Diffusion of hydrogen in cubic Laves phase HfTi₂Hx

Experimental data for proton nuclear spin relaxation and diffusion of H in HfTi₂Hx are 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 are significant for the relaxation. The diffusion data is 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 are 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 (DFT) calculations are used to provide constraints on them. Good fits to both the relaxation and diffusion data are obtained for energy parameters that are close to those from the DFT calculations. A complete set of jump parameters for H between the interstitial sites is deduced which provides a detailed microscopic description of the diffusion as a function of temperature.