Lifshitz point on the state diagram of ferroelectrics

An analysis is made of the shape of the phase diagrams of substances in the neighborhood of the Lifshitz point, which separates the second-order phase transitions going from the initial phase to a commensurate and to an incommensurate phase. The conclusions of the thermodynamic theory in the mean field approximation and in a treatment including the effects of the interaction of welldeveloped fluctuations are compared. The possible complication of the phase diagram by a crossing of the line of Lifshitz points with a line of tricritical points and the possible existence of a tricritical Lifshitz point are discussed. The changes in the character of the critical behavior in the neighborhood of the Lifshitz point are discussed in reference to the results of experimental studies of the transformation of anomalies of the thermodynamic characteristics, principally for the proper uniaxial ferroelectrics Sn$_2$P$_2$(Se$_x$S$_{1-x}$)$_6$.