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.

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A ferroelectric crystal [C3N2H5]5[Bi2Br11] has been synthesized and structurally characterized at 170 and 100 K. The crystal structure consists of discrete corner-sharing bioctahedra [Bi2Br11]5− and highly disordered imidazolium cations. The room temperature crystal structure has been determined as monoclinic, space group, P 21/n with: a = 9.257(2) Å, b = 15.157(3) Å, c = 13.865(3) Å and β = 97.73(3)°. The crystal undergoes two solid–solid phase transitions: at 355 K of first-order and at 155 K of second-order type. The later transition takes place between monoclinic phases: . The dielectric and pyroelectric measurements allow us to characterize the low temperature phase III as ferroelectric with the Curie temperature at 155 K. The saturated spontaneous polarization of the order of 2.6 × 10−3 C m−2 was measured along the a-axis (130 K). The ferroelectric phase transition mechanism at 155 K is due to the dynamics of three of five nonequivalent imidazolium cations.

The reaction of imidazole with hydroiodic acid leads to three products crystallizing as ionic salts; [C 3 N 2 H 5 + ][I 2 ], [C 3 N 2 H 5 + ] 2 [I 4 22 ] and [C 3 N 2 H 3 I 2 + ][I 2 ]. All the analogs were characterized by single-crystal X-ray diffraction, while the first two were additionally studied by calorimetric, dilatometric, dielectric and proton magnetic resonance methods. At room temperature (RT), [C 3 N 2 H 5 + ][I 2 ] adopts the centrosymmetric, trigonal space group (R3). The crystal structure consists of disordered imidazolium cations and discrete I 2 ions. [C 3 N 2 H 5 + ][I 2 ] undergoes two discontinuous phase transitions (PTs) at 180/185 K and 113/ 123 K (cooling-heating), both of them governed by the imidazolium cation dynamics. [C 3 N 2 H 5 + ] 2 [I 4 22 ] consists of disordered imidazolium cations and quite rare and exotic [I 4 ] 22 tetraiodide counterion. It undergoes continuous PT at 204 K of the ferroelastic type with a symmetry change from orthorhombic Fddd to monoclinic C2/c. The mechanism of PT is complex and consists of 'order-disorder' and 'displacive' contributions that are assigned to the dynamics of cations and to the distortion of the [I 4 22 ] rods, respectively. [C 3 N 2 H 3 I 2 + ][I 2 ] is built up of discrete 4,5-diiodoimidazolium cations and isolated I 2 ions. A characteristic feature of this compound is the presence of a layered structure in which moieties are held together by strong I … I halogen interactions and N-H … I hydrogen bonds.3 Electronic supplementary information (ESI) available: Details of the thermal (TGA), X-ray diffraction and dielectric and 1 H NMR measurements. CCDC 929449-929453. For ESI and crystallographic data in CIF or other electronic format see

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