Phenomenological theory of phase diagrams involving incommensurate phase

A phenomenological approach to the construction of phase diagrams with mult iple phase transitions (including incommensurate ones) is presented. The di agrams containing the Lifshitz point are considered. The theoretical temper ature-pressure and temperature-electric field phase diagrams for deuterated theourea and betaine calcium chloride dihydrate are found to be consistent with the diagrams obtained in experiments.