The formation of superabundant vacancies (Vac-H clusters) was 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 face-centered cubic structure, at 930 to 1350K and H pressures of 2.4 to 7.4GPa. The results show that the vacancy concentration was not so much dependent on temperature and H pressure as the H concentration. In Ni and Co, the vacancy concentration, xcl, increased linearly with the H concentration, xH, for the whole concentration range, reaching an xcl value of about 0.3 at an xH value of about 1. In Pd, the vacancy concentration was very small up to an xH value of about 0.6 and increased linearly thereafter with nearly the same slope as in Ni and Co. The maximum vacancy concentration reached in Pd was about 0.12. It was noted that the observed difference in the xH–xcl relationship was related to the magnitude of the formation energy of Vac-H clusters, which was very small in Ni and Co and relatively large in Pd.

A Relation between the Vacancy Concentration and Hydrogen Concentration in the Ni–H, Co–H and Pd–H Systems. S.Harada, S.Yokota, Y.Ishii, Y.Shizuku, M.Kanazawa, Y.Fukai: Journal of Alloys and Compounds, 2005, 404-406, 247-51