  We study the properties of a quarter-filled system of electrons on a square
lattice interacting through a local repulsion $U$ and a nearest-neighbour
repulsion $V$ in the limit $V=+\infty$. We identify the ground-state for $U$
large enough and show that domain walls appear below a critical value
$U_c=4t/\pi$. We argue that this corresponds to a metal-insulator transition
due to Mott localization.
