Phase behaviour of hard spheres confined between parallel hard plates: manipulation of colloidal crystal structures by confinement

We study the phase behaviour of hard spheres confined between two parallel hard plates using extensive computer simulations. We determine the full equilibrium phase diagram for arbitrary densities and plate separations from one to five hard-sphere diameters using free energy calculations. We find a first-order fluid-solid transition, which corresponds to either capillary freezing or melting depending on the plate separation. The coexisting solid phase consists of crystalline layers with either triangular (Delta) or square ( square) symmetry. Increasing the plate separation, we find a sequence of crystal structures from (...)n Delta -> (n + 1)square -> (n + 1)Delta(...), where n is the number of crystal layers, in agreement with experiments on colloids. At high densities, the transition between square to triangular phases is interrupted by intermediate structures, e. g., prism, buckled, and rhombic phases.