  Nucleation in systems with a metastable liquid-gas critical point is the
prototypical example of a two-step nucleation process, in which the appearance
of the critical nucleus is preceded by the formation of a liquid-like density
fluctuation. So far, the majority of studies on colloidal and protein
crystallization have focused on one-component systems, and we are lacking a
clear description of two-step nucleation processes in multicomponent systems,
where critical fluctuations involve coupled density and concentrations
inhomogeneities. Here, we examine the nucleation process of a binary mixture of
patchy particles designed to nucleate into a diamond lattice. By combining
Gibbs-ensemble simulations and direct nucleation simulations over a wide range
of thermodynamic conditions, we are able to pin down the role of the liquid-gas
metastable phase diagram on the nucleation process. In particular, we show that
the strongest enhancement of crystallization occurs at an azeotropic point with
the same stoichiometric composition of the crystal.
