Rational Design of Ceramic-Like Molecular Ferroelectric by Quasi-Spherical Theory

Abstract

Molecular ferroelectrics are attracting tremendous interest because of their easy and environmentally friendly processing, light weight, low acoustical impedance, and mechanical flexibility, which are viable alternatives or supplements to conventional ceramic ferroelectrics. However, reports of ceramic-like molecular ferroelectrics that can be applied in the polycrystalline form have been scarce. Here, according to the "quasi-spherical theory", we successfully synthesized a ceramic-like molecular ferroelectric with an m3mFmm2 type phase transition at 357 K, 1,5-diazabicyclo[3.2.1]octonium tetrafluoroborate ([3.2.1-dabco]BF 4 ), which can show excellent ferroelectric performance in the polycrystalline thin-film form at room temperature. On the basis of the reported molecular ferroelectric [2.2.2-dabco]BF 4 (2.2.2-dabco = 1,4-diazabicyclo[2.2.2]octonium) with an Aizu notation of 4/mmmFmm2 and two polar axes, we changed the [2.2.2-dabco] + cation to the [3.2.1-dabco] + cation to reduce the molecular symmetry and keep the quasi-spherical shape simultaneously, making the number of polar axes up to six. Moreover, the spontaneous polarization P s gets successfully increased from 4.9 μC cm −2 in [2.2.2-dabco]BF 4 to 5.5 μC cm −2 in [3.2.1-dabco]BF 4 . This precise molecular design strategy offers an efficient pathway to design ceramic-like molecular ferroelectrics.

An organic plastic ferroelectric with high Curie point

“…Then a series of perrhenate-based plastic/ferroelectrics were reported. 24,25 The globular organic molecules composed of tetramethylammonium or quinuclidinium cations have extensively been employed to construct the plastic ferroelectrics, such as tetraethylammonium perchlorate, 11 [3.2.1-dabco]BF 4 , 26 [Hdabco]ClO 4 , 27 [Hdabco]BF 4 , 28 ammonia borane derivatives, 21,29 etc. In addition to the above-mentioned molecular ionic crystals, single-component organic 2-(hydroxymethyl)-2-nitro-1,3-propanediol with low molecular mass exhibits plastic ferroelectricity and 48 crystallographically equivalent polarization directions, which is the most among all ferroelectrics.…”

Bistable State of Protons for Low-Voltage Memories

“…In the recent decade, great progress has been made in molecular ferroelectrics . Among them, the DABCO (1,4-diazabicyclo­[2.2.2]­octane) and quinuclidine families that generally contain hydrogen-bonds in structures have been extensively studied due to their superior ferroelectric performances (Table S1) such as superfast polarization switching of 10 kHz in [HDABCO]­[ClO 4 ], low coercive voltage of 12 V in [HDABCO]­[BF 4 ], large P s value of up to 22 μC/cm 2 in the metal-free perovskite MDABCO-NH 4 I 3 (MDABCO is N -methyl- N ′-diazabicyclo­[2.2.2]­octonium), duodecuple polarization directions in quinuclidinium periodate, and the ceramic-like molecular ferroelectric [3.2.1-DABCO]­[BF 4 ] (3.2.1-DABCO is 1,5-diazabicyclo­[3.2.1]­octonium) . However, the ferroelectric mechanisms in all these compounds are attributed to the order–disorder of cations or anions rather than the proton motion.…”

H/F substitution for advanced molecular ferroelectrics

“…Ferroelectrochemistry, conceptually employing chemical ideas and approaches to design molecular ferroelectrics as well as optimize their performance, has been widely accepted, including the methodologies of reducing molecular symmetry, introducing homochirality, and H/F substitution [8,15,[17][18][19][20][21]. Among them, H/F substitution, which refers to the substitution of an F atom for an H atom, plays an important role in the chemical design of high-performance molecular ferroelectrics as well as their ferroelectric and piezoelectric performance optimization (Figure 1).…”