Phase transformation of the La0.7Mg0.3Ni2.8Co0.5-H2 system studied by in situ X-ray diffraction

Phase transformation in the La0.7Mg0.3Ni2.8Co0.5-H2 system along the hydrogen absorption and desorption pressure-composition isotherm was studied using in situ X-ray diffraction. La0.7Mg0.3Ni2.8Co0.5 consisted mainly of the Ce2Ni7-type La3Mg(Ni,Co)14 phase. In hydrogen absorption of the Ce2Ni7-type phase, the hydrogen solid solution and hydride phases coexisted. The solid solution phase transformed to the hydride phase with isotropic expansion of the crystal lattice. In hydrogen desorption, coexistence of the two phases was not observed and the crystal lattice consecutively reduced with reduction of the hydrogen content. Lattice strain in the Ce2Ni7-type phase in hydrogen desorption was higher than that in hydrogen absorption.