On the formation and ordering of ‘superabundant’ vacancies in palladium due to hydrogen absorption

Abstract

The formation of superabundant vacancies in metals can be driven by hydrogen incorporation on the interstitial sublattice. In order to achieve this, it is essential to have a sufficiently high hydrogen chemical potential so that effectively all the available interstitial sites are occupied. The concomitant formation of high concentrations of vacancies on the metal sublattice is then controlled by the coupling of the vacancy chemical potentials on the metal and interstitial sublattices by the Schottky equilibrium condition. This formation of excessive concentrations of vacancies on the host metal lattice is expected to be a general phenomenon which, if the system can achieve vacancy equilibrium with external or internal sinks, will occur as soon as the interstitial sublattice approaches complete filling. This means that the hydrogen/metal ratio will exceed that normally expected for a complete filling of the interstitial sites being considered. Once a high concentration of vacancies has been formed, then an effective repulsive interaction between the vacancies on the metal sublattice may give rise to vacancy ordering at lower temperatures. This interpretation is in good agreement with the experimental results of Fukai and Okuma for the Pd-H system. The authors estimate that they have reached a hydrogen/metal ratio ofmore » {approx} 1.2, which they have shown to be quite feasible at the H{sub 2}(g) pressures and temperatures used in their experiments.« less