P. Szklarz

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.

[NH2(C2H4)2O]MX5: a new family of morpholinium nonlinear optical materials among halogenoantimonate(III) and halogenobismuthate(III) compounds. Structural characterization, dielectric and piezoelectric properties

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).

Structure, phase transitions and molecular?dynamics of?[C(NH2)3]3[M2I9], M = Sb, Bi

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.

The structure, phase transition and molecular dynamics of [C(NH2)3]3[Sb2Br9]

The crystal structures of [C(NH2)3]3[Sb2Br9] (Gu3Sb2Br9) at 300 K and of [C(NH2)3]3[Sb2Cl9] (Gu3Sb2Cl9) at 90 and 300 K are determined. The compounds crystallize in the monoclinic space group: C 2/c. The structure is composed of Sb2X93− (X = Cl, Br) ions, which form two-dimensional layers through the crystal, and guanidinium cations. In Gu3Sb2Br9 the structural phase transformation of the first-order type is detected at 435/450 K (on cooling/heating) by the DSC and dilatometric techniques. The dielectric relaxation process in the frequency range between 75 kHz and 5 MHz over the low temperature phase indicates reorientations of weakly distorted guanidinium cations. The proton 1H NMR second-moment and spin–lattice relaxation time, T1, temperature runs for the polycrystalline Gu3Sb2Br9 sample indicate a complex cation motion. A significant dynamical non-equivalence of two guanidinium cations was found. The possible mechanism of the phase transition in Gu3Sb2Br9 is discussed on the basis of the results presented.

Critical Behaviour in Ferroelectrics as Studied by Nonlinear Dielectric Effect. Invariants of the Electric Susceptibility in a Biasing Field

The simplest equation of state compatible with the Widom and Griffiths scaling hypothesis have been built for the uniaxial ferroelectrics TGS and MAPCB. The explicit forms have been found of the scaling invariants deduced from the curves of the electric susceptibility measured in a constant biasing field. Smooth and sharp inflection points of the susceptibility curve in the vicinity of the critical temperature have been classified. The invariants and the inflection points have been used to determine critical parameters of TGS and MAPCB.

Enormous lattice distortion through an isomorphous phase transition in an organic–inorganic hybrid based on haloantimonate(III)

Bis(diisobutylammonium) octabromodiantimonate(III), [(i-C4H9)2NH2]2Sb2Br8, has been synthesized. The differential scanning calorimetric measurements indicate a reversible, first-order phase transition at 222/229 K (cooling/heating). The single crystal X-ray diffraction studies reveal that the phase transition is isomorphous and is accompanied by a huge distortion of the crystal lattice. By comparison of the crystal structures of [(i-C4H9)2NH2]2Sb2Br8 and [(i-C4H9)2NH2]2Sb2Cl8, an analogous mechanism of the phase transition of the former is proposed. The change of the electronic structure of the complex during the phase transition was analyzed by UV-vis spectroscopy. A low-frequency dielectric relaxation process appears over phase I (below the room temperature) and corresponds to the dynamics of dipolar diisobutylammonium cations. The detailed analysis of the molecular motions of the organic cations studied by means of proton magnetic resonance (1H NMR) in a wide temperature range indicates a leading role of the methyl groups in the relaxation mechanism. A variable-temperature investigation of the infrared spectra of [(i-C4H9)2NH2]2Sb2Br8 confirms, in turn, the influence of the diisobutylammonium cation dynamics on the molecular mechanism of the structural transformation at 229 K.

On the ratio of Curie--Weiss constants in ferroelectrics undergoing second order phase transitions

Scaling hypothesis is used to construct the equations of state that provide the experimentally observed variety of the Curie--Weiss constants ratios (i.e., the ratios of the slopes between the temperature dependencies of inverse of susceptibility below and above the critical point) in uniaxial ferroelectrics. The results are exemplified by the TGS crystal and by the methylammonium salts MAPCB and MAPBB.

Scaling equation of state for uniaxial ferroelectrics: zero-field susceptibility and NDE effect

The simplest scaling equation of state and its limit case corresponding to the Landau theory turns out to accurately describe the temperature dependence of the electric susceptibility measured in a biasing static field (Nonlinear Dielectric Effect NDE) in uniaxial ferroelectrics. The same equation of state should be, in principle, valid at the zero-biasing field, in which case the susceptibility is singular , where Γ+ and Γ− are the inverse Curie–Weiss constants at T > T C and T < T C, respectively. The simplest scaling equation of state implies the ratio Γ−/Γ+ = δ − 1, where δ is a critical exponent: δ = 3 in the Landau theory. However, the real experimental data show serious discrepancies with this prediction even in systems following the Landau theory. We show how to improve the equation of state without infringing the scaling hypothesis so that this discrepancy is removed. An interesting result of the proposed modification is that the deviation of Γ−/Γ+ from δ − 1 is a signature of the critical exponent γ being different from 1. The experimental data on the uniaxial ferroelectrics MAPCB and MAPBB will be used as examples of the treatment.

Singularities and scaling invariants of susceptibility in biasing field near critical point: application to uniaxial ferroelectrics

The general shape of the temperature dependence of the static susceptibility in a biasing field conjugated to the order parameter is analysed with the use of the simplest equation of state compatible with the Widom and Griffiths scaling hypothesis. The corresponding curves are demonstrated to show from two to four inflection points, from which a discontinuous inflection point is found to occur exactly at the critical point whenever the critical exponent of susceptibility differs from one: . The unique inflection point occurring below the temperature of the maximum of the susceptibility in the case of the classical critical exponents, i.e. in the mean field theory, is also shown to be strictly independent of the biasing field. New scaling invariants related to the inflection points are found and their analytical expressions are given for the considered equation of state. The usefulness of the theoretical results to the analysis of experimental data is discussed.

The low-temperature phase of morpholinium tetra­fluoro­borate

The crystal structure of the low-temperature form of the title compound, C4H10NO+·BF4 −, was determined at 80 K. Two reversible phase transitions, at 158/158 and 124/126 K (heating/cooling), were detected by differential scanning calorimetry for this compound, and the sequence of phase transitions was subsequently confirmed by single-crystal X-ray diffraction experiments. The asymmetric unit at 80 K consists of three BF4 − tetrahedral anions and three morpholinium cations (Z′ = 3). Hydrogen-bonded morpholinium cations form chains along the [100] direction. The BF4 − anions are connected to these chains by N—H⋯F hydrogen bonds. In the crystal structure, two different layers perpendicular to the [001] direction can be distinguished, which differ in the geometry of the hydrogen bonds between cationic and anionic species.