X-ray diffraction measurements of the Ni-H system were performed by using synchrotron radiation at H pressures of 3 to 5GPa at about 1000C. Gradual lattice contraction, which occurred over several hours at high temperatures, revealed the formation of super-abundant vacancies (vacancy-H clusters). Superlattice reflections due to ordered arrangements of V-H clusters were also observed. The concentration (0.30) of V-H clusters, as deduced from the magnitude of the lattice contraction, was nearly independent of pressure and temperature, and indicated the maximum possible cluster concentration accommodated by the metal lattice.

Superabundant Vacancy Formation in Ni–H Alloys. Y.Fukai, Y.Shizuku, Y.Kurokawa: Journal of Alloys and Compounds, 2001, 329[1-2], 195-201