Nuclear-magnetic-resonance methods for identifying and studying diffusion of different spin species in heteronuclear systems

We introduce a new dipolar relaxation time ${T_{1D}}^{\prime }$ which characterizes the spin-lattice relaxation of secular dipolar interactions in the presence of a large rf field. Measurements of ${T_{1D}}^{\prime }$ are particularly useful for studying slow atomic motions in multispin systems, since such measurements enable us to vary the contribution of a particular spin species's motion relative to the contribution of the other spin species's, thus enabling us to identify the diffusing species. We also show that the anisotropy of the conventional dipolar relaxation time $T_{1D}$ can differ enormously for diffusion of different spin species in a multispin system and, accordingly, can be used to identify the dominant diffusing species. Finally, we show that the high-rf-field rotating-frame relaxation time $T_{1\rho }$, measured as a function of rf frequency, also enables us to identify the diffusing species. We demonstrated experimentally the validity of these techniques by measurements of potassium vacancy diffusion in a KF: ${\mathrm{Ca}}^{++}$ single crystal and measurements of fluorine diffusion in AgF powder.