Isotope effects of the mechanical relaxation of H and D in dilute PdY and PdGd alloys

Abstract

The internal friction spectra of dilute PdY and PdGd alloys charged with H or D show three relaxation maxima. The maxima attributed to the stress induced ordering of H or D pairs (Zener maximum) and to the reorientation of H–Y or H–Gd pairs (D–Y or D–Gd pairs, respectively) (reorientation maximum) show clear isotope effects concerning their temperature position as well as their activation parameters. In case of the reorientation maximum (activation enthalpy Ha,HM2=0.18–0.26 eV, Ha,DM2=0.20–0.26 eV, depending on H or D as well as Y or Gd concentration) which occurs at temperatures slightly above 100 K the pre-exponential factor of the jump frequency is of the same order of magnitude as the Debye frequency of the host lattice (νDeb=5.7·1012 s−1) for D, whereas it is one order of magnitude smaller for H. In case of the Zener maximum (Ha,HM1=0.11 eV, Ha,DM1=0.15 eV) which occurs at temperatures of about 80 K the pre-exponential factors for D and H are lower by two and four orders of magnitude, respectively, as compared to the Debye frequency. These results are interpreted in terms of thermally activated tunnelling jump processes which occur either adiabatically for higher temperatures or non-adiabatically for lower temperatures.