Ultrasonic evidence for strong isotope effects on the local motion of H(D) inTaV2H(D)x

Abstract

Ultrasonic measurements were performed on the $C15$ Laves-phase compounds ${\mathrm{TaV}}_{2}{\mathrm{H}}_{0.18}$ and ${\mathrm{TaV}}_{2}{\mathrm{D}}_{0.17}$ over the temperature range 3--345 K. Ultrasonic loss peaks were observed in both materials at 250--275 K for measurement frequencies in the range of 1 MHz. These peaks were interpreted as H(D) hopping between interstitial g-site hexagons, the motion responsible for long-range diffusion. This motion exhibited a small isotope effect with the H hopping rate being somewhat faster than that for D. The activation energies and attempt frequencies were in good agreement with earlier measurements by other techniques. In contrast to the similarity of the ${\mathrm{TaV}}_{2}{\mathrm{H}}_{0.18}$ and ${\mathrm{TaV}}_{2}{\mathrm{D}}_{0.17}$ results at high temperatures, the low-temperature behavior of the two materials differed substantially. A weak loss peak was observed in ${\mathrm{TaV}}_{2}{\mathrm{D}}_{0.17}$ at low temperatures. This peak was attributed to the local motion within g-site hexagons previously explored by nuclear magnetic resonance and quasielastic neutron scattering experiments. At a given temperature the rate of this motion is several orders of magnitude faster than that responsible for the high-temperature peak. No evidence of a low-temperature loss peak was found for ${\mathrm{TaV}}_{2}{\mathrm{H}}_{0.18}.$ This strong isotope effect suggests that the local motion is highly dominated by quantum effects. The most likely reason for the absence of an attenuation peak in ${\mathrm{TaV}}_{2}{\mathrm{H}}_{0.18}$ is that the local motion occurs at a rate much higher than that of the ultrasonic frequency (\ensuremath{\sim} 1 MHz) throughout the temperature range explored.