  The influence of electrostatic interactions on the ordering of sodium ions in
Na$_x$CoO$_2$ is studied theoretically through Monte-Carlo simulations. For
large $x$ small di- or tri-vacancy clusters are stable with respect to isolated
Na vacancies. At commensurate fillings these small clusters order in triangular
superstructures. These results agree with recent electron diffraction data at
$x=1/2$ and 3/4. We have performed neutron La\"ue diffraction experiments at
higher $x$, which confirm the predictions of this simple model. The
consequences on the properties of the electronic charges in the Co layers are
discussed.
