Ultrasonic evidence for strong isotope effects on the local motion of H(D)in TaV2H(D)(x) - art. no. 214302

Ultrasonic measurements were performed on the C15 Laves-phase compounds TaV 2H0.18 and TaV2D0.17 over the temperature range 3-345 K. Ultrasonic loss pe aks were observed in both materials at 250-275 K for measurement frequencie s in the range of 1 MHz. These peaks were interpreted as H(D) hopping betwe en interstitial g-site hexagons, the motion responsible for long-range diff usion. 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 tech niques, In contrast to the similarity of the TaV2H0.18 and TaV2D0.17 result s at high temperatures. the low-temperature behavior of the two materials d iffered substantially. A weak loss peak was observed in TaV2D0.17 at low te mperatures. This peak was attributed to the local motion within g-site hexa gons previously explored by nuclear magnetic resonance and quasielastic neu tron scattering experiments. At a given temperature the rate of this motion is several orders of magnitude faster than that responsible for the high-t emperature peak. No evidence of a low-temperature loss peak was found for T aV2H0.18. This strong isotope effect suggests that the local motion is high ly dominated by quantum effects. The most likely reason for the absence of an attenuation peak in TaV2H0.18 is that the local motion occurs at a rate much higher than that of the ultrasonic frequency (similar to1 MHz) through out the temperature range explored.