Diffusion of hydrogen and deuterium in titanium hydride and deuteride

In the hydrogen (H) concentration range 1.6 &le x &le 1.8, TiHx exists in a δ phase where Ti forms a fcc lattice in which the H occupies tetrahedral interstitial sites. We studied the diffusion of the H in this phase by mechanical spectroscopy (temperatures from 5 to 370 K, vibrating reed technique, frequencies from 169 to 1 290 Hz). We find a large H-induced Zener-relaxation peak between 240 and 270 K (for the present frequencies) from which we determined the jump rate of the H interstitials with the help of a recent theoretical model for the Zener relaxation. The jump rate follows an Arrhenius relation with an activation enthalpy of (0.49 ± 0.04) eV. An extrapolation of the present results agrees well with previous high-temperature data from neutron spectroscopy and nuclear magnetic resonance. For concentrations x > 1.8, a continuous (ferroelastic) phase transition takes place below approx.320 K, changing the fcc Ti lattice into fct (Ε phase). The phase transition is accompanied by a drastic softening of Young's modulus which allows a very precise determination of the transition temperature.