Introduction

The bases for consideration of fluctuations of various kinds in otherwise homogeneous phases have been developed in the previous chapters. In particular, heterophase fluctuations involve interfaces between different phases, as well as changes in the free energy of a system. Homophase or, more accurately, homostructural fluctuations are fluctuations in composition and/or density or strain. At equilibrium, there are distributions of both such fluctuations, called embryos. Nucleation of product phases from a metastable parent phase may occur by spontaneous growth of critically sized embryos belonging to an equilibrium distribution of embryos when the product phase is thermodynamically a more stable phase.

In the homophase system we can define a locus of spinodal compositions. A change of temperature from above to below the spinodal leads to instability of the host phase and the onset of composition fluctuations. In this regime equilibrium distributions of embryos cannot exist in the metastable host. Composition fluctuations develop to produce periodic waves of composition, one wavelength of which grows in amplitude to result in spinodal decomposition of the parent solution. The locus of spinodal compositions lies within the miscibility gap. Analogous to the spontaneous reaction along a composition path yielding spinodal decomposition there is one along a strain path that can decompose a host phase into periodic strain waves at constant composition. The onset of phonon softening with change in temperature makes the latter decomposition a realistic possibility. These homophase fluctuations will be considered after first considering heterophase fluctuations, which occur within a stable phase or...