Formation of virtual ordered states along a phase-decomposition path

The temporary appearance of a virtual ordered phase during a decomposition reaction is discussed in terms of thermodynamics and illustrated by a computer simulation that simultaneously describes ordering, clustering, and coarsening. It is particularly shown that the virtual ordered phase may temporarily appear within a miscibility gap during the decomposition of a homogeneous disordered phase into a mixture of two disordered phases. In such systems, an ordered phase existing along the interfaces between the two equilibrium disordered phases proves to be thermodynamically stable at low temperatures and an interfacial order-disorder phase transition is predicted. The possibility that the ordered phases observed in epitaxial layers of ternary and quaternary III-VI compound semiconductors and SiGe could be virtual phases is discussed.