A relation between the vacancy concentration and hydrogen concentration in the Ni-H, Co-H and Pd-H systems

The formation of superabundant vacancies (Vac-H clusters) has been observed in many M-H alloys, but the factors that determine the equilibrium concentration of vacancies have not been identified yet. To identify these factors, the equilibrium concentration of vacancies was estimated from lattice contraction measurements on Ni, Co and Pd having a fcc structure, at high temperatures (930-1350 K) and high hydrogen pressures (2.4-7.4 GPa). The results show that the vacancy concentration is not so much dependent on temperature and hydrogen pressure as the hydrogen concentration. In Ni and Co, the vacancy concentration (xcl) increases linearly with the hydrogen concentration (xH) for the whole concentration range, reaching xcl∼0.3 at xH∼1.0. In Pd, the vacancy concentration is very small up to xH∼0.6 and increases linearly thereafter with nearly the same slope as in Ni and Co. The maximum vacancy concentration reached in Pd is xcl∼0.12. It is noted that the observed difference in the xH -xcl relation correlates with the magnitude of the formation energy of Vac-H clusters, which is very small in Ni and Co and relatively large in Pd.