Electronic phase transitions and superconductivity in ferroelectric Sn2P2Se6 under pressure

Given the strong electron-phonon coupling observed during both ferroelectric (FE) and superconducting (SC) transitions, there is significant interest in investigating SC arising from FE instability. Sn2P2Se6 has garnered considerable attention due to its unique FE properties. Here, we report on the electronic phase transitions and SC in this compound based on high-pressure electrical transport measurement, optical absorption spectroscopy, and Raman-based structural analysis. Upon compression, the conductivity of Sn2P2Se6 increased monotonously, with an electronic phase transition occurring -5.4 GPa, as evidenced by optical absorption spectroscopy. The insulating state was fully suppressed -15 GPa, coinciding with the onset of SC -15.3 GPa. A zero-resistance state was achieved from 19.4 GPa onwards, with SC exhibiting continuous enhancement under pressure. The SC behavior was further confirmed by the magnetic field effect, and it exhibited a critical temperature (Tc) of 5.4 K at 41.8 GPa and a zero-temperature upper critical field of 6.55 T. Raman spectra supported the structural origin of the electronic transition -5.4 GPa, indicative of a transition from the paraelectric (PE) phase to the incommensurate phase-a distorted PE phase without symmetry change. Furthermore, a possible first-order phase transition was suggested during the semiconductor-metal transition -15 GPa. Comparison with the high-pressure behavior of sister compounds Sn2P2S6 and Pb2P2S6, along with the low-temperature behavior of Sn2P2Se6 at ambient pressure, suggests that SC in Sn2P2Se6 likely emerges in a FE or polar metal state. In this paper, we highlight the versatile physical properties of FEs and motivate further investigation into the correlation between FE instability and SC in the M2P2X6 family.