Record‐Low Compressibility in [N(C ₂ H ₅ ) ₃ CH ₃ ]FeCl ₄ Expands Phase Engineering Horizons in Hybrid Molecular Ferroelectrics

Advances in hybrid organic-inorganic ferroelectrics (HOIFs) plateau; high-pressure studies offer powerful approaches to deepen understanding of structure-property relationships and enable rational phase engineering of lead-free HOIFs as viable inorganic alternatives. To date, high-pressure studies focus on metal-organic frameworks (MOFs) and lead-based HOIFs, constrained by toxicity, stability, and limited pressure ranges. Structure-stability-compressibility correlations prove essential for understanding lead-free HOIFs under ultrawide pressure conditions, yet remain scarcely explored. We present high-pressure studies of the lead-free HOIF [N(C₂H₅)₃CH₃]FeCl₄ (EMAFC), which stays stable and mechanochromic up to 51.5 GPa with a reversible P6₃mc-to-P1 phase transition at 0.75 GPa. Pressure-triggered synchrotron powder X-ray diffraction, Raman, UV-vis, dielectric, and second-harmonic-generation switching data reveal coupling between structural changes and bandgap modulation. With a bulk modulus of K₀ = 42.0(5) GPa, EMAFC sets a record for compressibility among HOIFs. Beyond low compressibility, the tunability of EMAFC manifests through reversible retention of the SHG "on" state up to 9.5 GPa and transition to "off" by 20.0 GPa. Our results show that halide choice and lattice dynamics govern the compressibility and functional properties of HOIFs. EMAFC exhibits the lowest compressibility reported, establishing key structure-compressibility relationships and enabling advanced phase and property control in hybrid ferroelectrics.