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 (congruent-to 5 GPa) and temperatures (less-than-or-equal-to 800-degrees-C), anomalous lattice contraction of the hydride was found to occur in 2 approximately 3 h. This contraction, amounting to approximately 0.5 angstrom3 per a metal atom, remained in the recovered specimen even after the hydrogen was removed by heating to 400-degrees-C, but was annealed out at 800-degrees-C. The concentration of vacancies responsible for this effect is estimated at approximately 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.