Unconventional charge ordering in Na0.70CoO2 below 300K?

We report on measurements of the magnetic susceptibility χ(T) and 23Na nuclear magnetic resonance (NMR) on polycrystalline samples of Na0.7CoO2. The χ(T) data suggest that for T>75K, the Co ions adopt an effective configuration of Co3.4+. The temperature evolution of 2D exchange 23Na-NMR spectra and of the 23Na spin-lattice relaxation time T1 reveal significant anomalies between 230 and 300K. These features provide evidence for a crossover phenomenon or a phase transition and suggest that, upon decreasing temperature, a dramatic change occurs in the Co 3d-electron spin dynamics, starting just below 300K and being completed at 230K. This phenomenom may involve a partial charge ordering within the Co subsystem