Condensed Matter Physics, 1998, vol. 1, No 2, p. 331-338, English

Authors: A.Drobnich, Yu.Vysochanskii (Institute of Solid State Physics and Chemistry, Uzhgorod University, 54 Voloshyn St., UA-294000 Uzhgorod, Ukraine)

We consider a NaNO$_2$ crystal as to the peculiarities of Ising dipole models of ferroelectic crystals. The models with the local field of a macroscopic sample and without this field were considered. A dipole Ising model of Sn$_2$P$_2$S(Se)$_6$ ferroelectics has been developed. As a result of the calculations done by the Monte-Carlo method it has been found that when a certain short-range interaction coefficient in Sn$_2$P$_2$S$_6$ is selected, the intermediate modulated phase disappears in a model with the local field. In this case the temperatures of phase transitions (PT) are as follows: for the Sn$_2$P$_2$S$_6$ model T$_o^*$$\sim $10.5; for the Sn$_2$P$_2$Se$_6$ model T$_c^*$$\sim $8.5, T$_i^*$$\sim $9.5 (in effective temperatures). These data are in satisfactory agreement with the results of the real experiment. The concentration dependences of temperatures of PT in Sn$_2$P$_2$(S$_x$Se$_{1-x}$)$_6$ crystals are simulated and the Lifshitz point is observed. In mixed (Pb$_y$Sn$_{1-y}$)$_2$P$_2$S$_6$ and (Pb$_y$Sn$_{1-y}$)$_2$P$_2$Se$_6$ crystals the increase of Pb concentration causes reduction of temperatures of PT to 0 K. In this case the character of PT remains unchanged. By the method of the diluted Ising lattice the concentration dependences of temperatures of PT in these crystals are simulated.

Key words: ferroelectrics, phase transition, Monte-Carlo procedure, Ising lattice
Comments: Figs. 5, Refs. 16, Tabs. 0.

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