Ultrasonic attenuation and dispersion due to hydrogen motion in the C15 Laves-phase compound TaV2Hx

Hydrogen in the C15 Laves-phase material TaV2Hx has been studied by means o f resonant ultrasound spectroscopy over the temperature range of 15-345 K f or a series of hydrogen concentrations (x = 0.00-0.53). Ultrasonic loss pea ks and frequency shifts (dispersion) associated with the hydrogen motion we re observed, yielding parameters for the hydrogen motion. Hydrogen in these materials is known to occupy the tetrahedral g sites which form a series o f interlinked hexagons. The ultrasonic results were associated with H hoppi ng between g-site hexagons. The relaxation rates for x less than or equal t o 0.18 were best described as a sum of two Arrhenius processes. For x = 0.3 4 and 0.53 only a single Arrhenius process was needed to fit the results, a lthough the presence of a second Arrhenius mechanism could not be over-rule d. A single relaxation rate was sufficient to fit the data; a distribution of rates was not required. The magnitudes of the attenuation and dispersion depended linearly on the hydrogen concentration implying that it is the re laxation of isolated H atoms (the Snoek effect) that is responsible for the mechanical damping. The faster local motion of H reported from nuclear mag netic resonance measurements for motion within g-site hexagons was not obse rved in the present study. This suggests that the H hopping rate for the lo cal motion remains above the ultrasonic frequencies over the temperature ra nge of study, or perhaps that too few H atoms participate in the local moti on.