The presence of an anomalously large number of vacancies in alloys subjected to hydrogenation was revealed by using precision X-ray diffractometry. The alloys were found to undergo a non-monotonic structural evolution during long-term relaxation. The evolution was characterized by aperiodic time variations in the number of co-existing phases, in the volume of each of them, and in the defect structure; and by the cooperative motion of vacancies (as well as H in the early stages) between the matrix and defect regions. Key features of the evolution were an anomalously high concentration of H and vacancies, and a high concentration of defect regions, which caused thermodynamic instability of the system. The structural evolution had an oscillatory nature because the maxima of thermodynamic instability of the matrix, and of an ensemble of defect regions, were separated in time.

Role of Vacancies in the Structural Relaxation of Pd–Mo Alloys Saturated with Hydrogen. V.M.Avdyukhina, A.A.Anishchenko, A.A.Katsnelson, G.P.Revkevich: Physics of the Solid State, 2004, 46[2], 265-71