Formation of superabundant vacancies in Pd-H alloys

Temporal variation of the lattice parameter of Pd was measured under high hydrogen pressures (2-5 GPa) and temperatures (672-896 degreesC) by X-ray diffraction using a synchrotron radiation, and observed lattice contraction was interpreted as being due to the formation of a large number of vacancy-hydrogen (Vac-H) clusters, i.e. superabundant vacancies. Analysis of the: result led to the conclusion that a major part of Vac-II clusters (amounting to similar to 10 at.%) were introduced by diffusion from the surface, after a small number of them had been formed at some internal sources. The thermal-equilibrium concentration of Vac-H clusters at high temperatures shows a saturation behavior, which indicates the presence of a maximum possible concentration (ca.16 at.%) of the clusters. The formation energy, entropy and volume of a Vac-H cluster are found to be 0.72 eV, 7.2k and 0.60 Ohm, respectively, and the migration energy and volume are 1.20 eV and 0.49 Ohm respectively. Various other implications of the results are also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.