  We present a simple unifying model for crystallization and melting
temperatures by showing that homogeneous nucleation and phase transformations
driven by thickening of pre-existing surface layers are limiting conditions of
the more general heterogeneous nucleation case. Furthermore, to a first
approximation all these processes can be described by an extended classical
nucleation theory. The model can also be applied to phase transition
temperatures in confined volumes, provided reliable values for the interfacial
tensions within the systems are determinable. The expected melting and
crystallization temperature for any transformation pathway can then be
predicted.
