The formation of defect hydrides containing a large number of metal-atom vacancies was studied experimentally in the face-centred cubic phase of Fe, Co, Ni and Pd, under various conditions of H pressure and temperature. Two distinctly different behaviors were observed. In metals with small formation energies of Vac–H clusters, both H and vacancies readily entered the metal lattice to attain the ultimate composition, M3VacH4, whereas in metals with relatively large formation energies, the formation of this ultimate structure could become appreciable only at H concentrations which exceeded some critical value. This general trend was confirmed by a model calculation which included a long-range elastic interaction and short-range interatomic interactions between H atoms and vacancies.

The Defect Structure with Superabundant Vacancies to be Formed from FCC Binary Metal Hydrides - Experiments and Simulations. S.Harada, D.Ono, H.Sugimoto, Y.Fukai: Journal of Alloys and Compounds, 2007, 446-447, 474-8