Stability and first-order phase transitions

An approach is formulated to the problem of first-order phase transitions. It states that these transitions are caused by processes of stability loss at the microscopic level. A formal attribute of the loss of stability is the vanishing of the solution to the system of equations for distribution functions. Calculations for a model equation confirm this hypothesis. It is shown that the phase transition on the crystallization curve is caused by the instability of the system with respect to small perturbations of the distribution function at the first (the deepest) minimum of the thermal potential. The loss of stability on the liquid-evaporation curve is caused by the decay of the asymptotic behavior of the distribution function, while on the vapor-condensation curve it is again caused by the instability of the first minimum of the thermal potential. Criteria of stability loss are established. The results are generalized to the case of phase transitions in crystalline materials.