NMR and impedance studies of nanocrystalline and amorphous ion conductors: lithium niobate as a model system

Lithium niobate has been chosen as a model system for spectroscopic studies of the influence of different structural forms and preparation routes of an ionic conductor on its ion transport properties. The Li diffusivity in nanocrystalline LiNbO3, prepared either mechanically by high energy ball milling or chemically by a sol-gel route, was studied by means of impedance and solid state 7Li NMR spectroscopy. The Li diffusivity turned out to be strongly correlated with the different grain boundary microstructures of the two nanocrystalline samples and with the degree of disorder introduced during preparation, as seen especially by HRTEM and EXAFS. Although in both samples nanostructuring yields an enhancement of the Li diffusivity compared to that in coarse grained LiNbO3, the Li diffusivity in ball milled LiNbO3 is much higher than in chemically prepared nanocrystalline LiNbO3. The former LiNbO3 sample has a large volume fraction of highly disordered interfacial regions which seem to be responsible for fast Li diffusion and to have a structure very similar to that of the amorphous form. This is in contrast to the chemically prepared sample where these regions have a smaller volume fraction.