Abstract
Hydrogen diffusivities D in C14-ZrCr2H0.4 and C15-ZrCr2Hx (0.2<or=*<or=0.5) have been measured by means of pulsed-field-gradient nuclear magnetic resonance over the temperature range 130-430 K. In all the samples studied the temperature dependence of D follows an Arrhenius law above 200 K but shows marked deviations from Arrhenius behaviour below about 180 K. This suggests that different diffusion mechanisms dominate at high and low temperatures. The jump length of H atoms estimated by comparing D(T) and the proton spin-lattice relaxation rates appears to be greater than the closest distance between tetrahedral (Zr2Cr2) sites occupied by hydrogen in these compounds. A diffusion model that considers jumps between next-nearest-neighbour (Zr2Cr2) sites via metastable (ZrCr3) sites is discussed.
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