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

Fast Hydrogen Motion in the C15 Laves-Phase Compound ZrCr2H(D)x Studied by Ultrasound. J.E.Atteberry, R.G.Leisure, K.Foster, A.V.Skripov: Journal of Physics - Condensed Matter, 2004, 16[46], 8421-8