DIFFUSION OF HYDROGEN AND DEUTERIUM IN TITANIUM HYDRIDE AND DEUTERIDE

In the hydrogen (H) concentration range 1.6 less than or equal to x le ss than or equal to 1.8, TiHx exists in a delta phase where Ti forms a fee lattice in which the Ii occupies tetrahedral interstitial sites. We studied the diffusion of the H in this phase by mechanical spectros copy (temperatures from 5 to 370 K, vibrating reed technique, frequenc ies from 169 to 1290 Hz). We find a large II-induced Zener-relaxation peak between 240 and 270K (for the present frequencies) from which we determined the jump rate of the H interstitials with the help of a rec ent theoretical model for the Zener relaxation. The jump rate follows an Arrhenius relation with an activation enthalpy of (0.49 +/- 0.04) e V. An extrapolation of the present results agrees well with previous h igh-temperature data from neutron spectroscopy and nuclear magnetic re sonance. For concentrations x > 1.8, a continuous (ferroelastic) phase transition takes place below similar to 320 K, changing the fee Ti la ttice into fct (epsilon phase). The phase transition is accompanied by a drastic softening of Young's modulus which allows a very precise de termination of the transition temperature.