Temperature dependence of motion-induced nuclear spin relaxation in single crystals

The self-diffusion mechanism for NMR relaxation in solids is investigated both theoretically and experimentally. Use is made of an encounter model to calculate the dipolar pair correlation functions involved in NMR relaxation. The relaxation behavior and, in particular, the width and asymmetry of spin-lattice relaxation time ($T_1$ and $T_{1\rho }$) minima are sensitive to the microscopic atomic motion. Theoretical predictions for single crystals and powders obtained using models with correlated and uncorrelated random-walk mechanisms of self-diffusion are applied to diffusion on a fluorite lattice. Detailed experimental results for ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ relaxation in pure and doped barium fluoride are reported and used to verify the theoretical predictions. It is found that the predicted differences in behavior for the expected vacancy and interstitialcy mechanisms are such that the achievable experimental accuracy does not allow them to be distinguished. It is clear, however, that a correlated ionic motion is present and the predictions of the encounter-model theory are confirmed by experiment.