Alkylammonium halogenoferrates have attracted considerable attention due to their structural versatility and functional potential. Here, we report the growth of (C4H9)4NFeCl4 single crystals via slow evaporation under controlled conditions. X-ray powder diffraction (XRPD) confirmed the formation of a pure orthorhombic phase. Differential scanning calorimetry (DSC) revealed two reversible phase transitions between room temperature and 440 K, with a plastic crystalline state stabilized above 400 K. Temperature-controlled XRPD indicated a symmetry increase consistent with a phase transition, while Raman spectroscopy up to 423 K evidenced the reorientation of the tetrabutylammonium cation and displacement of the FeCl4- anion as the driving mechanisms. The large enthalpy changes and reversible order-disorder dynamics highlight the potential of (C4H9)4NFeCl4 for thermal energy storage applications. Electrical characterization, performed through complex impedance measurements in the temperature range 303-393 K and frequency range 200-1 × 107 rad s-1, confirmed the presence of two-phase transitions. The frequency-dependent AC conductivity follows Jonscher's universal power law and is governed by two hopping mechanisms: correlated barrier hopping (CBH) and non-overlapping small polaron tunneling (NSPT). Dielectric measurements further revealed relaxation processes consistent with the dynamic behavior of the cation-anion sublattice. These findings establish this hybrid halogenoferrate as a model system for probing phase-transition mechanisms in plastic crystalline materials and for designing next-generation thermal energy storage devices.
Brahim et al. (Thu,) studied this question.