  We report a numerical study of the rate of crystal nucleation in a binary
suspension of oppositely charged colloids. Two different crystal structures
compete in the thermodynamic conditions under study. We find that the crystal
phase that nucleates is metastable and, more surprisingly, its nucleation free
energy barrier is not the lowest one. This implies that, during nucleation,
there is insufficient time for sub-critical nuclei to relax to their lowest
free-energy structure. Such behavior is in direct contradiction with the common
assumption that the phase that crystallizes most readily is the one with the
lowest free-energy barrier for nucleation. The phenomenon that we describe
should be relevant for crystallization experiments where competing solid
structures are not connected by an easy transformation.
