Pressure-Induced Solid–Solid Phase Transitions in Barocaloric Organic Ionic Plastic Crystals

Barocaloric (BC) materials represent a highly promising family of solid-state refrigerants that offer real potential to replace environmentally damaging gas refrigerants. The rational design of BC materials relies on understanding solid–solid phase transitions (SSTs) under pressure. Organic ionic plastic crystals (OIPCs) emerge as promising BC candidates, but direct characterization of their pressure-dependent phase behavior remains a challenge. Herein, we employ high-pressure broadband dielectric spectroscopy (HP-BDS) as a tool to map the phase diagrams of three OIPCs: 4-ethyl-4-methylmorpholinium bis(fluorosulfonyl)imide ([C 2 mmor][FSI]), tetraethylammonium bis(fluorosulfonyl)imide ([N 2222 ][FSI]), and tetraethylammonium bis(trifluoromethanesulfonyl)imide ([N 2222 ][TFSI]). We demonstrate that the real part of the complex dielectric permittivity (ε′) is uniquely sensitive to SSTs in OIPCs, which are driven by changes in dipolar reorientation. By performing both isobaric cooling/heating and isothermal compression/decompression experiments, we directly determine the pressure sensitivity (dT SS /dP) of the SSTs and observe the transition hysteresis in terms of temperature and pressure, finding both properties to be strongly ion-dependent. Furthermore, hysteretic behavior at high pressure is found to be unique to each OIPC, despite similarities in ion structure. In particular, the pressure hysteresis directly observed for [C 2 mmor][FSI] is lower than that predicted from quasi-direct (isobaric) measurements, indicating the kinetic delay is smaller for a pressure change than a temperature change, which would further improve the already excellent BC metrics of this OIPC.