Flexible organic materials possessing useful electrical properties, such as ferroelectricity, are of crucial importance in the engineering of electronic devices. Up until now, however, only ferroelectric polymers have intrinsically met this flexibility requirement, leaving small-molecule organic ferroelectrics with room for improvement. Since both flexibility and ferroelectricity are rare properties on their own, combining them in one crystalline organic material is challenging. Herein, we report that trisubstituted haloimidazoles not only display ferroelectricity and piezoelectricity—the properties that originate from their non-centrosymmetric crystal lattice—but also lend their crystalline mechanical properties to fine-tuning in a controllable manner by disrupting the weak halogen bonds between the molecules. This element of control makes it possible to deliver another unique and highly desirable property, namely crystal flexibility. Moreover, the electrical properties are maintained in the flexible crystals.
A room-temperature ferroelectric, diisopropylammonium bromide (DPB), with dielectric constant e # 12 000 and a clear hysteresis loop at T c = 425 K is reported. At 417 K DPB undergoes the irreversible phase transition from nonpolar orthorhombic P2 1 2 1 2 1 to the ferroelectric monoclinic phase (P2 1 ) and subsequently, at 425 K, to the paraelectric prototype phase (P2 1 /m). The molecular mechanism of the paraelectric-ferroelectric transition is ascribed to the 'order-disorder' behaviour of the diisopropylammonium cations.
Two novel guanidinium iodoantimonate(III) and iodobismuthate(III) crystals,
[C(NH2)3]3[Sb2I9]
and [C(NH2)3]3[Bi2I9], have been synthesized and their structures have been determined by means
of single-crystal x-ray diffraction studies at three temperatures (293, 348
and 362 K). Both compounds appeared to be isomorphous in corresponding
phases. The crystal structure of the title compounds is composed of discrete
M2I93−
(M = Sb, Bi)
anions and C(NH2)3+
guanidinium cations. A non-equivalence of two guanidinium cations has been
found. Both guanidinium analogs exhibit a rich sequence of phase transitions. In
Gu3Sb2I9, three solid–solid structural phase transformations of the first order type are detected at
119/121, 341/344 and 355/362 K (on cooling/heating) by the DSC and dilatometric techniques.
Gu3Bi2I9
displays four first order phase transitions: 179/185, 202/215, 287/291 and 358/368 K. The
low temperature phases appear to have ferroic (ferroelastic) properties. The prototypic
paraelastic phase for both compounds belongs to hexagonal symmetry (space group
P63/mmc). The dielectric response has been measured in a wide frequency region (100 Hz–1 MHz), but
no dielectric dispersion has been detected. Possible mechanisms of the phase transitions in
Gu3M2I9
(M = Sb,
Bi) are discussed on the basis of the presented results.
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