Neutron spectroscopy on H-doped body-centered cubic Nb0.8Mo0.2 alloys demonstrated recently (i) a complete suppression of low-temperature H precipitation up to 5at%H, and (ii) an exclusive H occupation of tetrahedral interstitial sites, like pure body-centered cubic Nb. Nb0.8Mo0.2 alloys containing 0.85 and 3at%H were studied by mechanical spectroscopy and a H-induced low-temperature relaxation found at 2kHz and about 80K. Activation energy and reciprocal pre-exponential relaxation time were ~0.054eV and ~2.8·x 107/s, respectively. The relaxation peak was about 80% broader than for an ideal Debye relaxation, which revealed a spectrum of relaxation times. The height of the relaxation (Q-1) peaks was approximately linear to H concentration, thus indicating a Snoek-type relaxation of single H atoms. The heights of the peaks yielded a tetragonality |λ1–λ2| of about 0.04, which was an order of magnitude smaller than that of the Snoek effect of O in Nb. The results suggested that the observed relaxation reflected the low-temperature diffusive jumps of single H atoms between tetrahedral interstitial sites, with a jump rate above about 1.2·x 104/s at 80K.

Low-Temperature Snoek-Type Relaxation of Hydrogen Interstitial Atoms in Nb0.8Mo0.2. B.Leu, H.Wipf, B.Coluzzi, A.Biscarini, G.Mazzolai, F.M.Mazzolai, V.V.Sumin: Journal of Alloys and Compounds, 2003, 356-357, 322-5