Structural, electronic, and ferroelectric transitions in van der Waals ferroelectric CuInP2Se6 under high temperature and high pressure.

In this work, the high-temperature and high-pressure ferroelectric, structural, and electrical transport properties for CuInP2Se6 upon compression and decompression under different hydrostatic environments were comprehensively studied via Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy observations. Upon non-hydrostatic pressurization, CuInP2Se6 experienced two successive phase transitions at 5.4 and 14.1 GPa originating from the rapid compression of van der Waals gaps and the local structure variation of Se-P-Se bonds, followed by a metallization at 25.1 GPa. Furthermore, a ∼2.0 GPa pressure hysteresis was detected for the emergence of electronic transformation in CuInP2Se6 under hydrostatic conditions owing to the influence of hydrostaticity. Upon decompression, the phase transition of CuInP2Se6 was demonstrated to be reversible with considerable pressure hysteresis under different hydrostatic environments. In addition, the positive sinusoidal voltage-dependent electrical current relations with the nonlinearity factors of ∼1.0 manifested the Ohmic response of CuInP2Se6 under high pressure conditions. Meantime, the disappearance of the P1 Raman peak and the discontinuities in Raman shifts and full width at half-maximums offered robust evidence on the occurrence of ferroelectric crossover in CuInP2Se6. It is the first time that the phase boundary from the mixed antiferroelectric and ferroelectric (FE) orderings into the FE state of CuInP2Se6 is well established [i.e., TC (K) = 165.5 P (GPa) + 292.1] under the conditions of 298-873 K and 0.4-40.3 GPa. Our findings shed light on the ferroelectricity, crystalline structure, and electrical configuration of CuInP2Se6 under extreme conditions, which is of paramount significance to the fundamental research and potential applications for other metallic thio(seleno)phosphates.