23Na NMR Evidence for Charge Order and Anomalous Magnetism in NaxCoO2

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We have compared by ²³Na NMR and magnetic susceptibility the magnetic properties of oriented powder samples of NaₓCoO₂ compounds. The NMR spectra are found identical for various samples with nominal 0.50≤ x≤ 0.70, which establishes that the dominant phase has a single composition x≈ 0.70. Three Na sites are identified from their single valued quadrupole effects and magnetic shifts which implies a very definite order of the Na⁺ ions and of the Co charges in the CoO₂ planes. The local susceptibility of the magnetic cobalt sites displays a large enhancement below 100 K with respect to the usually found high T Curie-Weiss law. A very distinct much weaker Pauli like magnetism is detected for x≈ 0.35, the parent of the superconducting hydrated compound.