THEORETICAL INVESTIGATION OF THE THERMODYNAMIC STABILITY OF NANO-SCALE SYSTEMS .1. PERIODIC LAYER-STRUCTURES

The thermodynamic stability of periodic layer-structures is analyzed t heoretically using equilibrium statistical mechanics. While the system itself is not in complete thermodynamic equilibrium, it can be stable under appropriate constraint conditions and its stability can be dete rmined by minimizing the free energy in the constrained state. A model binary FCC system with a miscibility gap is treated using the pair ap proximation of the Cluster Variation Method. A symmetric system with a n overall average composition 50 atom % A and 50 atom % B is considere d. It is shown that the equilibrium compositions of two phases in a pe riodic layer-structure depend strongly on the periodicity when the com position wavelength is decreased down to a few nanometers. The result reveals that the mutual solubilities of two materials increase signifi cantly as the layer-thickness decreases. In an extreme case, they may become totally miscible.