Raman spectroscopy and XRD measurement of the ferrielectric-paraelectric phase transition in layered CuInP₂S₆and CuInP₂S₆- In_4/3P₂S₆heterostructures

CuInP₂S₆  (CIPS) is an emerging layered material that is ferrielectric at room temperature (Curie temperature T_C ~ 315 K). When synthesized with Cu deficiencies, CIPS spontaneously segregates into CuInP₂S_{6 ­}and In_4/3P₂S₆ domains (CIPS-IPS), which form a self-assembled heterostructures within the individual lamellae. This re-structuring raises the Curie temperature and, depending on the Cu concentration, can be up to ~340 K for the highest Cu deficiency. In both CIPS and CIPS-IPS, the loss of polarization through the ferrielectric-paraelectric transition is driven by the diffusion of Cu ions within the lattice. Here we uncover the microscopic origins underpinning the phase transitions in CIPS and CIPS-IPS by performing a temperature-dependent Raman and XRD study. We measured the frequencies and linewidths of various cation and anion phonon modes, and compared them to the extracted atomic positions from the refinement of the XRD data. Our analysis shows that in addition to the Cu ion diffusion, the anion octahedral cages experience significant strains as they deform to accommodate the redistribution of Cu ions upon heating. This process begins around 315 K and ends around 330 K in CIPS-IPS.