Vibrational Fingerprints of Competing Structural and Polar Phases in CuInP2S6
Layered van der Waals CuInP2S6 crystals exhibit a rich sequence of temperature- and pressure-driven phase transitions, giving rise to multiple structural phases with distinct physical properties. Despite extensive experimental and theoretical efforts, unambiguous identification of these phases remains challenging because of their structural similarities and complex vibrational responses. In this work, we employ first-principles density functional theory calculations to systematically investigate the vibrational properties of the reference paraelectric C2/c and P-31c phases and their corresponding dynamically stable ferroelectric Cc and P31c phases, as well as the antiferroelectric P-1 and P-3 phases. From calculated phonon spectra and symmetry analysis, we determine the Raman- and infrared-active modes of each candidate structure and identify characteristic vibrational fingerprints that enable their phase determination through vibrational spectroscopy. Furthermore, using Porto notation, we establish polarization- and crystal-orientation-dependent Raman selection rules for different backscattering geometries and identify the modes that are expected to emerge or vanish under specific experimental configurations. Thus, our results provide a comprehensive theoretical framework for interpreting Raman and infrared measurements of CuInP2S6 and for reliably identifying its competing structural, ferroelectric, and antiferroelectric phases across temperature- and pressure-driven phase transitions.