Investigating pressure-driven phase transitions in layered ferroelectric CuInP₂S₆

Recent developments in 2D van der Waals (vdW) ferroelectric materials have been driven by their intralayer and interlayer degrees of freedom, leading to unconventional properties. Among these materials, CuInP₂S₆​ (CIPS) stands out due to its negative piezoelectricity, polarization switching, ionic conductivity, and quadruple-well ferroelectricity. These unique properties stem from the displacive nature of Cu¹⁺ ions within the CIPS crystal structure. This study explores the influence of hydrostatic pressure on CIPS, employing first-principles density-functional theory (DFT) methods to investigate structural phase transitions in the pressure range 0-18 GPa. We results reveal a structural phase transitions near 6.8 GPa, from a non-centrosymmetric monoclinic Cc phase (spg#9) to a non-centrosymmetric trigonal P31c phase (#159). At higher pressures, a centrosymmetric phase, P-31c (#163), may coexist with the P31c phase due to their comparable enthalpy. The trigonal P-31c phase is identified as the paraelectric counterpart to the ferroelectric P31c phase. Furthermore, our DFT calculations predict that with increasing pressure, the electronic bandgap decreases. Overall, this study provides theoretical insights into the high-pressure phase transitions of vdW-layered CIPS ferroelectrics.