Raman spectroscopy of ferroelectric Sn2P2S6 under high pressure up to 40 GPa: Phase transitions and metallization

We report results of a Raman spectroscopy study of non-oxide ferroelectric tin-hypothiodiphosphate (Sn2P2S6) at ambient temperature under application of high pressure up to 40 GPa. Pressure evolution of the Raman spectra revealed several different compression regimes that are most likely related to phase transitions, with boundaries near <1, ∼5–7, ∼16–19, ∼26–29, and ∼39 GPa. Above 39 GPa, the Raman signal disappeared. Pronounced softening in some phonon modes prior to these crossovers also suggested that these features are related to structural phase transitions. In optical absorption spectroscopy, we confirmed the wide semiconductor band gap Eg of Sn2P2S6, and in particular, found indirect gap of Eg = 2.26 eV and direct one of Eg = 2.42 eV. In visual examinations, we observed that the sample color gradually changed with pressure from yellow to orange, red, and then to opaque. Eventually, at the maximal pressure achieved in our study, the sample demonstrated a metallic luster. Hence, the metallization at 39 GPa was proposed. At decompression, these characteristic features shifted to lower pressures. Thus, besides the known ferroelectric Pn and paraelectric P21/n phases, Sn2P2S6 can adopt several more structures in the pressure range up to 40 GPa, with electronic properties ranging from wide-gap semiconductor to metal. We found that high-pressure behavior of electronic band structure in Sn2P2S6 is, to significant extent, analogous to a case of elemental sulfur.