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

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 indu ced 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) s how clear isotope effects concerning their temperature position as well as their activation parameters. In case of the reorientation maximum (activati on enthalpy H-a,H(M2) =0.18-0.26 eV, H-a,D(M2) =0.20-0.26 eV, depending on H or D as a.H a,D well as Y or Gd concentration) which occurs at temperatur es slightly above 100 K: the pre-exponential factor of the jump frequency i s of the same order of magnitude as the Debye frequency of the host lattice (nu(Deb) =5.7.10(12) s(-1)) for D, whereas it is one order of magnitude sm aller for H. In case of the Zener maximum (H-a,H(M1)=0.11 eV, H-a,D(M1)=0.1 5 eV) which occurs at temperatures of about 80 K the pre-exponential factor s 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 adiabat ically for higher temperatures or non-adiabatically for lower temperatures. (C) 1999 Elsevier Science S.A. All rights reserved.