Hydrogen diffusion in titanium and zirconium hydrides

Titanium and zirconium form hydrides TiH(D)x and ZrH(D)x with hydrogen concentrations between x≈1.5 (Ti) or 1.6 (Zr) and x=2.0 (room temperature). In these hydrides, the metal atoms form a fcc (δ-phase) or a fct (ϵ-phase) lattice in which the hydrogen atoms occupy tetrahedral interstitial sites. All the tetrahedral sites are occupied at the maximum concentration x=2.0. The hydrogen atoms in titanium and zirconium represent a model system for a concentrated lattice gas. We studied hydrogen and deuterium diffusion in titanium and zirconium hydride by mechanical spectroscopy (vibrating reed technique, temperatures from 5 to 400 K, frequencies between 160 and 1300 Hz). The experiments yielded large hydrogen-induced Zener-relaxation peaks between 240 and 340 K from which the jump rates of the hydrogen interstitials were determined with the help of a theoretical model for the Zener relaxation in a concentrated lattice gas. The jump rates follow an Arrhenius relation with activation energies of 0.49±0.04 eV (H in titanium and zirconium), 0.60±0.04 eV (D in titanium) and 0.51±0.04 eV (D in zirconium). Extrapolation of the present jump rates to higher temperatures allows a comparison with diffusion data from previous high-temperature nuclear magnetic resonance and neutron-scattering measurements. The comparison yields a perfect agreement for titanium hydride, and a poor one for zirconium hydride. The poor agreement for zirconium hydride indicates differences in the microscopic diffusion mechanism between low and high temperatures, which do not exist in the case of titanium hydride.