23Na NMR evidence for charge order and anomalous magnetism in NaxCoO2.

Oriented powder samples of NaxCoO2 are studied by 23Na NMR and SQUID magnetometry. In nominal 0.50<or=x<or=0.70 solid state reacted samples the dominant phase synthesized has a single composition x(0) approximately 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. This contrasts with the Pauli-like magnetism detected for x approximately 0.35, the parent of the superconducting hydrated compound.