97-25U

Analytical method for the description at a phase transition in a three- dimensional spin system is suggesred. The method is based on the use of non-Gaussian distribution for fluctuations of spin moment densities and enables to represent the critical behaviour near a phase transition point. Explicit expressions for fundamental thermodynamical functions of an Ising-like system with exponentially dropping interparticle potential are obtained. Their dependence on the temperature is researched as well as that on microscopic parameters of a model (lattice constant, parameters of an interaction potential).