  Sodium ion ordering on in situ cleaved NaxCoO2 (x=0.84) surface has been
studied by ultra high vacuum scanning tunneling microscopy (UHV-STM) at room
temperature. Three main phases, with p(3x3), (root 7 x root 7), and (2 root 3 x
2 root 3) hexagonal unit cells and surface Na concentration of 1/3, 3/7, 1/2,
respectively, were identified. One surprising finding is that Na trimers act as
the basic building blocks that order in long range. The stability of Na trimers
is attributed to the increased Na coordination with oxygen as indicated by ab
initio calculations, and possibly at finite temperature by configuration
entropy.
