Diatomic Hydrogen-Metal Complexes in Dilute Palladium-Rare Earth Alloys

Abstract
The internal friction of hydrogen-doped dilute Pd_100-xRE_x alloys (RE = Gd, Ce) (x = 1 ...9) shows three maxima in the temperature range between 10 K and 300 K. These maxima are interpreted as Zener peak of hydrogen in the β-phase (H_a = 0.11 eV), Snoek-Köster peak (H_a = 0.18 eV) and reorientation peak of diatomic H-RE complexes. The relaxation strength of the latter peak is independent of the RE concentration and depends linearly on the hydrogen content. A value of |λ₁-λ₂| = 0.036 for the elastic anisotropy of the H-RE complexes is obtained. In alloys with RE = Gd this peak saturates when the hydrogen content is comparable with the RE concentration, whereas in the case of RE = Ce saturation at much lower hydrogen contents is observed. The activation enthalpy H_a of the H-RE reorientation varies with the RE as well as with the H concentration and lies between 0.18 eV and 0.26 eV. The activation enthalpy and the width of the peak increase with increasing RE content. Increasing the H content leads to a decrease of H_a. These features of the reorientation of diatomic H-RE complexes are discussed in terms of a distribution of site enthalpies caused by elastic long-range interaction between a H-RE complex and further RE atoms in its vicinity.