Theoretical temperature-pressure phase diagram for {N(CH3)(4)}(2)ZnCl4

A phenomenological approach for construction theoretical phase diagrams wit h multiple phase transitions including incommensurate transitions is develo ped for the case when a special triple point, a Lifshitz-type point, is ass umed to be present in the experimental diagram For (N(CH3)(4))(2)ZnCl4 crys tal family. The theoretical temperature-pressure phase diagram for (N(CH3)( 4))(2)ZnCl4 is plotted and is found to be in agreement with the experimenta l phase diagram. The approximations and assumptions made in the constructio n of the diagrams are discussed.