  We consider a system of spherical particles interacting by means of a pair
potential equal to a finite constant for interparticle distances smaller than
the sphere diameter and zero outside. The model may be a prototype for the
interaction between micelles in a solvent [C. Marquest and T. A. Witten, J.
Phys. France 50, 1267 (1989)]. The phase diagram of these penetrable spheres is
investigated using a combination of cell- and density functional theory for the
solid phase together with simulations for the fluid phase. The system displays
unusual phase behavior due to the fact that, in the solid, the optimal
configuration is achieved when certain fractions of lattice sites are occupied
by more than one particle, a property that we call `clustering'. We find that
freezing from the fluid is followed, by increasing density, by a cascade of
second-order, clustering transitions in the crystal.
