Interesting experimental properties of the incommensurate phase in ferroelectric ${\mathrm{Sn}}_2{\mathrm{P}}_2{\mathrm{Se}}_6$ are explained in the framework of Landau-type phenomenological theory. The unique procedure of multilevel genetic-algorithm optimization is implemented to find parameters of the thermodynamic potential. It is shown that the temperature dependence of the polarization wave number in the incommensurate phase can be correctly described simultaneously with the anomalous heat capacity only when the nonlinear properties of the order parameter distribution are taken into account.

• Received 27 April 2000

DOI:https://doi.org/10.1103/PhysRevB.64.064103