Competitive Halogen Bond in the Molecular Ferroelectric with Large Piezoelectric Response

Molecular piezoelectrics
are attracting tremendous interest because
of their easy processing, light weight, low acoustical impedance,
and mechanical flexibility. However, reports of molecular piezoelectrics
with a piezoelectric coefficient d33 comparable
to piezoceramics such as barium titanate (BTO, 90–190 pC/N)
have been scarce. Here, we present a uniaxial molecular ferroelectric,
trimethylchloromethylammonium tribromocadmium­(II) (TMCM-CdBr3), in which the halogen bonding might be a possible critical point
for the stabilization of one-dimensional (1D) {CdBr3}− chain and further reservation of its ferroelectricity
in such organic–inorganic hybrid crystalline systems. It has
a large d33 of 139 pC/N, 1 order of magnitude
higher than those of most classically uniaxial ferroelectrics such
as LiNbO3 (6–16 pC/N) and Rochelle salt (∼7
pC/N), and comparable with those of multiaxial ferroelectrics such
as BTO and trimethylbromomethylammonium tribromomanganese­(II) (112
pC/N). Moreover, the simple single-crystal growth and easy-to-find
polar axis enable it to hold a great potential for applying in the
single-crystal form. In light of the strong, specific, and directional
halogen-bonding interactions, this work provides possibilities to
explore new classes of molecular piezoelectrics and contribute to
further developments.