Hydrogen-Induced Anelastic Relaxation in Pd-1.9 at % MgO

Abstract
In Pd, MgO precipitates with diameters in the nm range can be produced by internal oxidation of PdMg_x samples that are annealed in an O₂ atmosphere. The Pd-MgO interfaces can be decorated with excess oxygen, depending on the O₂ gas pressure in the final annealing procedure. We studied by mechanical spectroscopy the influence of hydrogen (H) interstitials on the anelastic behavior of internally oxidized Pd-1.9 at%MgO samples (with and without oxygen decoration of the Pd-MgO interfaces, temperature range 4 to 380 K, frequencies between 213 and 685 Hz). The H causes a broad relaxation peak with an activation energy of ~ 0.15 eV for the underlying relaxation process. The peak does not exist in ordinary (pure) Pd doped with H. We conclude that the relaxation peak results from H interstitials that are trapped in the interfacial regions between the MgO precipitates and the Pd. This supports the results of a recent electrochemical study which similarly reports a very effective trapping of H interstitials in these regions. Surprisingly, the relaxation peak was not found to depend on whether the interfacial regions were decorated with oxygen or not.