Superstructure, sodium ordering and antiferromagnetism in NaxCoO2 (0.75<x<1.0)

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.