Ultrasonic attenuation and dispersion due to hydrogen motion in the C15Laves-phase compound TaV2Hx

Abstract

Hydrogen in the C15 Laves-phase material TaV2Hx has been studied bymeans of resonant ultrasound spectroscopy over the temperature range of15-345 K for a series of hydrogen concentrations (x = 0.00-0.53). Ultrasonicloss peaks and frequency shifts (dispersion) associated with the hydrogenmotion were observed, yielding parameters for the hydrogen motion. Hydrogen inthese materials is known to occupy the tetrahedral g sites which form aseries of interlinked hexagons. The ultrasonic results were associated with Hhopping between g-site hexagons. The relaxation rates for x⩽0.18 werebest described as a sum of two Arrhenius processes. For x = 0.34 and 0.53 onlya single Arrhenius process was needed to fit the results, although thepresence of a second Arrhenius mechanism could not be over-ruled. A singlerelaxation rate was sufficient to fit the data; a distribution of rates wasnot required. The magnitudes of the attenuation and dispersion dependedlinearly on the hydrogen concentration implying that it is the relaxation ofisolated H atoms (the Snoek effect) that is responsible for the mechanicaldamping. The faster local motion of H reported from nuclear magnetic resonancemeasurements for motion within g-site hexagons was not observed in thepresent study. This suggests that the H hopping rate for the local motionremains above the ultrasonic frequencies over the temperature range of study,or perhaps that too few H atoms participate in the local motion.