Journal of Physical Studies 2(1), 65–69 (1998) DOI: https://doi.org/10.30970/jps.02.65 INVESTIGATIONS OF THE PHASE TRANSITIONS IN DIPOLE MODEL OF Sn2P2S6 AND Sn2P2Se6 FERROELECTRICS BY MONTE CARLO METHOD

A. Drobnich, Yu. Vysochanskii
Institute of Solid State Physics and Chemistry, Uzhgorod State University
Pidhirna str., 46, 294000, Uzhhorod, Ukraine

A dipole Ising model of the Sn$_2$P$_2$S(Se)$_6$ ferroelectrics has been developed on the basis of the structural data of these crystals in both ferroelectric and paraelectric phases. The basis of this model is as follows: a dipole which is parallel to the crystalographic axis $\bbox {a}$ and located in the Sn position in the paraelectric phase, corresponds to each of the four atoms of the active Sn-sublattice. To consider the dipole long-range interaction around the dipole an area of simple form is singled out. Within this area the interaction between the dipoles is calculated precisely. The interaction with the other dipoles is considered by introducing a local field. For better accordance with the real experiment an amendment on the short-range interaction with two nearest neighbours has been introduced. As a result of the calculations made by 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 this case the temperatures of the phase transitions are as follows: for Sn$_2$P$_2$S$_6$ model $T_0^* \sim 10.5$; for Sn$_2$P$_2$Se$_6$ model $T_c^* \sim 8.5$, T$_i^* \sim 9.5$ (in the effective temperatures). These data are in satisfactory agreement with the results of the real experiment. Taking into consideration the fact that the two pairs of dipoles are not identical, the change in their moments ratio has been found to cause a reorientation of the modulation vector of the incommensurate phase from its position in a symmetry plane along the axis $\bbox {c}$ to the orthogonal position along the axis $\bbox {b}$.

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