2-DIMENSIONAL PHASE-TRANSITIONS ASSOCIATED WITH SURFACE MISCIBILITY GAPS

This article provides a thermodynamic treatment of two-dimensional phase transitions associated with a surface miscibility gap, in the regular solution approximation. The treatment is an extension of an approach due to Helms. In the context of surface segregation, this type of phase change manifests itself as an abrupt change in surface composition at a transition temperature which depends, among others, on the bulk composition of the solution. It is also shown that the surface compositions corresponding to the end points of the abrupt change are identical with the limits of miscibility of a hypothetical two-dimensional closed system. In addition, the article summarizes the results of previous Monte Carlo simulation, and reports on some previous as well as some new experimental measurements of this transition in Cu-Ag alloys.