Hydrogen and deuterium tunnelling in scandium

A detailed study of the relaxation processes occurring below room temperature has been carried out by charging polycrystalline Sc with different amount of hydrogen and deuterium. For all the concentrations, we have observed relaxation effects ascribed to tunnelling of H (D) between the two equivalent first neighbour tetrahedral sites in the c direction, which strongly depend on the H (D) content. In diluted Sc–H (up to few hundreds ppm H), due to the presence of interstitial O, the main anelastic peak is related to the tunnelling of H trapped by O. Increasing the H (D) concentration the traps are saturated and new peaks appear at lower temperature. A marked isotope effect is observed; the peaks shift in temperature and are at least four times smaller with H than with D, when measured at the same frequency. In deuterated sample the main peak has been analysed in terms of the two-level-systems (TLS). A good fit for the case of 1 at.% D could be obtained with a relaxation time τ∝T−7 and a small distribution of the asymmetries. This indicates that, quite surprisingly, the rate in this temperature range is dominated by multiple phonons processes.