Pressure-tuned plethora of ferroelectric phases in CuInP₂S₆

Layered chalcogenides like CuInP₂S₆ are room temperature ferroelectrics. Modest compression even increases the electric polarization, raising questions about the origin of this unusual trend and other properties under pressure. In this work, we combine synchrotron-based infrared and Raman scattering spectroscopies, x-ray diffraction, and resistivity measurements with first-principles calculations of the lattice dynamics and energy landscape to unravel the influence of pressure on symmetry, polarization, and metallicity. We uncover a remarkable series of phase transitions across a series of polar space groups: monoclinic Cc  → trigonal P31c (prismatic sulfur) → P31c (octahedral sulfur). True metallicity develops above 63 GPa, significantly higher than in related MPS₃ materials (M = Mn, Co, Fe, Ni), offering a picture of competing states of matter that is different than previously supposed.  Detailed examination of pressure trends within the Cc phase also reveals phonon lifetime changes and streaking of satellite x-ray peaks that correlate with the maximum polarization. We discuss these tendencies in terms of Cu⁺¹ ion migration, phase formation, and the overall energy landscape. Our findings place the high pressure behavior of CuInP₂S₆ on a firm foundation and pave the way for the development of structure-property relations in this family of complex chalcogenides.