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.