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

TL;DR

This paper models the anharmonic quantum oscillations in Sn$_2$P$_2$S$_6$ ferroelectrics using a three-well potential, successfully explaining negative thermal expansion near the phase transition.

Contribution

It introduces a pseudospin fluctuation model based on a three-well potential to describe anharmonic properties and thermal expansion in ferroelectric materials.

Findings

Negative Grüneisen parameters near phase transition

Agreement with experimental negative thermal expansion

Pseudospin fluctuation spectra match observed behavior

Abstract

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.