Ferroelectricity and Piezoelectricity in Free-Standing Polycrystalline Films of Plastic Crystals

Plastic
crystals represent a unique compound class that is often
encountered in molecules with globular structures. The highly symmetric
cubic crystal structure of plastic crystals endows these materials
with multiaxial ferroelectricity that allows a three-dimensional realignment
of the polarization axes of the crystals, which cannot be achieved
using conventional molecular ferroelectric crystals with low crystal
symmetry. In this work, we focused our attention on malleability as
another characteristic feature of plastic crystals. We have synthesized
the new plastic/ferroelectric ionic crystals tetramethylammonium tetrachloroferrate­(III)
and tetramethylammonium bromotrichloroferrate­(III), and discovered
that free-standing translucent films can be easily prepared by pressing
powdered samples of these compounds. The thus obtained polycrystalline
films exhibit ferroelectric polarization switching and a relatively
large piezoelectric response at room temperature. The ready availability
of functional films demonstrates the practical utility of such plastic/ferroelectric
crystals, and considering the vast variety of possible constituent
cations and anions, a wide range of applications should be expected
for these unique and attractive functional materials.