  The phase diagram of colloidal hard superballs, of which the shape
interpolates between cubes and octahedra via spheres, is determined by
free-energy calculations in Monte Carlo simulations. We discover not only a
stable face-centered cubic (fcc) plastic crystal phase for near-spherical
particles, but also a stable body-centered cubic (bcc) plastic crystal close to
the octahedron shape, and in fact even coexistence of these two plastic
crystals with a substantial density gap. The plastic fcc and bcc crystals are,
however, unstable in the cube and octahedron limit, suggesting that the rounded
corners of superballs play an important role in stablizing the rotator phases.
In addition, we observe a two-step melting phenomenon for hard octahedra, in
which the Minkowski crystal melts into a metastable bcc plastic crystal before
melting into the fluid phase.
