Investigation of Composition-Dependent Phonon Spectra in In-Plane Heterostructured Cu_(1–x)In_(1+x/3)P₂S₆ by Brillouin Light Scattering

CuInP₂S₆ (CIPS) is a two-dimensional van der Waals material that is ferrielectric at room temperature (<i>T</i>_C of 315 K). This <i>T</i>_C can be raised up to 335 K by synthesizing CIPS with Cu deficiencies (Cu_1-xIn_1+x/3P₂S₆, CIPS-IPS), which causes the material to self-segregate into separate CIPS and In_4/3P₂S₆ (IPS) domains. Using Brillouin light scattering microscopy, we examine the phonon spectra of CIPS, IPS, and CIPS-IPS (x = 0.2, 0.3, 0.5, 0.6, 0.8) at room temperature and across <i>T</i>_C. We observe unique longitudinal acoustic (LA) phonon signatures for pure CIPS and IPS; however, the CIPS-IPS samples host LA phonons corresponding to both CIPS and IPS, due to the formation of the in-plane heterostructures. These phonons soften in CIPS and CIPS-IPS near their respective values of <i>T</i>_C, and there are sharp discontinuities in the phonon frequencies at <i>T</i>_C, indicative of the ferrielectric-to-paraelectric phase transition. The temperature and width of this transition is dependent on composition, with pure CIPS showing the sharpest transition at 40.0 °C, while reduction in Cu leads to broadening and an increased <i>T</i>_C, caused by the strain exerted on the CIPS domains by the IPS domains. This strain also manifests in IPS domains, as the phonons soften to accommodate the structural change in the CIPS domains.