  Transmission-electron-microscopy investigation reveals the presence of a
superstructure with a doubled period of 2d110 in the NaxCoO2 materials for x
ranging from 0.75 to 1.0. Systematic analyses suggest that this superstructure
in general appears just below a phase transition which could yield an anomalous
kink in resistivity. We herein interpret this superstructure in terms of sodium
ordering occurring at low temperatures. Measurements of magnetic susceptibility
for all NaxCoO2 (0.75<x<1.0) materials show an antiferromagetic state with Neel
temperature TN~4K.
