Migration mechanism in defect metal hydrides containing superabundant vacancies

1. Introduction In the presence of interstitial H atoms, the concentration of M-atom vacancies is enhanced dramatically, forming a defect structure containing superabundant vacancies (SAVs). The diffusivity of M atoms is enhanced accordingly. Physically, these phenomena are the result of the lowering of the formation energy of a vacancy by trapping H atoms [1, 2]. A Monte Carlo calculation on the SAV formation process revealed that, in hydrides of fcc metals, two distinct defect phases are formed; a vacancy-ordered phase of high concentrations of vacancies on the L1 2 structure, and a vacancy-disordered phase of relatively low concentrations where vacancies are randomly distributed over the M lattice. Transitions between these two phases take place, as shown in Fig.1 [2].