Switchable Dielectric Phase Transition Induced by Ordering of Twisting Motion in 1,4-Diazabicyclo[2.2.2]octane Chlorodifluoroacetate

A novel molecular electric ordered compound [(Hdabco)+ClF2CCOO–] (complex 1, dabco = 1,4-diazabicyclo[2.2.2]­octane) has been discovered as a switchable dielectric material, in which the N–H···O hydrogen bonds connect the anions and cations together to form a dimer structure. It undergoes a reversible second order solid state phase transition at 165 K (Tc), which was confirmed by thermal analyses including differential scanning calorimetry (DSC) and specific heat (Cp) and dielectric measurements. Variable temperature single crystal X-ray diffraction analyses revealed its order–disorder transformation of twisting motion characteristic behaviors for the phase transition. Owing to the ordering of twisting motions of the chlorodifluoroacetate anions, the structure of 1 demonstrates a phase transition from high crystallographic symmetry with space group of Pnma to the low state with space group of P21/c; that is, an Aizu notion of mmmF2/m occurs. Emphatically, the dielectric constant of 1 displays a distinctive step-like anomaly; namely, it possesses a high dielectric state in the room temperature phase (RTP) and a low state in the low temperature phase (LTP). Such distinctive dielectric performances disclose that 1 might be considered as a potential switchable dielectric material. We believe that all the findings would afford a useful strategy in exploring new electric ordering materials.