  In frameworks of the phenomenological approach we analyze of the phase
diagram of mixed compounds. We obtain space groups of symmetry of the real
structures as result of phase transition from close-packed degenerate
structure. The theory of a sequence of phase transitions with an incommensurate
phase is constructed, and influence of an electronic subsystem and distortions
on phase transitions is investigated. We investigate of dependence of the
temperature interval existence of an incommensurate phase on the electron
concentration and the component deformation tensor in doped semiconductors. The
comparison of theoretical results with optical, dielectric and structural data
is carried out and parameters of thermodynamic potential are determined.
