  Crystallographic, magnetic and NMR properties of a NaxCoO2 single crystal
with x~1 are presented. We identify the stoichiometric Na1CoO2 phase, which is
shown to be a non-magnetic insulator, as expected for homogeneous planes of
Co3+ ions with S=0. In addition, we present evidence that, because of slight
average Na deficiency, chemical and electronic phase separation leads to a
segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase.
The importance of phase separation is discussed in the context of magnetic
order for x~0.8 and the occurrence of a metal-insulator transition for x->1.
