Hydrogen diffusion in titanium and zirconium hydrides

Titanium and zirconium form hydrides TiH(D)(x) and ZrH(D)(x) with hydrogen concentrations between x approximate to 1.5 (Ti) or 1.6 (Zr) and x=2.0 (roo m temperature). In these hydrides, the metal atoms form a fcc (epsilon-phas e) or a fct (epsilon-phase) lattice in which the hydrogen atoms occupy tetr ahedral interstitial sites. All the tetrahedral sites are occupied at the m aximum concentration x=2.0. The hydrogen atoms in titanium and zirconium re present a model system for a concentrated lattice gas. We studied hydrogen and deuterium diffusion in titanium and zirconium hydride by mechanical spe ctroscopy (vibrating reed technique, temperatures from 5 to 400 K, frequenc ies between 160 and 1300 Hz). The experiments yielded large hydrogen-induce d 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 rate s 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 tem peratures allows a comparison with diffusion data from previous high-temper ature nuclear magnetic resonance and neutron-scattering measurements. The c omparison yields a perfect agreement for titanium hydride, and a poor one f or zirconium hydride. The poor agreement for zirconium hydride indicates di fferences in the microscopic diffusion mechanism between low and high tempe ratures, which do not exist in the case of titanium hydride. (C) 2000 Elsev ier Science S.A. All rights reserved.