The freezing transition of hard spheres confined between two parallel hard plates is studied for different plate separations interpolating between two and three spatial dimensions. Using Monte Carlo simulations and free volume theory, the full phase diagram is obtained exhibiting solid-to-solid transitions between buckled, layered, and rhombic crystals. While the fluid-solid transition is always strongly first order, both strong and very weak transitions occur between different crystalline structures. All predicted transitions should be experimentally observable in confined colloids.

• Received 26 February 1996

DOI:https://doi.org/10.1103/PhysRevLett.76.4552