Prof. You Yumeng’s team from Southeast University discovered the first proton transfer DABCO-based ferroelectric

By 吴婵Viewed 32 2020-04-27

in exploring the low-coercivity field molecular-based ferroelectrics, and thus discovered many excellent ferroelectrics. To date, those compounds based on spherical organic cations highlight the best performance, such as 1,4-diazabicyclo [2.2.2] octane (DABCO) and the quinine ring family. Their low rotational barriers enable them to have low coercive voltage. Including [HDABCO] [ClO4] and [HDABCO] [BF4] previously discovered by the team, the ferroelectricity of these materials is all derived from the order-disorder transition of cations or anions in the molecules.

In this study, the team modified the anion of bi-trimethyl sulfoacid by using the “toluene effect” to design and synthesize (trifluoromethylsulfonyl) ammonium acid 1,4-diazabicyclobis [2.2.2] octane: [HDABCO] [TFSA] and its deuterium: [DDABCO] [TFSA]. The study found that [HDABCO] [TFSA] features a 2/mF2 ferroelectric phase transition. Unlike other DABCO and quinine ring ferroelectrics, the ferroelectricity of [HDABCO] [TFSA] comes from the ordering of protons in hydrogen bonds (Fig. 1 (a)). The potential energy curve satisfies the soft phonon double-well potential model (Fig. 1 (b)). In the high-temperature phase, the protons with larger kinetic energy traverse more frequently between the two wells; thus the protons have almost the same probability in both wells and the crystal is in a disordered phase. In the low-temperature phase, the protons are much more likely to stay in one of the wells, so the crystal enters an ordered phase. In the ordered phase, the protons have to stride over relatively lower energy barriers for transfer; therefore, this ferroelectric mechanism enables [HDABCO] [TFSA] to have a lower coercive voltage (Fig. 1 (c)). However, the working temperature of [HDABCO] [TFSA] is lower than the room temperature, which has severely limited its application in devices. The researchers further obtained the deuterium of [HDABCO] [TFSA] based on the deuterium isotope effect: [DDABCO] [TFSA], and successfully raised the working temperature above the room temperature. Fig. 2 shows the change curve of the two dielectrics with temperature and the phase-transition temperatures are 274.3K and 327.4K respectively. Such great deuterium isotope effects (ΔT = ~ 53K) have further verified that these compounds are hydrogen-bonded ferroelectrics. This article provides a new insight for designing and synthesizing low-coercive voltage ferroelectrics, which will be beneficial to the application of molecular-based ferroelectrics in low-power storage devices.

Fig. 2. [HDABCO] [TFSA] and [DDABCO] [TFSA] dielectric variation curve with temperature

This study was jointly accomplished by doctoral students Song Xianjiang, Zhang Zhixu and Chen Xiaogang, etc.. Southeast University is the sole completion institute and Prof. You Yumeng et al. are the corresponding authors. This achievement was supported by the “Cultivation Fund for the Top Ten Science and Technology Issues of Southeast University”.

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Submitted by: School of Chemistry and Chemical Engineering

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