Sodium ion dynamics in the nonstoichiometric layer-type oxide Na0.67Ni0.33Ti0.67O2 studied by 23Na NMR

Sodium ion dynamics in the nonstoichiometric layer-type oxide Na0.67Ni0.33Ti0.67O2 powder was studied by 23Na NMR techniques. Even though 23Na is a quadrupole nucleus, the spectra are dominantly governed by magnetic susceptibility anisotropy of paramagnetism rather than quadrupole interaction. The static resonance line becomes narrower at higher temperature manifesting motional narrowing. The line shape is not axially symmetric below 270 K but becomes symmetric at higher temperature. This indicates that the Na+ motion is faster than 10−8 s and isotropic in two-dimension above 270 K resulting in averaging out the anisotropy in the ab-planes. Two different prismatically coordinated Na sites of the sample are reported by XRD but the fast and isotropic Na motion in the ab-planes averages out the locational difference in the NMR spectra. At lower temperature, the Na+ motion slows down and becomes anisotropic in the ab-planes enough to see the anisotropy in the plane.