Compositional design rules for tuning functionalities in CuInP 2 X 6 (X = S, Se) van der Waals semiconductor ferroelectrics

(CIPS), offer unique opportunities for lightweight, scalable, low-power nanoscale electronic devices. However, the limited pool of functional 2D-vdW ferroics highlights the need for clear design principles that can be used to guide experiments. Here, we use first-principles density functional theory (DFT) to study how isovalent atomistic substitution at In and P sites modifies structure, polarization, and electronic properties in CIPSe and CIPS. When substituting In with Sb and Bi, and P with As and Sb, we reveal how ionic radius mismatch, electronegativity differences, and stereochemical lone pair activity shape the ferroelectric and semiconducting response in both rigid (S-based) and soft (Se-based) 2D lattices. In CIPSe, Bi doping at In sites widens the band gap to ∼1.07 eV without reducing polarization or switching performance, provided the Bi atoms are placed in a balanced zigzag arrangement that limits local strain. In CIPS, polarization values drop more noticeably than in CIPSe, and dopant location has a stronger effect on switching behavior. P-site substitution leads to stronger distortions in CIPSe, where larger dopants destabilize the P-P dimer network and in many instances, we predict it to be a metal. However, with As doping in CIPS, polarization remains stable, and the band gap decreases without major structural disruption. Our results establish structure-property design rules based on dopant size and location, host lattice stiffness, and chemical compatibility, offering a route to engineer new 2D ferroelectrics in which polarization and band gap can be tuned together through specific compositional changes.