  The layered sodium cobalt oxide NaxCoO2 is studied by electron diffraction
for a wide range of sodium contents, 0.15<x<0.75. An extensive series of
ordered Na ion-Na vacancy superlattices is found beyond the simple hexagonal
average structure. The most strongly developed superlattice is found for the
composition Na0.5CoO2, which displays Co3+/Co4+ charge ordering at low
temperatures. The structural principle for some of the observed ordering
schemes, particularly near x=0.5, is, surprisingly, the presence of lines of Na
ions and vacancies rather than simply maximized Na-Na separations.
