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

Abstract

In situ X-ray and neutron powder diffraction were carried out for the La 4 MgNi 19 alloy sample, which was obtained by annealing under controlled Mg-vapor pressure and temperature. The sample contained five phases: Ce 5 Co 19 -type La 4 MgNi 19 , Pr 5 Co 19 -type La 4 MgNi 19 , Gd 2 Co 7 -type La 3 MgNi 14 , Ce 2 Ni 7 -type La 3 MgNi 14 , and CaCu 5 -type LaNi 5 . 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 La 4 MgNi 19 D x had a rhombohedral structure (space group: R3m) with a = 0.53990(3) nm, c = 5.2458(3) nm. Lattice expansions Aa/a and Ac/c in hydrogenation were 7.3% and 8.8%, and volume expansion ΔV/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 MgZn 2 -type cell and the CaCu 5 -type cell.