Investigation of composition-dependent phonon spectra in in-plane heterostructured Cu_(1−x)In_(1+x/3)P₂S₆ by Brillouin light scattering

CuInP₂S₆ is a two-dimensional layered material that is ferrielectric at room temperature (Curie temperature T_C of 315 K). The Curie temperature 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 CIPS and In_4/3P₂S₆ (IPS) domains. Here 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) both at room temperature and across T_C. At room temperature, we observe unique longitudinal acoustic phonon signatures for pure CIPS and IPS, however the CIPS-IPS samples host both the CIPS and IPS phonons, indicative of the separate CIPS and IPS domains. We observe softening of the phonons of CIPS and CIPS-IPS near their respective values of T_C, and there are sharp discontinuities in the phonon frequencies at T_C indicative of the ferrielectric-to-paraelectric phase transition. The temperature and width of the phase transition is highly dependent on the stoichiometry of the sample, with pure CIPS showing the sharpest transition at 40.0 °C (313.0 K) while reduction in Cu leads to broadening and an increased transition temperature. The broadness of the phase transition in the CIPS-IPS samples is caused by the strain exerted on the CIPS domains by the IPS domains. This strain is also evident in the IPS phonons of the CIPS-IPS samples, which have to soften to accommodate for the structural change in the CIPS domains at T_C.