four 4j positions. At least 2.05 D atoms are found in tetrahedral positions and the remaining ones are in octahedral positions. The deuterium ordering is responsible for the symmetry lowering observed for the LaCu5 compound upon hydrogenation. It induces small shifts of the metallic atoms compared to the CaCu5-type structure leading to the supercell description. Finally, the impact of this structure on the unusual diffusion properties previously observed for LaCu5H3.4 during nuclear magnetic resonance (NMR) studies will be discussed.