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

We have compared by ${}^{23}$Na NMR and magnetic susceptibility the magnetic properties of oriented powder samples of Na${}_x$CoO${}_2$ compounds. The NMR spectra are found identical for various samples with nominal $0.50\leq x\leq 0.70,$whichestablishesthatthedominantphasehasasinglecomposition$x\approx 0.70$. 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 CoO${}_2$ 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 $x\approx 0.35$, the parent of the superconducting hydrated compound.