Structural and Thermal Properties of All-Organic Dielectric Material MDABCO(NH ₄ )(NO ₃ ) ₃

Organic ferroelectrics are attracting attention as flexible, lightweight, and environmentally benign alternatives to inorganic perovskite ferroelectrics. We synthesized and characterized a metal-free dielectric material, MDABCO (N-methyl-N′-diazabicyclo[2.2.2]octonium)(NH4)X3, by substituting the iodide ion (X = I) with nitrate (NO3). Heat capacity measurements conducted with a relaxation calorimeter (PPMS) indicated a phase transition (α → β) occurring near 273 K. Detailed differential scanning calorimetry (DSC) analysis revealed that the β-phase is metastable and that a more stable phase (γ-phase) exists. Single-crystal X-ray diffraction showed that, unlike the iodide analogue, the β-phase adopts an antiferroelectric structure, attributed to hydrogen-bonding interactions that govern MDABCO molecular orientation. Dielectric measurements further identified a relaxation process in the α-phase associated with the rotational dynamics of MDABCO cations, underscoring the critical role of hydrogen bonding in modulating structure and dielectric behavior in all-organic perovskites.