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

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posted on 31.08.2016, 00:00 authored by Zi-Shuo Yao, Kaoru Yamamoto, Hong-Ling Cai, Kazuyuki Takahashi, Osamu Sato
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.