Small-parameter series for the surface tension in a lattice model

For a lattice model a new method of description is proposed. It is based on the use of the particle number density and the vacancy number density simultaneously and makes it possible to use the well-known diagram technique of the theory of a classical liquid for two-phase states. In the low-temperature region an expansion for the surface tension with respect to a small parameter is obtained. The critical temperature and critical exponent of the surface tension are determined by the standard methods from the coefficients of the low-temperature expansion. The problem of an adsorption layer is considered.