Ultrasonic attenuation and dispersion due to hydrogen motion in the C15 Laves-phase compound TaV₂H_x

Abstract

Hydrogen in the C15 Laves-phase material TaV2Hx has been studied by means of resonant ultrasound spectroscopy over the temperature range of 15-345 K for a series of hydrogen concentrations (x = 0.00-0.53). Ultrasonic loss peaks and frequency shifts (dispersion) associated with the hydrogen motion were observed, yielding parameters for the hydrogen motion. Hydrogen in these materials is known to occupy the tetrahedral g sites which form a series of interlinked hexagons. The ultrasonic results were associated with H hopping between g-site hexagons. The relaxation rates for x⩽0.18 were best described as a sum of two Arrhenius processes. For x = 0.34 and 0.53 only a single Arrhenius process was needed to fit the results, although the presence of a second Arrhenius mechanism could not be over-ruled. 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 relaxation of isolated H atoms (the Snoek effect) that is responsible for the mechanical damping. The faster local motion of H reported from nuclear magnetic resonance measurements for motion within g-site hexagons was not observed in the present study. This suggests that the H hopping rate for the local motion remains above the ultrasonic frequencies over the temperature range of study, or perhaps that too few H atoms participate in the local motion.

Hydrogen diffusion in the Laves-phase compound TiCr1.78

“…2. The activation energy W and the limit relaxation time 0 deduced from the Arrhe- [7]. In the range 130-180 K also the activation energy for long-range diffusion of H in the C14 compound ZrCr 2 H 0.4 has a value (0.068 eV [6]) close to the present one.…”

“…Inspection of this figure shows that the width of the experimental curve is larger than that of the theoretical one and slightly increases with increasing n H . This is a distinctive feature with respect to the attenuation peak occurring in the C15 TaV 2 H x compound [7], which has been found to be associated with a single-time relaxation. Assuming a Gaussian distribution of times, the strength E of a relaxation can be estimated from the height Q −1 M of the peak according to the following relationship:…”

“…These studies have revealed the existence of a H(D) Snoek-type peak at around 250 K, whose relaxation time, similar to the case of long-range diffusion [6], exhibits two temperature regimes [7], which have been accounted for by the two tunnelling processes between interhexagon states mentioned above.…”

“…Short-range motions of H have been studied by resonant ultrasonic spectroscopy at frequencies of about 1 MHz in the C15 Laves-phases TaV 2 H x (D x ) [7,8] and ZrCr 2 H(D) x [9]. These studies have revealed the existence of a H(D) Snoek-type peak at around 250 K, whose relaxation time, similar to the case of long-range diffusion [6], exhibits two temperature regimes [7], which have been accounted for by the two tunnelling processes between interhexagon states mentioned above.…”

Hydrogen jump motion in Laves-phase hydrides: Two frequency scales

“…The ultrasonic attenuation is also well suited for probing isotope effects on hydrogen jump motion since the coupling between the ultrasound and the hydrogen motion is the same for both H and D. Measurements of the ultrasonic attenuation in TaV 2 H x (D x ) in the frequency range of 1 MHz [35,39,40] have revealed even more dramatic isotope effects than those found in the 51 V NMR experiments. For example, for TaV 2 D 0.17 the ultrasonic attenuation shows a distinct peak near 20 K which can be attributed to localized D motion; however, for TaV 2 H 0.18 the low-T attenuation peak is observed near 1 K, and its amplitude is about eight times lower than that for TaV 2 D 0.17 [35].…”