Investigating Strain between Phase-Segregated Domains in Cu-Deficient CuInP₂S₆

CuInP₂S₆ (CIPS) is an emerging layered ferroelectric material with a Curie temperature above room temperature. When synthesized with Cu deficiencies (i.e., Cu_1–xIn_1+x/3P₂S₆), the material segregates into CIPS and In_4/3P₂S₆ (IPS) self-assembled heterostructures within the same single crystal. This segregation results in significant in-plane and out-of-plane strains between the CIPS and IPS phases as the volume fraction of the CIPS (IPS) domains shrinks (grows) with a decreasing Cu fraction. Here, we synthesized CIPS with varying amounts of Cu (x = 0, 0.2, 0.3, 0.4, 0.5, 0.7, 0.8, and 1) and measured the strains between the CIPS and IPS phases through the evolution of the respective Raman, infrared, and optical reflectance spectra. Density functional theory calculations revealed vibrational modes that are unique to the CIPS and IPS phases, which can be used to distinguish between the two phases through two-dimensional Raman mapping. A comparison of the composition-dependent frequencies and intensities of the CIPS and IPS Raman peaks showed interesting trends with a decreasing CIPS phase fraction (i.e., Cu/In ratio). Our data reveal red- and blue-shifted Raman and infrared peak frequencies that we correlate to lattice strains arising from the segregation of the material into CIPS and IPS chemical domains. The strain is highest for a Cu/In ratio of 0.33 (Cu_0.4In_1.2P₂S₆), which we attribute to the equal and opposite strains that the CIPS and IPS phases exert on each other. In addition, the bandgaps we extracted from the optical reflectance spectra revealed a decrease in values, with Cu_0.4In_1.2P₂S₆ having the lowest value of ∼2.3 eV.