G. Bator

Structure and phase transition in (CH3NH3)3Bi2Br9. A novel improper ferroelectrics

Abstract The crystals of (CH3NH3)3Bi2Br9 have been grown which at room temperature are isomorphous with Cs3Bi2Br9 and (CH3NH3)3Sb2Br9. The space group is P3m1, a = 0.807, c = 1.008 nm, V = 0.5685 nm3, Z = 1. The crystals reveal, when cooling, three phase transitions at 188 ± 0.5, 140 ± 1, and 101.5 ± 0.5 K. The high temperature phase (I) behaves as a plastic one with respect to a freedom of rotation of methylammonium cations. The two high temperature transitions I II and II III can be related, similarly to (CH3NH3)3Sb2Br9, to a freezing of reorientation of those cations. At 101.5 K a transition to the improper ferroelectric phase IV was discovered. The features of this phase are non-typical and the nature of the transition is unknown.

Ferroelectric properties of (C5H5NH)5Bi2Br11

A new pyridinium compound, (C5H5NH)5Bi2Br11, has been synthesized. The x-ray diffraction studies indicate that its structure is built up of four independent pyridinium cations and isolated Bi2Br115− groups. At room temperature it crystallizes in the monoclinic space group P21/n. Two phase transitions of second- and first-order type are detected by the differential scanning calorimetry measurements at 118 and 405/403 K (on heating–cooling), respectively. The corresponding anomalies in the linear thermal expansion at 118 and 405 K are observed along three crystallographic directions. The dielectric investigations reveal dielectric absorption and dispersion in the radio-frequency region. The experimental results were analyzed on the basis of dielectric formula which represents the sum of two independent relaxators. The pyroelectric measurements reveal that (C5H5NH)5Bi2Br11 becomes ferroelectric below 118 K and is characterized by the appearance of spontaneous polarization of the order of 3⋅10−3 C/m2 along th...

A ferroelectric inorganic–organic hybrid based on NLO-phore stilbazolium

A ferroelectric inorganic–organic hybrid based on in situ substituted stilbazolium cation (TAMS2+ = trimethylamino-N-methyl stilbazolium) has been obtained. Single-crystal X-ray structure analysis demonstrates that the NLO-phore organic component of the complex is embedded into the inorganic polymeric [Bi2Cl8]2− framework and shows a polar arrangement with the cooperation of the nonsymmetric inorganic component. The measurement of ferroelectric and nonlinear optical properties show typical ferroelectricity and second harmonic generation (SHG) responses as well as a significant improvement of the laser damage threshold and thermal stability of relative organic material.

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.

Dielectric dispersion and vibrational studies of a new ferroelectric, glycinium phosphite crystal

The dielectric dispersion in glycine phosphite crystal over the frequency range 30 - 1000 MHz is presented. Ferroelectric dispersion of the Debye type along the b-axis occurs in the microwave-frequency region and is caused by a single relaxational soft mode. The powder IR (at low temperatures between 300 and 14 K) and FT Raman spectra in the region between 4000 and are measured. DSC measurements for the GPI and deuterated crystals are performed. The results confirm the order - disorder nature of a proper ferroelectric phase transition of second order.

Structure, phase transitions and molecular motions in 4-aminopyridinium perchlorate

The crystal structure of the 4-aminopyridinium perchlorate (4-apyH)ClO4 has been determined at 100 K by means of x-ray diffraction as monoclinic, with space group P 21, with Z = 8. The crystal undergoes two structural phase transitions: one of first-order type, reversible, at 241/243 K (on cooling/heating respectively) and one of weakly first-order type, irreversible, at 277 K (on heating). The crystal dynamics is discussed on the basis of the temperature dependence of the 1 H nuclear magnetic resonance second moment (M2) and spin–lattice relaxation time T1. Both phase transitions are interpreted in terms of the changes in the motional state of (4-apyH)+ cations and ClO4− anions. The dielectric dispersion studies disclose a relaxation process over the high-temperature phase (above 241 K) in the audio-frequency region. The dielectric results are described by a Cole–Cole equation. The title crystal reveals pyroelectric properties below 241 K. The ferroelastic domain structure of (4-apyH)ClO4 is observed over the whole temperature range studied.

Structure, phase transition and molecular motions in (C5H5NH)BiCl4

The crystal structure at 293 K of the new pyridinium compound, (C5H5NH)BiCl4, has been determined by X-ray diffraction as monoclinic, space group Cc, Z = 4. The crystal is built up of one-dimensional (BiCl4−)n chains and pyridinium C5H5NH+ cations. A structural phase transition of first-order type is detected by differential scanning calorimetry (DSC) and dilatometric measurements at 114/110 K (on heating–cooling, respectively). Proton NMR second moment and spin–lattice relaxation time studies confirmed the order–disorder mechanism of the phase transition at 110 K. It was connected with the reorientational motion of the pyridinium cation. Dielectric investigations reveal absorption and dispersion in the audio-frequency region in both the high and low temperature phases. The experimental results were analysed in the high temperature phase in terms of the Havriliak–Negami formula. In the low temperature phase the Cole–Cole relation for a single relaxator was applied. Above the phase transition point the macroscopic relaxation time exhibits non-Arrhenius behaviour, whereas below Tc it is perfectly described by a pure Arrhenius relation.

The structure and vibrational spectra of some ferroelectric and ferroelastic alkylammonium halogenoantimonates(III) and bismuthates(III)

Abstract Crystals from the rich family of alkylammonium halogenoantimonates(III) and bismuthates(III) containing small bioctahedra as well as infinite two- or one-dimensional polyoctahedral units attract particular attention from the point of view of possible applications. Such crystals exhibit a wealth of phase transitions including those to ferroelectric and ferroelastic phases. The analysis of vibrational spectra performed in this paper with respect to modes assigned to alkylammonium group shows that their interactions with polyanionic sublattice is of medium strength. The absorption pattern and particularly the splitting of bands on cooling very well correlates with other anomalies of physical properties and particularly with the behaviour of second moment of PMR lines and T 1 and T 1 ρ relaxation times as functions of temperature. The temperature behaviour of modes is well described in terms of pseudospin–phonon coupling model that we presented for different modes in various crystals. The role of anionic dynamics is not sufficiently recognised but in one case, namely for the low temperature transition in (MA) 3 Bi 2 Br 9 crystal this role is predominant, according to vibrational and NQR spectra of methylammonium and isostructural caesium salt.

CRYSTAL STRUCTURE AND PHASE TRANSITION OF [(CH3)2NH2]GaCl4

Abstract A new [(CH 3 ) 2 NH 2 ]GaCl 4 crystal of the family of alkylammonium halogenogallates (III) was grown. X-ray studies showed that the crystals were orthorhombic, space group Pmcn . The lattice consists of (CH 3 ) 2 NH 2 + cations and isolated GaCl 4 − anions. Differential scanning calorimetry, dilatometric, dielectric and infrared studies revealed a structural phase transition of the first-order type at 100 K. The possible mechanism of the phase transition is discussed.

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.