REORIENTATION OF DIATOMIC H-Y COMPLEXES IN DILUTE PD-Y ALLOYS

The reorientation of diatomic H-Y complexes in hydrogen-doped dilute Pd100-xYx alloys (n=1, 2, 4.5, 8) has been studied by means of internal friction (IF) in an inverted torsion pendulum and in a vibrating reed apparatus. As reported earlier, the internal friction of hydrogen-doped dilute Pd100-xYx alloys shows three maxima that are identified as the Zener peak, the reorientation peak of diatomic H-Y complexes and the Snoek-Koster peak. The width of the reorientation peak exceeds that of a single Debye maximum, and increases with the Y content as well as with the hydrogen content of the sample. For a fixed hydrogen content, the maximum shifts to higher temperatures if the Y content of the alloys increases; in contrast, for a fixed Y content, there is a shift to lower temperatures caused by an increasing hydrogen content. The measured reorientation peak can be fitted by a superposition of Debye maxima using a constant pre-exponential factor and spectra of activation enthalpies for the relaxation times. The analysis of the hydrogen content dependence of the relaxation strength yields a value of \lambda(1)-lambda(2)\=0.036 for the elastic anisotropy of the diatomic H-Y complexes. The activation parameters for the relaxation time of the reorientation of an isolated complex are 0.18 eV and 3x10(-12) s. The properties of the activation enthalpy spectra can be interpreted in terms of a model which takes into account the elastic long-range interaction between an H-Y complex and the Y atoms in the vicinity of this complex.