Large Piezoelectric Effect in a Lead-Free Molecular Ferroelectric Thin Film

Piezoelectric materials have been
widely used in various applications,
such as high-voltage sources, actuators, sensors, motors, frequency
standard, vibration reducer, and so on. In the past decades, lead
zirconate titanate (PZT) binary ferroelectric ceramics have dominated
the commercial piezoelectric market due to their excellent properties
near the morphotropic phase boundary (MPB), although they contain
more than 60% toxic lead element. Here, we report a lead-free and
one-composition molecular ferroelectric trimethylbromomethylammonium
tribromomanganese­(II) (TMBM-MnBr3) with a large piezoelectric
coefficient d33 of 112 pC/N along polar
axis, comparable with those of typically one-composition piezoceramics
such as BaTiO3 along polar axis [001] (∼90 pC/N)
and much greater than those of most known molecular ferroelectrics
(almost below 40 pC/N). More significantly, the effective local piezoelectric
coefficient of TMBM-MnBr3 films is comparable to that of
its bulk crystals. In terms of ferroelectric performance, it is the
low coercive voltages, combined with the multiaxial characteristic,
that ensure the feasibility of piezo film applications. Based on these,
along with the common superiorities of molecular ferroelectrics like
light weight, flexibility, low acoustical impedance, easy and environmentally
friendly processing, it will open a new avenue for the exploration
of next-generation piezoelectric devices in industrial and medical
applications.