Hysteresis of the α – β Phase Transition in Palladium Hydride

In-situ x-ray diffraction analyses of the hydriding process of metal hydrides can provide insights into the connection between lattice strain and pressure hysteresis. We have examined the hydriding transition of the palladium-hydrogen system, a model case due to the fact that the isostructural transition occurs at modest temperatures and hydrogen partial pressures. By measuring the lattice parameter changes for the α and β phases during both hydriding and dehydriding, we have determined the extent of coherency of the phase transition as a function of hydride fraction, temperature, and cycle number. Precise in-situ XRD measurements have revealed a hysteresis in the lattice parameters during hydriding and dehydriding. At the same time, the formation of dislocations or defects has been qualitatively identified through refinement of microstrain parameters. The experimental results will be presented in the context of theories of the origin of hysteresis in metal hydrides; either due to coherency strains or dislocation creation/annihilation.