A. Piecha

Novel organic–inorganic hybrid ferroelectric: bis(imidazolium) pentachloroantimonate(III), (C3N2H5)2SbCl5

Electric permittivity, reaching ca. 4800 units at Tc, of the first haloantimonate(III) ferroelectric with R2MX5 composition characterized by unprecedented SbCl5 chains of cis-connected octahedra results from the mixed ‘order–disorder’ and ‘displacive’ mechanism of the phase transition.

Structure and ferroelectric properties of (C 3 N 2 H 5 ) 5 (Bi 2 Br 11 )

Pentakis (imidazolium) undecachlorodibismuthate(III) (C{sub 3}N{sub 2}H{sub 5}){sub 5}Bi{sub 2}Cl{sub 11}, has been synthesized and studied by means of the x-ray, differential scanning calorimetry (DSC), and dielectric methods over a wide temperature range. The room temperature crystal structure has been determined as monoclinic, space group P2{sub 1}/n with a=13.616 A, b=14.684 A, c=9.045 A, and {beta}=96.85 deg. Two solid-solid phase transitions: P4n2{yields}{sup 360K}P2{sub 1}/n (I{yields}II), and P2{sub 1}/n{yields}{sup 166K}P2{sub 1} (II{yields}III), discontinuous and continuous in nature, respectively, have been revealed. The dielectric and pyroelectric measurements allow us to characterize the lowest temperature phase (III) as ferroelectric with the Curie point at 166 K and saturated spontaneous polarization value of the order of 6x10{sup -3} C m{sup -2} along the b axis. The ferroelectric phase transition mechanism is governed by the dynamics of imidazolium cations. The room temperature, paraelectric phase (II) demonstrates additionally the ferroelastic properties.

Room-temperature ferroelectricity in diisopropylammonium bromide

A room-temperature ferroelectric, diisopropylammonium bromide (DPB), with dielectric constant e ≈ 12 000 and a clear hysteresis loop at Tc = 425 K is reported. At 417 K DPB undergoes the irreversible phase transition from nonpolar orthorhombic P212121 to the ferroelectric monoclinic phase (P21) and subsequently, at 425 K, to the paraelectric prototype phase (P21/m). The molecular mechanism of the paraelectric–ferroelectric transition is ascribed to the ‘order–disorder’ behaviour of the diisopropylammonium cations.

Structure and ferroelectric properties of [C3N2H5]5[Bi2Br11]

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) A, b = 15.157(3) A, c = 13.865(3) A 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.

Synthesis, crystal structure and phase transitions of a series of imidazolium iodides

The reaction of imidazole with hydroiodic acid leads to three products crystallizing as ionic salts; [C3N2H5+][I−], [C3N2H5+]2[I42−] and [C3N2H3I2+][I−]. 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), [C3N2H5+][I−] adopts the centrosymmetric, trigonal space group (R). The crystal structure consists of disordered imidazolium cations and discrete I− ions. [C3N2H5+][I−] 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. [C3N2H5+]2[I42−] consists of disordered imidazolium cations and quite rare and exotic [I4]2− 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 [I42−] rods, respectively. [C3N2H3I2+][I−] is built up of discrete 4,5-diiodoimidazolium cations and isolated I− 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.

Critical slowing down of low-frequency dielectric relaxation in ferroelectric (C3N2H5)5Bi2Cl11

Dielectric dispersion studies in the frequency range between 220 Hz and 1 MHz have been performed on pentakis(imidazolium) undecachlorodibismuthate(III), (C3N2H5)5Bi2Cl11, single crystals. The measurements have been made over the temperature range between 300 and 140 K. It is found that the dielectric dispersion, observed along the b-axis of the monoclinic symmetry (P 21/c), is monodispersive between 166.5 and 168 K, and polydispersive in the close vicinity of Tc = 166 K, for T<Tc + 0.2 K. The observed dielectric dispersion is analysed using a single relaxation mode of the Debye type. Over the paraelectric phase the process of the dielectric critical slowing down is observed. This is characteristic of the order–disorder ferroelectric phase transition.

Structure and ferroelectric properties of(C3N2H5)5Bi2Cl11

Pentakis (imidazolium) undecachlorodibismuthate(III) (C{sub 3}N{sub 2}H{sub 5}){sub 5}Bi{sub 2}Cl{sub 11}, has been synthesized and studied by means of the x-ray, differential scanning calorimetry (DSC), and dielectric methods over a wide temperature range. The room temperature crystal structure has been determined as monoclinic, space group P2{sub 1}/n with a=13.616 A, b=14.684 A, c=9.045 A, and {beta}=96.85 deg. Two solid-solid phase transitions: P4n2{yields}{sup 360K}P2{sub 1}/n (I{yields}II), and P2{sub 1}/n{yields}{sup 166K}P2{sub 1} (II{yields}III), discontinuous and continuous in nature, respectively, have been revealed. The dielectric and pyroelectric measurements allow us to characterize the lowest temperature phase (III) as ferroelectric with the Curie point at 166 K and saturated spontaneous polarization value of the order of 6x10{sup -3} C m{sup -2} along the b axis. The ferroelectric phase transition mechanism is governed by the dynamics of imidazolium cations. The room temperature, paraelectric phase (II) demonstrates additionally the ferroelastic properties.

Critical behaviour and the order parameter saturation in uniaxial ferroelectrics with the polar axes in and out of a crystallographic direction

The equations of state for two uniaxial systems: the first with the polar direction identical with one of the crystallographic axes and the second with the polar direction within a crystallographic plane giving a realistic Curie–Weiss constant ratio and the experimentally obtained non-classical critical exponents are used to describe the behaviours of spontaneous polarization and of the susceptibility. The theories are exemplified by the behaviour of molecular-ionic crystals (CH3NH3)5Bi2Cl11 (MAPCB), (CH3NH3)5Bi2Br11 (MAPBB) and (C3N2H5)5Bi2Br11 (IBB).

Structure and ferroelectric properties of (C{sub 3}N{sub 2}H{sub 5}){sub 5}Bi{sub 2}Cl{sub 11}

Pentakis (imidazolium) undecachlorodibismuthate(III) (C{sub 3}N{sub 2}H{sub 5}){sub 5}Bi{sub 2}Cl{sub 11}, has been synthesized and studied by means of the x-ray, differential scanning calorimetry (DSC), and dielectric methods over a wide temperature range. The room temperature crystal structure has been determined as monoclinic, space group P2{sub 1}/n with a=13.616 A, b=14.684 A, c=9.045 A, and {beta}=96.85 deg. Two solid-solid phase transitions: P4n2{yields}{sup 360K}P2{sub 1}/n (I{yields}II), and P2{sub 1}/n{yields}{sup 166K}P2{sub 1} (II{yields}III), discontinuous and continuous in nature, respectively, have been revealed. The dielectric and pyroelectric measurements allow us to characterize the lowest temperature phase (III) as ferroelectric with the Curie point at 166 K and saturated spontaneous polarization value of the order of 6x10{sup -3} C m{sup -2} along the b axis. The ferroelectric phase transition mechanism is governed by the dynamics of imidazolium cations. The room temperature, paraelectric phase (II) demonstrates additionally the ferroelastic properties.