A STUDY OF HYDROGEN DIFFUSION IN HEXAGONAL YHX SINGLE-CRYSTALS (X-LESS-THAN-OR-EQUAL-TO-0.10) MECHANICAL SPECTROSCOPY

We studied the diffusion of H in hcp YHx single crystals (x less than or equal to 0.1), by mechanical spectroscopy (vibrating-reed technique , 280 to 1750 Hz). In agreement with previous data from polycrystallin e YH2, we find an anelastic relaxation peak at similar to 280 K. Our s ingle crystals show a large anisotropy in the relaxation strength whic h demonstrates that this peak results from a Zener relaxation of H pai rs. Two types of H jumps determine the relaxation kinetics. Comparison with previous quasielastic neutron scattering data identifies the jum ps that cause the relaxation, showing further that their jump rates fo llow, over ten orders of magnitude, an Arrhenius law with an activatio n energy of(0.60 +/- 0.03) eV.