A PULSED-FIELD-GRADIENT NMR-STUDY OF HYDROGEN DIFFUSION IN THE LAVES-PHASE COMPOUNDS ZRCR2HX

Hydrogen diffusivities D in C14-ZrCr2H0.4 and C15-ZrCr2Hx (0.2 less-than-or-equal-to x less-than-or-equal-to 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.