Dielectric Phase Transition Induced by Halogen Substitution Based on 1,4-Diazabicyclo[2.2.2]octane-Derivatives

We successfully synthesized (X-EtHDabco)[ZnBr4] (X=H, F, Cl for compound 1, 2, and 3, EtDabco=N-ethyl-1,4-diazoniabicyclo[2.2.2]octane) through precise molecular design. Systematic measurements (powder X-ray diffraction, differential scanning calorimetry measurements, dielectric measurements) were performed on these compounds. It is noteworthy that, no hint of heat anomaly was observed for 1, while both 2 and 3 displayed intriguing isostructural order-disorder phase transition near 230 K. Through the analysis of the crystal structure, the order-disorder transformation of organic ammonium plays a major role in the phase transition, in addition, the displacement of the inorganic framework also contributes to dielectric response.