Hydrogen diffusivity and the anomalous transverse spin relaxation in C14-type HfCr2Hx

The hydrogen diffusivity D in C14-type HfCr2H0.4 has been measured by means of the pulsed-field-gradient nuclear magnetic resonance over the temperature range 141-428 K. The temperature dependence of D in this range is found to deviate strongly from the Arrhenius behavior. This is consistent with a quantum nature of H diffusion in HfCr2Hx. Measurements of the proton spin-spin relaxation rate R2 in HfCr2Hx (x = 0.4, 0.6 and 0.74) have revealed the anomalous R2 peak near 230 K for all the samples studied. The experimental results indicate that this peak originates from slow fluctuations of the proton resonance frequency. Such fluctuations are expected for protons diffusing through a spatially inhomogeneous medium on the length scale of about 500 Å. The origin of the inhomogeneity suggested by the R2 data in HfCr2Hx remains to be elucidated.