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.

Novel room-temperature ferroelectric crystals with complex sequences of phase transitions.

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.

This paper presents the structural features of ionic complexes formed by morpholine and metal ions which belong to group VA, namely Sb(III) and Bi(III). A series of target inorganic-organic hybrid compounds of the general formula [NH(2)(C(2)H(4))(2)O](2)MX(5) (where M = Sb, Bi; X = Cl, Br) has been synthesized by incorporating the organic component (morpholine) into the highly polarizable one-dimensional halogenoantimonate(III)/halogenobismuthate(III) chain network. Among the studied compounds, four were found to crystallize in the room temperature phase in the piezoelectric, orthorhombic space group P2(1)2(1)2(1), Z = 4, the feature being confirmed by the powder second harmonic generation of light and piezoelectric measurements. Dielectric dispersion studies between 200 Hz and 2 MHz disclosed a relaxation process below room temperature well described by the Cole-Cole equation. Based on crystal structures available in Cambridge Structural Database (version 5.32, November 2010) we attempt to show a relationship between the acentric symmetry of compounds and the type of anionic network within the R(2)MX(5)-subgroup (where R denotes organic cation) of halogenoantimonates(III) and halogenobismuthates(III).

No abstract

Ferroelectric properties of haloantimonates­(III) and halobismuthates­(III) have been detected for as much as 40 structures belonging to 7 different types of anionic networks, with RMX4, R2MX5, R3M2X9, and R5M2X11 stoichiometries being the most frequently reported to host these properties. We report on the first ferroelectric of the halobismuthate­(III) family with a R3MX6 stoichiometry, that is, tris­(acetamidinium)­hexabromobismuthate­(III), (CH3C­(NH2)2)3[BiBr6] (ABB), characterized by a one-component organic network. While the stoichiometry and crystal packing of ABB might seem uncomplicated, the temperature-resolved structural and spectroscopic studies paint a different picture in which rich polymorphism in the solid state occurs between tetragonal (paraelastic) and triclinic (ferroelastic) crystal phases: I (P42/n) → II (P1̅) at 272/277 K (cooling/heating), II (P1̅) → III (P1̅) at 207 K, and III (P1̅) → IV (P1) at 98/127 K. The ferroelectric properties of phase IV have been confirmed by the pyroelectric current and hysteresis loop measurements; additionally, the acentric symmetry has been further supported by second harmonic generation measurements. Crystallographic analysis of phase III reveals the antiparallel alignment of acetamidinium dipoles, pointing to the antiferrroelectric nature of this phase. In turn, the character of the ferroelectric transition (III → IV) should be considered as “displacive” for both cationic and anionic substructures.) In this report, we also explore the two-photon absorption property of ABB at 800 nm, a property that is unexplored for any halobismuthate­(III) thus far. We also present periodic ab initio calculations for ABB crystals. The Berry-phase approach at the Hartree–Fock and density functional theory (DFT-D3) method levels is employed for spontaneous polarization calculations. The origin of ferroelectric polarization is studied using DFT-D3 and RHF electronic structure calculations, emphasizing the relationship between P s and the relative orientation of organic/inorganic components.

No abstract