Critical Behavior of Sn2P2S6 and Sn2P2(Se0.28S0.72)6 Crystals under High Hydrostatic Pressures

TL;DR

This study investigates how high hydrostatic pressures influence the phase transition behaviors of Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals, revealing critical points and transition character changes.

Contribution

It provides the first experimental evidence of a tricritical point in Sn2P2S6 and details the pressure-induced transition character changes in both crystals.

Findings

Sn2P2S6 exhibits a tricritical point at (4.3 kbar, 259 K).

Pressure transforms the phase transition from second-order to first-order.

Increasing pressure causes phase transition splitting in Sn2P2(Se0.28S0.72)6.

Abstract

Basing on the temperature dependences of optical birefringence for Sn2P2S6 and Sn2P2(Se0.28S0.72)6 crystals subjected to hydrostatic pressures, we prove unambiguously that Sn2P2S6 reveals a tricritical point on its (p,T)-phase diagram with the coordinates (p,T) = (4.3 kbar, 259 K), so that the second-order phase transition transforms into the first-order one whenever the pressure increases above 4.3 kbar. We also find that increasing hydrostatic pressure applied to Sn2P2(Se0.28S0.72)6 leads to the change in the phase transition character from tricritical to first-order. Further increase in the pressure up to ~ 2.5 kbar imposes splitting of the first-order paraelectric-to-feroelectric phase transition into two phase transitions, a second-order paraelectric-to-incommensurate one and a first-order incommensurate-to-ferroelectric transition.