  We report computational evidence for a new type of disordered phase in
crystals resulting from entropy driven self-assembly of hard convex polyhedra.
The disorder was reflected in the orientations of the anisotropic particles and
not in the positions of the centers of geometry. Despite the lack of order,
particle orientations were not random, and exhibited strong correlations. The
orientational correlations possessed certain special properties that remained
constant across the pressure range where this phase was observed. The
rotational motions of all particles became discrete, characterized by a fixed
number of absolute orientations. The particles were equally partitioned among
them at all time, while maintaining specific values of pairwise orientational
differences. The sense of ``order'' in the disorder was conserved irrespective
of the extent of rotational mobility of the particles. This gave rise to a
discretely mobile phase in the low density solid and a frozen disordered state
at high pressure. The newly identified ``discrete plastic crystal'' phase
transitioned into a freely rotating one, where rotational degrees of freedom
were completely random. The resulting solid-solid transition was purely
orientational in nature without the change in the face centered cubic
crystalline arrangements of the centers of the anisotropic particles. This
finding can be interpreted as the simplest example of correlated disorder in
colloidal crystals.
