Adaptive Ferroic Bi‐Sb Hybrid Iodides for Multi‐Modal Energy Harvesting and Phototronics

ABSTRACT

Organic–inorganic hybrid halides offer a versatile platform for programmable multimodal functionalities, through structural softness and lattice‐charge coupling, despite the challenge of balancing the competing ferroic order, carrier transport, and thermal stability requirements. We present a composition‐driven lattice engineering strategy in a 1D iodide series based on 1,6‐diaminohexane (HDA), comprising orthorhombic non‐centrosymmetric phases of HDABiI ₅ (HBI) and HDASbI ₅ (HSI) together with a lower‐symmetry non‐centrosymmetric monoclinic phase of HDABi _0.5 Sb _0.5 I ₅ (HBSI) identified by its birefringent response, allowing systematic control of symmetry, lattice dynamics, and coupled transport phenomena. Through variable‐temperature crystallography and ¹ H NMR, multi‐perturbation Raman spectroscopy, scanning probe microscopy, photophysical measurements, and electronic structure calculations, we demonstrate a progression from a rigid Bi‐dominated lattice exhibiting strong piezoelectric and ferroelectric coupling to a softer Sb‐rich framework with enhanced anharmonicity, pyroelectricity, and photonic activity, while HBSI combines efficient thermal robustness and phonon transport. Density functional theory calculations show that Bi/Sb substitution tunes the electronic structure from indirect to direct bandgap, modulates oscillator strength, spontaneous polarization, pressure‐dependent electronic properties, thermoelectric and piezoelectric responses, while preserving low iodine‐vacancy formation energies. A trilayer HBI‐HSI‐HBSI architecture couples piezoelectric, triboelectric, pyroelectric, thermoelectric, ferroelectric, and photonic effects, showing 5.1‐fold synergistic enhancement in current generation through interfacial polarization and multimodal coupling.