Description of lattice anharmonicity observed in ferroelectrics with unusual three-well local potential

For Sn$_2$P$_2$S$_6$ family of uniaxial ferroelectrics, the anharmonic quantum oscillators model, based on local three-well potential for spontaneous polarization fluctuations, has been used for a description of anharmonic properties, namely thermal expansion. The calculated pseudospin fluctuations spectra in ferroelectric phase demonstrate negative Gr\"{u}neisen parameters for excitations that satisfy Curie-Weiss-like temperature and pressure dependencies in the vicinity of second order phase transition. Thermal expansion coefficient is calculated by evaluation of pseudospins entropy baric dependence. Negative thermal expansion in the ferroelectric phase of Sn$_2$P$_2$S$_6$ crystal is obtained which is in agreement with observed experimental data.Comment: 6 pages, 5 figure

Nonlinear dynamics of ferroelectrics with three-well local potential

For Sn$_2$P$_2$S$_6$ ferroelectrics, the appearance of spontaneous polarization is related to stereoactivity of tin cations and valence fluctuations of phosphorous cations. Here, the continuous phase transition and its behavior under pressure is determined by thee-well local potential and can be described in an anharmonic quantum oscillator model. For such a model, the spectrum of pseudospin fluctuations at different temperatures and pressures has been calculated and compared with the data of Raman spectroscopy. It was revealed that the ferroelectric lattice instability is related to several low energy optic modes.Comment: 8 pages, 8 figure

The phase transitions character and microscopic models for Sn₂P₂S(Se)₆ ferroelectrics

The paper considers an example of concrete objects, i.e. ferroelectrics of Sn₂P₂S₆ 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 Sn₂P₂S₆ compound is considered.В статті досліджена кореляція між особливостями фазової діаграми (наявність на діаграмі точки Ліфшиця та трикритичної точки) та кросовером характеру фазових переходів від типу зміщення до типу лад-безлад на прикладі конкретних об’єктів, а саме сегнетоелектриків родини Sn₂P₂S₆. Аналізуються результати комп’ютерного моделювання з використанням мікроскопічних моделей, що найбільш адекватно відображають дипольне впорядкування в цих кристалах. Розглядається ефект розмірної залежності температури сег-нетоелектричного фазового переходу для мікрокристалів сполуки Sn₂P₂S₆

Highly efficient acoustooptic diffraction in Sn2P2S6 crystals

We have studied the acoustooptic (AO) diffraction in Sn2P2S6 crystals and found that they manifest high values of AO figure of merit. The above crystals may therefore be used as highly efficient materials in different AO applications.Comment: 5 pages, 1 figur