Neutron scattering study of ferroelectric Sn2P2S6 under pressure

TL;DR

This neutron scattering study investigates the ferroelectric phase transition in Sn2P2S6 under pressure, revealing a direct transition without intermediate phases and supporting theoretical models of tricritical Lifshitz points.

Contribution

The paper provides experimental evidence contradicting previous phase diagrams, showing a direct ferroelectric-paraelectric transition in Sn2P2S6 under pressure.

Findings

No intermediate incommensurate phase observed.

Polarization fluctuations decrease in the ordered phase.

Order parameter follows a power law with logarithmic corrections.

Abstract

Ferroelectric phase transition in the semiconductor Sn2P2S6 single crystal has been studied by means of neutron scattering in the pressure-temperature range adjacent to the anticipated tricritical Lifshitz point (p=0.18GPa, T=296K). The observations reveal a direct ferroelectric-paraelectric phase transition in the whole investigated pressure range (0.18 - 0.6GPa). These results are in a clear disagreement with phase diagrams assumed in numerous earlier works, according to which a hypothetical intermediate incommensurate phase extends over several or even tens of degrees in the 0.5GPa pressure range. Temperature dependence of the anisotropic quasielastic diffuse scattering suggests that polarization fluctuations present above TC are strongly reduced in the ordered phase. Still, the temperature dependence of the (200) Bragg reflection intensity at p=0.18GPa can be remarkably well modeled assuming the order-parameter amplitude growth according to the power law with logarithmic corrections predicted for a uniaxial ferroelectric transition at the tricritical Lifshitz point.