  The preferential formation of one solid over the other, as it precipitates
out from the melt at specific temperatures, is often explained by invoking a
competition between thermodynamic and kinetic control. A quantitative theory,
however, could not be developed because of the lack of accurate values of
relevant surface tension terms. Motivated by the observations that wetting of
the interface between two stable phases by multiple metastable phases of
intermediate order can reduce the surface tension significantly
(Kirkpatrick-Thirumalai-Wolynes (KTW), Phys. Rev. A 1989, 40 (2), 1045; Santra
et al., J. Phys. Chem. B, 2013, 117, 13154 ), we develop a statistical
mechanical approach based on a Landau-Ginzburg type free energy functional to
calculate the surface tension between two stable phases in the presence of N
number of metastable phases. Simple model calculations are performed that show
the surface tension between two coexisting stable phases (melt and the stable
crystalline forms) depends significantly on the number, relative depths and
arrangements of the free energy minima of the metastable phases, in addition to
the size of the nucleus. We provide an explanation of the quickly disappearing
polymorphs (QDPMs) that often melt back to the liquid (or, the sol) phase. It
is shown that our model systems could describe some aspects of solid formation
in real polymorphic systems, like phosphates and zeolites.
