The ferrielectric properties of layered CuInP₂S₆ (CIPS) are highly sensitive to stoichiometry and growth-induced defects. Here we systematically compare crystals grown by chemical vapor transport and the Bridgman–Stockbarger method with Cu- and In-enriched samples, combining compositional analysis, dielectric spectroscopy, uniaxial-pressure measurements, Raman spectroscopy, piezoresponse force microscopy and first-principles calculations. The transition temperature increases systematically from 307 K (Bridgman) to 312 K (Cu-enriched), 317 K (gas transport) and 322 K (In-enriched). In enrichment is consistent with spontaneous CIPS/In₄/₃P₂S₆ phase segregation that generates effective positive chemical pressure, stabilizing the ferrielectric state and producing a strongly biased, pinned domain configuration. Cu enrichment produces the opposite response—lower TC, enhanced dielectric loss and a fragmented domain morphology—characteristic of a negative-pressure-like perturbation of the Cu-ion energy landscape. Landau–Ginzburg–Devonshire analysis and density functional calculations provide a unified microscopic picture based on Cu-ion configurational freedom and interlayer coupling. Growth defects act as an independent source of domain-wall pinning. Together, these results establish stoichiometry and synthesis conditions as practical tools for engineering ferrielectric order in van der Waals CIPS.

The ferrielectric properties of layered CuInP₂S₆ (CIPS) are highly sensitive to stoichiometry and growth-induced defects. Here we systematically compare crystals grown by chemical vapor transport and the Bridgman–Stockbarger method with Cu- and In-enriched samples, combining compositional analysis, dielectric spectroscopy, uniaxial-pressure measurements, Raman spectroscopy, piezoresponse force microscopy and first-principles calculations. The transition temperature increases systematically from 307 K (Bridgman) to 312 K (Cu-enriched), 317 K (gas-transport) and 322 K (In-enriched). In enrichment is consistent with spontaneous CIPS/In₄/₃P₂S₆ phase segregation that generates effective positive chemical pressure, stabilizing the ferrielectric state and producing a strongly biased, pinned domain configuration. Cu enrichment produces the opposite response—lower TC, enhanced dielectric loss and a fragmented domain morphology—characteristic of a negative-pressure-like perturbation of the Cu-ion energy landscape. Landau–Ginzburg–Devonshire analysis and density-functional calculations provide a unified microscopic picture based on Cu-ion configurational freedom and interlayer coupling. Growth defects act as an independent source of domain-wall pinning. Together, these results establish stoichiometry and synthesis conditions as practical tools for engineering ferrielectric order in van der Waals CIPS.