Mechanism of the alpha -to- beta phase transformation in the LaNi sub(5)-H sub(2) system

High-energy synchrotron in situ X-ray powder diffraction has been used to elucidate the mechanism of the hydriding phase transformation in a LaNi sub(5) model hydrogen storage intermetallic in real time. The transformation proceeds at 10 degree C via the transient growth of an interfacial phase, the gamma phase, with lattice parameters intermediate between those of the alpha (dilute solid solution) and beta (concentrated hydride) phases. The gamma phase forms to partially accommodate the 24% change in unit cell volume between the alpha and beta phases during hydriding and dehydriding. The alpha , gamma and beta phases coexist at the nanoscopic level.