It was recalled that an explanation had previously been given, for the formation of high vacancy concentrations, in terms of a simple statistical model. That model was confirmed more quantitatively here, and it was demonstrated that it could also explain an ordering of such vacancies in the present metal. It was concluded that the formation of super-abundant vacancies in metals could be driven by H incorporation on the interstitial sub-lattice. In order to achieve this, it was essential to have a sufficiently high H chemical potential so that all of the available interstitial sites were effectively occupied. The concomitant formation of concentrations of vacancies on the metal sub-lattice was then controlled by a coupling of the vacancy chemical potentials, on the metal and interstitial sub-lattices, via the Schottky equilibrium condition. The formation of excessive concentrations of vacancies on the host metal lattice was expected to be a general phenomenon which, if the system could achieve vacancy equilibrium with external or internal sinks, would occur as soon as the interstitial sub-lattice approached complete filling. This implied that the H/metal ratio would exceed that which was normally expected for complete filling of the interstitial sites under consideration. After a high concentration of vacancies had formed, an effective repulsive interaction between the vacancies on the metal sub-lattice could then give rise to vacancy ordering at lower temperatures. This interpretation was in good agreement with experimental results for the Pd-H system. It was estimated that the latter had involved a H/metal ratio of about 1.2; a value which was quite feasible at the H pressures and temperatures which were used.

W.A.Oates, H.Wenzl: Scripta Metallurgica et Materialia, 1995, 33[2], 185-93