STUDY OF HYDROGEN DIFFUSION IN ZR-NI ALLOYS BY TA-181 PERTURBED ANGULAR-CORRELATION SPECTROSCOPY

The diffusion of hydrogen in the crystallized hydride Zr//2NiH//3//. //8 has been studied by perturbed angular correlation (PAC) spectroscopy using **1**8**1Ta as a probe in substitution for zirconium. The temperature dependence of the quadrupole relaxation of the **1**8**1Ta spins between 200 K and 470 K could be properly described by assuming the jump probability of hydrogen atoms to result from the coexistence of two thermally activated processes. The values obtained for the activation energies and pre-exponential frequency factors of both processes suggest that diffusion is dominated by a tunnelling mechanism for T less than 300 K and by a 'classical' over-barrier jump mechanism for T greater than 370 K.