Tuning the electronic and vibrational properties of Sn2P2Se6 and Pb2P2S6 crystals and their metallization under high pressure
External stimuli enabling either a continuous tuning or an abrupt switching of the basic properties of materials that are utilized in various industrial appliances could significantly extend their range of use. The key characteristics of semiconductors are basically linked to their electronic and optical properties. In this study, we experimentally demonstrated that two kindred wide-band-gap semiconductors, ferroelectric Sn2P2Se6 and paraelectric Pb2P2S6, which are commonly used in optical technologies, have remarkably different and unusual responses in their electronic band structures to applied moderate pressures. The electrical resistance of Sn2P2Se6 smoothly decreased with pressure by about eight orders of magnitude to 10 GPa, thereby suggesting a progressive shrinkage in its band gap; whereas, the resistance of Pb2P2S6 was only insignificantly lowered with pressure to 20 GPa. By means of Raman spectroscopy, we observed several distinct crossovers in the compression behaviour of both crystals and attributed them to phase transitions. These Raman studies provided evidence for the metallization of Sn2P2Se6 at 29 GPa and Pb2P2S6 at 49 GPa. We inferred that, namely, the metal cations in these crystals control the pressure responses of their band structures and proposed that the other M2P2X6 compounds, those already known and those not yet reported (e.g., with M = Cu, In, Fe, Co, Mn, Cr, Ca, Sr, and Mg), could also exhibit the diverse and non-trivial pressure responses of their electronic band structures. Thus, our study has revealed the significant potential for the stress-related technologies of this poorly-studied class of materials, thereby stimulating both the synthesis and investigation of new members.