Abstract
A prototype phase diagram for the family of the ordered superstructures in fcc lattices is calculated in the tetrahedron-octahedron approximation of the cluster variation method. The tetrahedron-octahedron cluster combination allows for first- and second-neighbor interactions, both essential for the superstructures to be ground states. Calculations are carried out for positive (antiferromagnetic) nearest-neighbor pair interactions and for a ratio of second- to first-neighbor interaction energies of 0.25. Salient features of the resulting phase diagram are tricritical points in the fcc to and the fcc to transitions, and the presence of a bicritical point at the junction of the and critical lines with the to first-order transition line. The phase of the family is found to be stable at relatively low temperatures and in a very narrow composition range. Stability of the disordered state is investigated in space and the ordering spinodal is determined.
- Received 19 January 1979
DOI:https://doi.org/10.1103/PhysRevB.21.216
©1980 American Physical Society