Role of vacancies in the structural relaxation of Pd-Mo alloys saturated with hydrogen

The presence of an anomalously large number of vacancies in Pd-Mo alloys subjected to hydrogenation is revealed using precise x-ray diffractometry. These alloys are found to undergo nonmonotonic structural evolution during long-term relaxation. The evolution is characterized by aperiodic time variations in the number of coexisting phases, in the volume of each of them, and in the defect structure and by the cooperative motion of vacancies (as well as hydrogen in the early stage) between the matrix and defect regions. The key features of this evolution are an anomalously high concentration of not only hydrogen but also vacancies and a high concentration of defect regions, which causes thermodynamic instability of the system. The structural evolution has an oscillating character, because the maxima of thermodynamic instability of the matrix and of an ensemble of defect regions are separated in time.