Determining the lithium local environments in the lithium manganates LiZn0.5Mn1.5O4 and Li2MnO3 by analysis of the 6Li MAS NMR spinning sideband manifolds

Li2MnO3 and the spinel LiZn0.5Mn1.5O4 each contain a subset of lithium ions in very, similar octahedral environments, in terms of the Li-O-Mn connectivities (i.e., numbers of nearby manganese ions and Li-O-Mn bond angles). We show, however, that the different geometric arrangements of the Mn ions about the lithium ions lead to very different spinning sideband manifold patterns, providing a simple method for distinguishing between the two different local environments. The dipolar coupling tensors which describe the coupling between the electron and nuclear magnetic moments are determined for the 2b site in Li2MnO3 and the octahedral site in LiZn0.5Mn1.5O4, on the basis of the crystallographic structures, and the spinning sideband patterns are calculated. Two types of axial symmetry, i.e., oblate and prolate symmetry, for the sites in Li2MnO3 and LiZn0.5Mn1.5O4, respectively, are obtained from the simulations, consistent with the experimental spectra. A good fit to the experimental spectrum was obtained, without considering the quadrupolar interaction, since the size of this interaction is small in comparison to the dipolar interaction even at the low field strengths used here (4.7 T). This study demonstrates that the Li-6 MAS NMR spinning sideband manifolds are very sensitive to the arrangements of the first cation coordination sphere of Mn ions around the Li cations, providing a method for obtaining local structural information from these classes of materials.