The defect structure with superabundant vacancies to be formed from fcc binary metal hydrides: Experiments and simulations

Abstract The process of formation of defect hydrides containing a large number of metal-atom vacancies was studied experimentally in the fcc phase of Fe, Co, Ni and Pd, under different conditions of hydrogen pressure and temperature. Two distinctly different behaviors were observed: In metals with small formation energies of Vac–H clusters, both H and vacancies readily enter the metal lattice to attain the ultimate composition M 3 VacH 4 , whereas in metals with relatively large formation energies, the formation of this ultimate structure may become appreciable only at H concentrations exceeding some critical value. This general trend was confirmed by a model calculation including a long-range elastic interaction and short-range interatomic interactions between H atoms and vacancies.