Superabundant vacancy formation in Ni–H alloys

Abstract

X-ray diffraction measurements on the Ni–H system were made using synchrotron radiation at high hydrogen pressures p(H 2)=3∼5 GPa and high temperatures T≲1000°C. Gradual lattice contraction occurring over several hours at high temperatures revealed the formation of superabundant vacancies (vacancy-hydrogen clusters). Superlattice reflections due to ordered arrangements of Vac-H clusters were also observed. The concentration of Vac-H clusters ( x cl≅0.30), deduced from the magnitude of the lattice contraction, was very nearly independent of pressure and temperature, and indicates the maximum possible cluster concentration to be accommodated by the metal lattice. A simple enlightening description of the physics of superabundant vacancy formation is given in Appendix A.