IN-SITU X-RAY-DIFFRACTION - A USEFUL TOOL TO INVESTIGATE HYDRIDE FORMATION REACTIONS

A high-pressure X-ray diffraction (XRD) cell has been designed which allowed us to study simultaneously hydrogen absorption/desorption isotherms and XRD powder diffraction patterns on (de)hydrided intermetallic compounds. The hydride formation reaction was investigated in the case of LaNi5 under both dynamic and steady-state conditions. The alpha-to-beta phase transition was monitored throughout the plateau region. The discrete alpha-to-beta lattice expansion, which amounted to more than 21 vol.%, was found to be much larger than the crystallographic expansion of both solid solutions in the pressure range measured (less-than-or-equal-to 10 bar). The discrete phase transformation is thought to be responsible for the formation of planar-like defects, as observed by transmission electron microscopy, and, subsequently, for the cracking of powder particles. The formation of these crystallographically oriented defects has been correlated with the reduced coherence length and increased strain, which could be calculated from the anisotropic peak broadening found in the XRD diagrams. Annealing of dehydrided LaNi5 induced the planar-like defects to disappear.and, consequently, to sharpen the relevant XRD reflections.