Electronic phase transitions and superconductivity in ferroelectric S n 2 P 2 S e 6 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. $\mathrm{S}{\mathrm{n}}₂{\mathrm{P}}₂\mathrm{S}{\mathrm{e}}₆$ 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 $\mathrm{S}{\mathrm{n}}₂{\mathrm{P}}₂\mathrm{S}{\mathrm{e}}₆$ increased monotonously, with an electronic phase transition occurring \ensuremath{\sim}5.4 GPa, as evidenced by optical absorption spectroscopy. The insulating state was fully suppressed \ensuremath{\sim}15 GPa, coinciding with the onset of SC \ensuremath{\sim}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 (${T}_c$) 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 \ensuremath{\sim}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 \ensuremath{\sim}15 GPa. Comparison with the high-pressure behavior of sister compounds $\mathrm{S}{\mathrm{n}}₂{\mathrm{P}}₂{\mathrm{S}}₆$ and $\mathrm{P}{\mathrm{b}}₂{\mathrm{P}}₂{\mathrm{S}}₆$, along with the low-temperature behavior of $\mathrm{S}{\mathrm{n}}₂{\mathrm{P}}₂\mathrm{S}{\mathrm{e}}₆$ at ambient pressure, suggests that SC in $\mathrm{S}{\mathrm{n}}₂{\mathrm{P}}₂\mathrm{S}{\mathrm{e}}₆$ 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 ${M}₂{\mathrm{P}}₂{X}₆$ family.