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.

塑性晶体（plastic crystals）是一类独特的化合物类群，常见于具备球状结构的分子中。其高度对称的立方晶体结构赋予该类材料多轴铁电性（multiaxial ferroelectricity），可实现晶体极化轴的三维重排，这是晶体对称性较低的传统分子铁电晶体无法达成的特性。本研究将关注点投向塑性晶体的另一项典型特征——可延展性。我们成功合成了两种新型塑性/铁电离子晶体：四甲基铵四氯合铁(III)酸盐与四甲基铵溴三氯合铁(III)酸盐，并发现通过压制成型该类化合物的粉末样品，可便捷制备自立式透明薄膜。所制得的多晶薄膜在室温下展现出铁电极化翻转特性与较为显著的压电响应。这类功能性薄膜的易得性印证了此类塑性/铁电晶体的实用价值；考虑到可选用的阳离子与阴离子种类极为丰富，这类独特且极具应用潜力的功能材料有望拥有广阔的应用前景。