DIATOMIC HYDROGEN-METAL COMPLEXES IN DILUTE PALLADIUM-RARE EARTH ALLOYS

The internal friction of hydrogen-doped dilute Pd(100-x)RE(x) alloys ( RE = Gd, Ce) (x = 1...9) shows three maxima in the temperature range b etween 10 K and 300 K. These maxima are interpreted as Zener peak of h ydrogen in the beta-phase (H-a = 0.11 eV), Snoek-Koster peak (H-a = 0. 18 eV) and reorientation peak of diatomic H-RE complexes. The relaxati on strength of the latter peak is independent of the RE concentration and depends linearly on the hydrogen content. A value of /lambda(1) - lambda(2)/ = 0.036 for the elastic anisotropy of the H-RE complexes is obtained. In alloys with RE = Gd this peak saturates when the hydroge n content is comparable with the RE concentration, whereas in the case of RE = Ce saturation at much lower hydrogen contents is observed. Th e 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 i ncreasing RE content. Increasing the H content leads to a decrease of H-a. These features of the reorientation of diatomic H-RE complexes ar e discussed in terms of a distribution of site enthalpies caused by el astic long-range interaction between a II-RE complex and further RE at oms in its vicinity.