Title: Description of lattice anharmonicity observed in ferroelectrics with unusual three-well local potential
Author(s):

	R. Yevych (Institute for Solid State Physics and Chemistry, Uzhgorod National University, 54 Voloshyn St., 88000 Uzhgorod, Ukraine) ,
	Yu. Vysochanskii (Institute for Solid State Physics and Chemistry, Uzhgorod National University, 54 Voloshyn St., 88000 Uzhgorod, Ukraine)

For Sn₂P₂S₆ 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 Grüneisen 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 Sn₂P₂S₆ crystal is obtained which is in agreement with observed experimental data.

Key words: ferroelectrics, lattice models in statistical physics, thermal expansion
PACS: 05.50.+q, 05.70.Fh, 63.70.+h, 77.84.-s, 65.60.+a

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