  We report a numerical calculation of the elastic constants of the fcc and hcp
crystal phases of monodisperse hard-sphere colloids. Surprisingly, some of
these elastic constants are very different (up to 20%), even though the free
energy, pressure and bulk compressibility of the two crystal structures are
very nearly equal. As a consequence, a moderate deformation of a hard-sphere
crystal may make the hcp phase more stable than the fcc phase. This finding has
implications for the design of patterned templates to grow colloidal hcp
crystals. We also find that, below close packing, there is a small, but
significant, difference between the distances between hexagonal layers (c/a
ratios) of fcc and hcp crystals.
