Plastic crystalline [(C4H9)4N]FeCl4: reversible order–disorder phase transitions and potential for thermal energy storage

Abstract

Alkylammonium halogenoferrates have attracted considerable attention due to their structural versatility and functional potential. Here, we report the growth of [(C₄H₉)₄N]FeCl₄ 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)₄N]FeCl₄ 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.