Hydrogen Vacancy Diffusion Parameters in ScH2_x and ScD

The temperature dependence of the nuclear spin-lattice relaxation rate R\ of Sc has been measured in scandium dihydrides and dideuterides. These data are satisfactorily accounted for by two contributions to /?,, R]e due to conduction electrons, and R]Q due to quadrupo le interaction fluctuations resulting f rom vacancy diffusion on the hydrogen (deuter ium) sublattice. Describing these fluctuations by a Lorentzian spectral density and Arrhenius behavior for the vacancy hopping, analysis of the data yields values of the Korringa product A = 77/? ] e and the activation energy £ a and attempt frequency v0 for vacancy-diffusion. In contrast to v0 values based on proton measurements, the Sc results agree satisfactorily with the value based on inelastic neutron scattering measurements of hydrogen optic mode vibration frequencies, v0 = 2 x 10s. We conclude that hydrogen-hydrogen interactions are responsible for the lower v0 values derived from proton R\ measurements.