It was recalled that, in the phase diagram of NaxCoO2, the composition, x = 0.71,

exhibited particular stability and was termed so-called Curie-Weiss metal due to its

anomalous properties. Similarly, the composition with x = 0.84, prepared from the

high-temperature melt, was a special end-point beyond which the system

underwent phase separation. Using synchrotron X-ray diffraction and single

crystals, √[12]a and √[13]a superlattice structures were discovered which were

interpreted in terms of ordering of Na (vacancy) clusters. These results led to a

picture of coexisting local moments and itinerant carriers and formed the first step

towards understanding the many anomalous properties of cobaltates.

Sodium Ion Ordering and Vacancy Cluster Formation in NaxCoO2 (x=0.71 and

0.84) Single Crystals by Synchrotron X-Ray Diffraction. F.C.Chou, M.W.Chu,

G.J.Shu, F.T.Huang, W.W.Pai, H.S.Sheu, P.A.Lee: Physical Review Letters , 2008,

101[12], 127404