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In many systems, nucleation of a stable solid may occur in the presence ofother (often more than one) metastable phases. These may be polymorphic solidsor even liquid phases. In such cases, nucleation of the solid phase from themelt may be facilitated by the metastable phase because the latter can "wet"the interface between the parent and the daughter phases, even though there maybe no signature of the existence of metastable phase in the thermodynamicproperties of the parent liquid and the stable solid phase. Straightforwardapplication of classical nucleation theory (CNT) is flawed here as itoverestimates the nucleation barrier since surface tension is overestimated (byneglecting the metastable phases of intermediate order) while the thermodynamicfree energy gap between daughter and parent phases remains unchanged. In thiswork we discuss a density functional theory (DFT) based statistical mechanicalapproach to explore and quantify such facilitation. We construct a simple orderparameter dependent free energy surface that we then use in DFT to calculate(i) the order parameter profile, (ii) the overall nucleation free energybarrier and (iii) the surface tension between the parent liquid and themetastable solid and also parent liquid and stable solid phases. The theoryindeed finds that the nucleation free energy barrier can decrease significantlyin the presence of wetting. This approach can provide a microscopic explanationof Ostwald step rule and the well-known phenomenon of "disappearing polymorphs"that depends on temperature and other thermodynamic conditions. Theory revealsa diverse scenario for phase transformation kinetics some of which may beexplored via modern nanoscopic synthetic methods.

Nucleation of a stable solid from melt in the presence of multiple  metastable intermediate phases: Wetting, Ostwald step rule and vanishing  polymorphs