Title: THE PHASE TRANSITIONS CHARACTER AND MICROSCOPIC MODELS FOR Sn_2P_2S(Se)_6 FERROELECTRICS
Author(s): Yu.Vysochanskii, A.Drobnich (Institute for Solid State Physics and Chemistry, Uzhgorod National University, 46 Pidgirna Str., 88000 Uzhgorod, Ukraine)

The paper considers an example of concrete objects, i.e. ferroelectrics of Sn$_2$P$_2$S$_6$ family, for which, according to general estimations, a correlation between the shape of a phase diagram (the presence of a Lifshitz point and of a tricritical point) and a crossover ``displacive to order/disorder'' is observed. Simulations by means of microscopic models, which most adequately represent a dipole ordering in these crystals, are analyzed. A dimension dependence of a ferroelectric phase transition temperature for microcrystals of Sn$_2$P$_2$S$_{6}$ compound is considered.

Key words: ferroelectrics, displacive vs. order/disorder crossover, Lifshitz point, incommensurate phase, microscopic models, Monte Carlo simulations
PACS: 64.60.Cn