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

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]­BF4), 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]­BF4 (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 Ps gets successfully
increased from 4.9 μC cm–2 in [2.2.2-dabco]­BF4 to 5.5 μC cm–2 in [3.2.1-dabco]­BF4. This precise molecular design strategy offers an efficient
pathway to design ceramic-like molecular ferroelectrics.