Abstract
Ultrasonic attenuation has been measured as a function of temperature approximately over the range 10-300 K at 5, 10, 15 and 45 MHz in a single crystal of a Pd85Pt15 alloy. The measurements have been carried out for five different propagation modes, both in the H-free and the H-loaded (H/(Pd+Pt)=0.29 at.%) state of the material. An anelastic relaxation has been observed at around 225 K (f=10 MHz), whose activation energy W and limit relaxation time tau 0 are: W=0.24+or-0.02 eV; tau 0=(1.4+or-0.5)*10-14 s. The mode dependence of the relaxation strength reveals the existence of a hydrostatic relaxation due to reactions between dipoles associated with H-H and H-Pt complexes. The results can be accounted for in terms of stress-induced changes in the short-range order of both the H-H and the H-Pt bonds (or antibonds) as well as of the occupancy probabilities of octahedral interstitial sites by H.
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