Superabundant vacancy formation in Nb–H alloys; resistometric studies

The formation of superabundant vacancies (SAVs; vacancy-hydrogen clusters) was studied in Nb-H alloys by means of resistivity measurements as a function of temperature, pressure and H concentration. The formation energy of a vac-H cluster (0.3#+0.1 eV), which is 1/10 of the formation energy of a vacancy in Nb, is explained tentatively as being the consequence of six H atoms trapped by a vacancy with the average binding energy of 0.46 eV/H atom. The SAVs were introduced from the external surface, and transported into the interior by direct bulk diffusion and/or by fast diffusion along dislocations. The activation volumes for the formation and migration of vac-H clusters were determined to be 3.7 and 5.3 A3, respectively.