INTERNAL-FRICTION STUDY OF NITROGEN-HYDROGEN AND NITROGEN-DEUTERIUM PAIR IN TANTALUM

Low frequency (approximately 1 Hz) internal friction measurement was performed in tantalum-nitrogen (approximately 1 at%) alloys loaded with hydrogen (H) or deuterium (D). A well defined internal friction peak was observed at 50 K for H and at 65 K for D. Magnitude of the peak was measured as a function of H (D) concentration and found to saturate to a constant value when H (D) concentration becomes comparable with that of nitrogen. This indicates that a simple N-H or N-D pair is formed in Ta and the jump of H (D) around N gives rise to the peak. The activation energy of the peak was measured by a peak shift method giving E = 65 +/- 10 for H and 95 +/- 15 meV for D. Debye function was calculated assuming two different temperature dependence of T7 or exp (-E/kT) for hydrogen jump (a short range diffusion). Comparison with the experimental result reveals that the latter one is valid around 50 K for hydrogen jump in Ta. Referring experimental results on Nb-N-H single crystals where delta-s' = 0 has been found, the site where H (D) atom is trapped to N atom is seeked. The 2T state which consists of two t-sites located at the 3rd or 4th neighbor distance from N atom is found to be the most likely candidate. Also the anelastic strain due to hydrogen trapped to nitrogen atom was estimated with assuming the trigonometric symmetry for the N-H pair and no significant difference was found between H and D.