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Toward the Targeted Design of Molecular Ferroelectrics: Modifying Molecular Symmetries and Homochirality
journal contribution
posted on 2019-04-15, 00:00 authored by Han-Yue Zhang, Yuan-Yuan Tang, Ping-Ping Shi, Ren-Gen XiongConspectusAlthough the first ferroelectric discovered
in 1920 is Rochelle
salt, a typical molecular ferroelectric, the front-runners that have
been extensively studied and widely used in diverse applications,
such as memory elements, capacitors, sensors, and actuators, are inorganic
ferroelectrics with excellent electrical, mechanical, and optical
properties. With the increased concerns about the environment, energy,
and cost, molecular ferroelectrics are becoming promising supplements
for inorganic ferroelectrics. The unique advantages of high structural
tunability and homochirality, which are unavailable in their inorganic
counterparts, make molecular systems a good platform for manipulating
ferroelectricity. Remarkably, based on the Neumann’s principle
and the Curie symmetry principle defining the group-to-subgroup relationship,
we have found some outstanding high-temperature molecular ferroelectrics,
like diisopropylammonium bromide (DIPAB) with a large spontaneous
polarization up to 23 μC/cm2 (Fu, D. W.; et al. Science 2013, 339, 425). However, their application potential is severely limited by the
uniaxial nature, leading to major issues in finding proper substrates
for thin-film growth and achieving high thin-film performance. Inspired
by the commercialized inorganic ferroelectrics like Pb(Zr, Ti)O3 (PZT), where the multiaxial nature contributes greatly to
the optimized ferroelectric and piezoelectric performance, developing
high-temperature multiaxial molecular ferroelectrics is an imminent
task.In this Account, we review our recent research progress
on the
targeted design of multiaxial molecular ferroelectrics. We first propose
the “quasi-spherical theory”, a phenomenological theory
based on the Curie symmetry principle, to modify the spherical cations
to a low-symmetric quasi-spherical geometry for acquiring the highly
symmetric paraelectric phase and the polar ferroelectric phase of
multiaxial ferroelectrics simultaneously. Besides the sizes and weights
of the cation and anion, the intermolecular interactions are particularly
crucial for decelerating the molecular rotation at low temperature
to reasonably induce ferroelectricity. It means that the momentums
of the cation and anion should be matched, so we describe the “momentum
matching theory”. In particular, introducing homochirality,
a superiority of molecular materials over the inorganic ones, was
demonstrated as an effective approach to increase the incidence of
ferroelectric crystal structures.Thanks to the striking chemical
variability and structure–property
flexibility of molecular materials, our research efforts outlined
in this Account have led to and will further motivate the richness
and the application exploration of high-temperature, high-performance
multiaxial molecular ferroelectrics, along with the implementation
and perfection of the targeted design strategies.
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Molecular Ferroelectricscrystal structures.ThanksPZTRochelle saltresearch progressdesign strategiesCurie symmetry principlediisopropylammonium bromideresearch effortschemical variabilitymultiaxial natureModifying Molecular SymmetriesTargeted Designmultiaxial ferroelectricsapplication explorationgroup-to-subgroup relationshiplow-symmetric quasi-spherical geometryuniaxial naturethin-film performanceparaelectric phaseDIPABmemory elementsthin-film growthHomochirality ConspectusAlthough
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