Crystallization mediated by a metastable phase

Almost any solution can undergo a non-trivial crystal growth process given a sufficiently deep quench. For instance, the quench shown in the phase diagram in Fig. 1, from temperature T_A, into the fluid-crystal miscibility gap at temperature T_B, results in a non-equilibrium uniform solution at point i. When a crystal is introduced (or nucleated) at point v, the bulk fluid (at i) cannot locally coexist with it, as its chemical potential and osmotic pressure do not match the crystal's. Instead the fluid near the cystal surface is depleted to concentration ii at which it forms a sharp interface with a very rarefied solution iii which has a concentration gradient down to iv, the concentration that locally coexists with the crystal.

The result is a crystal coated with a macroscopic amount of a rarefied metastable phase that mediates its growth. An example, for simple colloidal particles, is shown in Fig. 3, taken from Ref.[1]. (In fact, Fig.3 was generated at a nearby, unstable concentration within the miscibility gap, but results in the same compound crystal-liquid-liquid interface.).

The intervention of a metastable phase boundary during crystal growth is surprisingly common, for many different substances and types of phase diagram. For instance, it is also thermodynamically required in Fig. 2, for a quench that passes a triple point. More on this topic can be found in the following two journal articles:

[1] The effectsof polydispersity and metastability on crystal growth kinetics
J J Williamson and R M L Evans,
Soft Matter 9, 3600–3612 (2013)
Preprint at arxiv.org/abs/1208.3804

[2] Classificationof ordering kinetics in three-phase systems
R. M. L. Evans, W. C. K. Poon and F. Renth,
Phys. Rev. E 64, 031403 (2001).