Ultrasonic Study of Hydrogen Motion in Palladium - and Palladium-Deuteride

Ultrasonic attenuation measurements have been performed on single crystal PdH_{rm x} and PdD_{rm x } over the temperature range 77-300K, where.64 <=q x <=q.76. The longitudinal and two independent transverse waves were propagated along the (110) crystal axis. Attenuation loss peaks are observed for all three modes and interpreted as a relaxation attenuation involving stress-induced hopping of pairs or complexes of vacancies on the H(D) sublattice. The investigation revealed structure in the ultrasonic attenuation data that was more obvious as concentrations increased. An asymmetric distribution in the activation energy does not give a correct description of the data nor does a distribution using the Zener relation. The data are therefore not characterized by a single relaxation time or a distribution of a relaxation times. Structure in some of the data suggests that two or more individual Debye peaks may fit the data. The data were than fit using sum of two Debye peaks with a total of six parameters. The activation energy of the high temperature peak is close to the values found from other measurements. A sum of the hopping rates gives a break in the data about 230K. The two peak fit is interpreted as classical barrier hopping and tunneling from an excited state. The data were also fit with a sum of three peaks based on the idea of different configurations around a tetrahedral site. The activation energy is close to values found from other measurements. The activation energy of the peaks of either fit shows no obvious concentration dependence in the concentration range studied. While measured activation energies of H and D are in agreement with NMR and long range diffusion measurements, the attempt frequencies are not. The relaxation strengths calculated from the magnitudes of the peak heights of both fits indicate the central forces do not describe the H-H (D-D) interactions.