A thermodynamic theory of condensed matter is developed based on the concept of coupled order parameters-density and ordering .These parameters are sufficient to describe the system at least in the vicinity of the vapor-liquid-solid triple point. To describe a wider region, containing other lines of phase transitions ,it is necessary to include coupling with the corresponding order parameters. A procedure is proposed for constructing the effective Hamiltonian of the system-free-energy density. the major thermodynamic functions of the system are calculated, and equation of state allowing three equilibrium phases--vapor, liquid, and solid--is derived. The structure of the interfaces between these phases is described. It is shown,in particular, that, near the triple point ,two types of interface are possible--with and without the liquid plateau corresponding to surface melting. The state with the plateau is energetically more favorable. Movement of the interface with a plateau corresponds to the vapor-liquid-solid mechanism of crystal growth. A technique is proposed for calculating temperature-density and pressure-temperature phase diagrams.
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