Formation of Superabundant Vacancies in Pd Hydride under High Hydrogen Pressures

In situ x-ray diffraction on Pd hydride under 5 GPa of hydrogen pressure show that lattice contraction due to vacancy formation occurs in 2-3 h at 700-800 °C, and two-phase separation into PdH and a vacancy-ordered phase of ${\mathrm{Cu}}_3$Au structure (${\mathrm{Pd}}_3$ ${\mathrm{VacH}}_4$) on subsequent cooling. After recovery to ambient conditions and removal of hydrogen, the vacancy concentration in Pd metal was determined by measuring density and lattice parameter changes to be 18 ± 3 at.%. This procedure provides a new method of introducing superabundant vacancies in metals.