Structural Study of La4MgNi19 Hydride by In Situ X-ray and Neutron Powder Diffraction

In situ X-ray and neutron powder diffraction were carried out for the La4MgNi19 alloy sample, which was obtained by annealing under controlled Mg-vapor pressure and temperature. The sample contained five phases: Ce5Co19-type La4MgNi19, Pr5Co19-type La4MgNi19, Gd2Co7-type La3MgNi14, Ce2Ni7-type La3MgNi14, and CaCu5-type LaNi5. Those mass fractions were 51%, 19%, 16%, 4%, and 10%, respectively. The crystal structures for the samples before and after hydrogenation were refined by the Rietveld method. The full hydride of the main phase La4MgN19Dx had a rhombohedral structure (space group: R (3) over barm) with a = 0.53990(3) nm, c = 5.2458(3) nm. Lattice expansions Delta a/a and Delta c/c in hydrogenation were 7.3% and 8.8%, and volume expansion Delta V/V was 25.3%. The hydrogen content D/M obtained from the refinement was 0.91(5), and hydrogen was evenly located in both the MgZn2-type cell and the CaCu5-type cell.