Above Room Temperature Organic Ferroelectrics: Diprotonated 1,4-Diazabicyclo[2.2.2]octane Shifts between Two 2‑Chlorobenzoates

A pure
organic single crystal, [H2dabco]·[2CB]2 ([H2dabco]2+ = diprotonated 1,4-diaza­bicyclo­[2.2.2]­octane,
2CB– = 2-chloro­benzoate), which undergoes
a ferroelectric-to-paraelectric phase transition above room temperature
(∼323 K upon heating), was prepared and characterized. This
ferroelectric crystal possesses a distinctive supramolecular architecture
composed of discrete H-bonded trimeric units (two 2CB– anions bridged by one [H2dabco]2+ cation through
N–H···O hydrogen bond interactions). In the
paraelectric phase, the [H2dabco]2+ cation is
rotationally disordered and lies at the symmetric center of the trimer.
Upon cooling, it is frozen in an ordered state and deviates toward
a 2CB– anion at one end along the H-bond. The collective
displacement of the cations leads to a polarization of the single
crystal along the crystallographic c axis, which
is confirmed by the temperature dependence of the second harmonic
generation and spontaneous polarization. A significant increase in
the phase transition temperature of the deuterated analogue suggests
that the proton plays an important role in the ferroelectric phase
transition.