Impact of organic cation dynamics on the optoelectronic and charge transport properties of the lead-free hybrid semiconductor tetraethylammonium tetrabromoferrate(iii) (TEAFeBr4).

Hybrid organic-inorganic halometallates have emerged as a compelling class of multifunctional materials, offering a unique platform for tailoring electronic and dielectric responses through structural engineering. In this work, the hybrid organic-inorganic compound tetraethylammonium tetrabromoferrate(iii), [N(C2H5)4][FeBr4] (TEAFeBr4), was synthesized via a solution-evaporation route and characterized by powder X-ray diffraction (PXRD) with Rietveld refinement, UV-Vis-NIR spectroscopy, and broadband impedance spectroscopy (1 Hz-1 MHz, 313-403 K). Rietveld refinement confirms a single-phase hexagonal structure (space group P63 mc, no. 186) with lattice parameters a = 8.3501 Å and c = 13.5815 Å. Tauc analysis reveals a direct optical band gap of 1.88 eV, placing TEAFeBr4 within the visible-light-responsive regime and positioning it favorably among non-toxic, lead-free halide semiconductors. The frequency-dependent AC conductivity follows Jonscher's universal power law, and the monotonic decrease of the frequency exponent s with temperature uniquely identifies the Correlated Barrier Hopping (CBH) mechanism as the dominant charge transport process. Arrhenius analysis of the DC conductivity yields an activation energy (E a) of 0.664 eV, consistent with adiabatic small-polaron hopping mediated by transient Fe3+/Fe2+ redox events on isolated [FeBr4]- tetrahedra. The effective Mott-Davis binding energy W M ≈ 0.51 eV confirms a moderate coulombic trapping potential in this ionic organic-inorganic lattice. These findings establish a coherent structure-transport-optical property landscape for TEAFeBr4, highlighting its potential as a lead-free semiconductor for further exploration in visible-light-responsive optoelectronic applications.