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

Abstract

For Sn_2P_2S_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 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 Sn_2P_2S_6 crystal is obtained which is in agreement with observed experimental data.

Keywords

• ferroelectrics
• lattice models in statistical physics
• thermal expansion