ABSTRACT
In this contribution we present a constant time version of the well known REDOR pulse sequence which enables us to determine the second moments in multiple spin systems with strong dipolar couplings. From the resulting dipolar evolution curves, accurate values for the second moments can be obtained without the need to incorporate the full information about the detailed spin geometry of the multiple spin systems into the simulation protocol.
ABSTRACT
We present a detailed study on the exact location and dynamics of Li ions in the garnet-type material Li(5)La(3)Nb(2)O(12) employing advanced solid state NMR strategies. Applying temperature-dependent (7)Li-NMR, (6)Li-MAS-NMR, (6)Li-{(7)Li}-CPMAS-NMR, (6)Li-{(7)Li}-CPMAS-REDOR-NMR as well as 2D-(6)Li-{(7)Li}-CPMAS-Exchange-NMR spectroscopy, we were able to quantify the distribution of the Li cations among the various possible sites within the garnet-type structure and to identify intrinsic details of Li migration. The results indicate a sensitive dependence of the distribution of Li cations among the tetrahedral and octahedral sites on the temperature of the final annealing process. This distribution profoundly affects the mobility of the Li cations within the garnet-type framework structure. Extended Li mobility at ambient temperature is only possible if the majority of the Li cations is accommodated in the octahedral sites, as observed for the sample annealed at 900 degrees C. Octahedrally-coordinated Li cations could be identified as the mobile Li species, whereas the tetrahedral sites seem to act as a trap for the Li cations, rendering the tetrahedrally-coordinated Li cations immobile on the time scale of the NMR experiments.
