Charge Ordering in Na0.70CoO2 below 300 K

Abstract We report on measurements of the magnetic susceptibility χ ( T ) and 23 Na nuclear magnetic resonance (NMR) on polycrystalline samples of Na0.7CoO2. The χ ( T ) data suggest that for T > 75 K , the Co ions adopt an effective configuration of Co 3.4 + . The temperature evolution of 2D exchange 23 Na -NMR spectra and of the 23 Na spin-lattice relaxation time T 1 reveal significant anomalies between 230 and 300 K. 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 300 K and being completed at 230 K. This phenomenom may involve a partial charge ordering within the Co subsystem.