Molecular Ferroelectric Material Realizes Two-Step Wide-Temperature Second-Harmonic Generation Response

In this study, a molecular ferroelectric material [Me3NCMe3]FeCl4 (1) was synthesized via steric modification of ammonium cations. Structural characterization reveals two reversible phase transitions at 273/259 and 343/330 K, accompanied by distinct symmetry changes. The crystal structure evolves from the centrosymmetric space group Pbcm at low temperature, to the polar Pmc21 at intermediate temperature, and finally to the cubic Pm3̅m paraelectric phase at high temperature. The intermediate polar phase demonstrates ferroelectricity and second-harmonic generation (SHG) response. A reversible “off–on–off” SHG behavior and dielectric anomalies were observed between 270 and 346 K. These features are consistent with two distinct phase transitions within this temperature range.