Temporal variations in the lattice parameters of Pd were measured, under high H pressures (2 to 5GPa) and temperatures (672 to 896C), by means of synchrotron X-ray diffraction. The observed lattice contraction was interpreted as being due to the formation of a large number of vacancy-H clusters (super-abundant vacancies). An analysis of the results led to the conclusion that a large fraction (10at%) of the vacancy-H clusters was introduced by diffusion from the surface, after a small number of them had been formed by some internal source. The thermal equilibrium concentration of the clusters at high temperatures exhibited a saturation behavior which suggested the presence of a maximum possible concentration (about 16at%). The formation energy, entropy and volume of a vacancy-H cluster were found to be 0.72eV, 7.2k and 0.60Ω, respectively. The migration energy and volume were 1.20eV and 0.49Ω, respectively.

Formation of Superabundant Vacancies in Pd-H Alloys. Y.Fukai, Y.Ishii, Y.Goto, K.Watanabe: Journal of Alloys and Compounds, 2000, 313, 121-32