A possible generalization of the phenomenological theory of phase transitions in type II ferroelectrics with incommensurate phase

We develop a generalized phenomenological theory of the paraelectric - incommensurate - commensurate phase transitions in type II ferroelectrics. The linear temperature dependence of the sixth-power term coefficient in the thermodynamic potential expansion is proposed. Contrary to the pre-existing theoretical approaches, the developed model eliminates a number of long-standing problems in the theory of type II ferroelectrics and can provide a correct description of the experimental data on the temperature dependence of the properties of type II ferroelectrics in the incommensurate phase.