Study of phase transitions and the incommensurate phase in ACBX4 crystals by the Monte Carlo method

The statistical model of successive phase transitions in the family of crystals described by the general formula ACBX4 has been studied. It is assumed that tetrahedral BX4 groups in the disordered tetragonal phase can be located in four equally probable equilibrium positions. Five representatives with different sequences of phase transitions in this family of crystals are considered. The effective equilibrium constants of interactions between tetrahedral groups are calculated within the framework of the electrostatic model. The corresponding phase diagrams and the thermodynamic characteristics of the phase transitions are studied by the Monte Carlo method. The experimental data provided the establishment of the sequence of phase transitions in Rb2ZnCl4 and K2SeO4 crystals with the formation of an intermediate incommensurate phase. The character of the structure modulation in this phase is established. The calculated thermodynamic characteristics of the phase transitions agree quite well with the experimental data for all the crystals under consideration.