  We report simulations on the homogeneous liquid-fcc nucleation of charged
colloids for both low and high contact energy values. As a precursor for
crystal formation, we observe increased local order at the position where the
crystal will form, but no correlations with the local density. Thus, the
nucleation is driven by order fluctuations rather than density fluctuations.
Our results also show that the transition involves two stages in both cases,
first a transition liquid-bcc, followed by a bcc-hcp/fcc transition. Both
transitions have to overcome free energy barriers, so that a spherical bcc-like
cluster is formed first, in which the final fcc-like structure is nucleated
mainly at the surface of the crystallite. This means that the bcc-fcc phase
transition is a heterogeneous nucleation, even though we start from a
homogeneous bulk liquid. The height of the bcc-hcp/fcc free energy barrier
strongly depends on the contact energies of the colloids. For low contact
energy this barrier is low, so that the bcc-hcp/fcc transition happens
spontaneously. For the higher contact energy, the second barrier is too high to
be crossed spontaneously by the colloidal system. However, it was possible to
ratchet the system over the second barrier and to transform the bcc nuclei into
the stable hcp/fcc phase. The transitions are dominated by the first liquid-bcc
transition and can be described by Classical Nucleation Theory using an
effective surface tension.
