Comments on the region of applicability of the landau theory for structural phase transitions

The Landau theory of phase transitions1 is known to ignore (in its main approximation) long-wave fluctuations of the order parameter. In this aspect this theory is equivalent to the theory of the mean (molecular, self-consistent) field. True, starting from the Landau theory one can calculate contributions of the long-wave fluctuations as well but only if the contributions are small enough. This is not the case in the immediate vicinity of a second-order phase transition. At present the fluctuation theory of phase transitions, or more specifically, the scaling1,2 or renormalization group3-5 theory, is widely used for this region