of a given (r,t) cluster can be occupied by any one of

In a binary system [5], the points of a given (r,t) cluster can be occupied by any one of the two "atomic species," thus determining 2' distinguishable configurations of (r,t). In principle, a particular (r,t) cluster configuration can be specified by a set of r numbers that take values l and -1 for each of the two components in the system. However, all configurations of (r,t) cluster which are related by the symmetry operations have the same probability (concentration in the crystal). Thus we can label the (r,t) cluster configurations by a single index i where, in general, i= 1 , ... , s with s :<::: 2' . The configuration i of (r,t) will have associated with it a degeneracy factor a; defined as the number of distinguishable configurations which can be generated by the symmetry operations of the (r,t) cluster. The configurational entropy is given by

S = RLXY'La;x; lnx; where R is the universal gaseous constant and x; the concentration of the (r, t) cluster in the i-th configuration. If the largest clusters to be considered, henceforth referred to as the basic clusters, contain n points, the coefficients y" are given by

and

where U is the internal energy of the system including the strain energy, x;' and a;" are the concentration and degeneracy factor of the i-kind of n-point cluster, respectively. The miscibility gap, estimated for the binary alloys with the diamond lattice, is shown on Fig. l. It is presented in the strictly regular (SR) and quasichemical (QC) approximations and in CVM. The degree of clusterization defined through the short-range order equal to the concentrations of different pairs of atoms is shown on Fig. 2. The estimated values show that there are large clusters enriched with atoms of one type in the alloy. Ternary alloys of binary compounds AxBl-xC (InxGa,_xAs, InxGa,_xN alloys) and ABxCl-x (GaSbxAs1_x alloys) are considered as binary alloys with the structure of mixed sublattice. Ternary alloy with zinc-blende structure corresponds to the binary alloy with facecentered-cubic (fcc) lattice, as it was described in [12]. Ternary alloy with wurtzite

structure is considered as binary alloy with hexagonal-close-packed (hcp) lattice in mixed sub lattice [ 12].