Ferrielectric-paraelectric phase transitions in layered CuInP2S6 and CuInP2S6

To better control the properties of emerging ferroelectric materials, it is important to understand the microscopic origins of their phase transitions. Here, we focus on ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ (CIPS), which is an emerging layered material that exhibits ferrielectric ordering well above room temperature (Curie temperature ${T}_{\mathrm{C}}$ $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}315\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). When synthesized with Cu deficiencies, CIPS spontaneously segregates into ${\mathrm{CuInP}}_{2}{\mathrm{S}}_{6}$ and ${\mathrm{In}}_{4/3}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ domains (CIPS-IPS), which form self-assembled heterostructures within the individual lamellae. This restructuring raises the Curie temperature up to $\ensuremath{\sim}340\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for the highest Cu deficiency. In both CIPS and CIPS-IPS, the loss of polarization through the ferrielectric-paraelectric transition is driven by the movement of Cu ions within the lattice. Here, we studied the phase transitions in pure CIPS and CIPS-IPS $({\mathrm{Cu}}_{0.4}{\mathrm{In}}_{1.2}{\mathrm{P}}_{2}{\mathrm{S}}_{6})$ through temperature-dependent Raman spectroscopy and x-ray diffraction (XRD). We measured the frequencies and linewidths of various cation and anion phonon modes and compared them with the extracted atomic positions from the refinement of XRD data. Our analysis shows that, in addition to the Cu cation movement, the anion octahedral cages experience significant strains as they deform to accommodate the redistribution of Cu ions upon heating. This results in several discontinuities in peak frequencies and linewidths close to 315 K in CIPS. In the CIPS-IPS heterostructure, this process begins $\ensuremath{\sim}315\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ends $\ensuremath{\sim}330\phantom{\rule{0.16em}{0ex}}\mathrm{K}$.