  We study the liquid-solid transition in a collection of interacting particles
moving through a dissipative medium under the action of a constant, spatially
uniform external force, e.g. a charge-stabilized suspension in a fluidized bed
or a flux-point lattice moving through a thin, current-carrying slab of type II
superconductor. The mobility of a given region in these systems is in general a
function of the local concentration. We show that the structure factor peak is
suppressed in an anisotropic manner as a result of this effect, resulting in a
shift of the crystal-liquid phase boundary towards the crystal side. A
nonequilibrium phase diagram is presented.
