THE MECHANISM OF HYDROGEN DIFFUSION IN ZIRCONIUM DIHYDRIDES

Hydrogen diffusion in ZrH(x) (1.58 less-than-or-equal-to x less-than-o r-equal-to 1.98) in the temperature range 600 K to 970 K has been meas ured by means of pulsed-field-gradient nuclear magnetic resonance. The activation enthalpy for hydrogen diffusion, H(a), obtained by fitting an Arrhenius expression D = D0 exp(-H(a)/k(B)T) to the diffusivities, increases sharply as x approaches the limiting value of two in good a greement with results deduced from the proton spin-lattice relaxation rate, GAMMA1, measured on the same samples. Hydrogen atoms jump predom inantly between nearest-neighbour tetrahedral sites in ZrH(x). The obs erved concentration dependence of both the effective value of H(a) and the pre-exponential factor D0 suggests, however, that at high hydroge n concentrations and high temperatures another interstitial site is oc cupied in addition. At x --> 2 a small fraction of hydrogen atoms loca ted on an interstitial site other than the tetrahedral site appears to contribute significantly to the diffusivity. The temperature and conc entration dependence of the diffusion data can quantitatively be descr ibed by such a model. The activation enthalpies for all possible jumps in this system with two different kinds of site are shown to be indep endent of the hydrogen concentration x. The corresponding attempt freq uencies v(a) almost-equal-to 10(13) s-1 are compatible with the pictur e of a classical diffusion mechanism.