Critical behaviour of Sn₂P₂S₆ and Sn₂P₂(Se_0.28S_0.72)₆ crystals under high hydrostatic pressures

Abstract

Basing on the temperature dependences of optical birefringence for Sn₂P₂S₆ and Sn₂P₂(Se_0.28S_0.72)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(Se_0.28S_0.72)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-ferroelectric phase transition into two phase transitions, a second-order paraelectric-to-incommensurate one and a first-order incommensurate-to-ferroelectric transition.

Keywords:

• ferroelectrics
• critical exponents
• phase diagram
• tricritical point
• triple point

Acknowledgement

We thank Prof. Yu. Vysochanskii for useful discussions and supplying the crystals.

Additional information

Funding

This work was supported by the Ministry of Education and Science of Ukraine under the [Project # 0111U010236].