Fast hydrogen motion in the C15 Laves-phase compound ZrCr2H(D)x studied by ultrasound

Abstract

Ultrasonic measurements were made on the C15 Laves-phase material ZrCr2H(D)x over the temperature range 4–295 K for x(H) = 0.09, 0.15 and 0.31 and x(D) = 0.12. Attenuation peaks associated with H (D) motion between g-site hexagons were observed in all of these materials for measurement frequencies of approximately 1.5 MHz. A strong isotope effect was observed for similar concentrations of H and D and interpreted in terms of quantum mechanisms of diffusion. For temperatures below 200 K, the dominant diffusion mechanism appears to be tunnelling transitions between ground states of hydrogen in adjacent interstitial sites. Comparison with earlier ultrasonic and NMR results suggests that such phonon-assisted tunnelling between ground states is the dominant mechanism of long-range diffusion in many C15 Laves-phase compounds for temperatures below about 200 K. The elastic shear moduli of ZrCr2H0.09, ZrCr2H0.15 and ZrCr2D0.12 were also measured. A small shift was observed in the modulus for each material at a temperature corresponding to the relevant peak in the ultrasonic loss, which is consistent with the interpretation that these peaks are due to H (D) relaxation.