Abstract
The formation of defect hydrides containing a large number of M-atomvacancies (superabundant vacancies; SAVs) was studied in bccNbHx and in thefcc phase of FeHx, CoHx, NiHx and PdHx, by resistivity and XRD measurements under different conditions of hydrogen pressure andtemperature, with/without allowing for exchange of hydrogen with environment(open-/closed-system methods). Two distinctly different behaviors were observed: in metalswith small formation energy of Vac–H clusters, both H and vacancies enter abundantly intothe M-lattice to form the ultimate defect-ordered structure, whereas in metalswith relatively large formation energies, vacancy concentrations remain relativelysmall. This general trend was examined by Monte Carlo simulations based on alattice–gas model. The result showed the occurrence of two distinct phases in thevacancy distribution caused by the combined action of the long-range elasticinteraction and local Vac–H interactions, in accordance with the observation.Conditions for the occurrence of these ‘vacancy-rich’ and ‘vacancy-poor’ states areexamined.
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