Evidence of Copious Vacancy Formation in Ni and Pd under a High Hydrogen Pressure

From in situ observation of X-ray diffraction of Ni and Pd under a high hydrogen pressure (≃5 GPa) and temperatures (≤800°C), anomalous lattice contraction of the hydride was found to occur in 2∼3 h. This contraction, amounting to ∼0.5 Å3 per a metal atom, remained in the recovered specimen even after the hydrogen was removed by heating to 400°C, but was annealed out at 800°C. The concentration of vacancies responsible for this effect is estimated at ∼20% of metal-atom sites. Anomalous concentration dependence of the hydrogen-induced volume and enhanced diffusion of metal atoms are explained in terms of this effect.