Experimental data for proton nuclear spin relaxation and diffusion of H in HfTi2Hx were analysed by simultaneously fitting the temperature-dependent relaxation and diffusion data with a common set of parameters. HfTi2Hx has the C15 structure with the H occupying the inequivalent interstitial e and g sites. The fitting of the relaxation data uses a rigorous theory of nuclear spin relaxation between inequivalent sites and makes no assumptions about which types of H jumps were significant for the relaxation. The diffusion data was fitted by developing the theory of diffusion between the inequivalent e and g interstitial sites, which enables the diffusivity to be calculated rigorously as a function of temperature from the H jump rates in the low concentration limit. Monte Carlo simulations were used to estimate the effect of diffusion correlation effects at higher H concentrations. Models for diffusion between inequivalent sites involve a large number of parameters and density functional theory calculations were used to provide constraints on them. Good fits to both the relaxation and diffusion data were obtained for energy parameters that were close to those from the density functional theory calculations. A complete set of jump parameters for H between the interstitial sites was deduced which provides a detailed microscopic description of the diffusion as a function of temperature.

Diffusion of Hydrogen in Cubic Laves Phase HfTi2Hx. B.Bhatia, X.Luo, C.A.Sholl, D.S.Sholl: Journal of Physics - Condensed Matter, 2004, 16[49], 8891-903