Marek Szafrański

Giant Dielectric Anisotropy and Relaxor Ferroelectricity Induced by Proton Transfers in NH+···N-Bonded Supramolecular Aggregates

A huge dielectric effect has been observed in a pure and water-soluble hydrogen-bonded organic crystal, 1,4-diazabicyclo[2.2.2]octane hydroiodide [C6H13N2]+.I(-) (dabcoHI). In this structure, the dabco cations are NH+...N bonded into linear aggregates, where the protons are disordered at two nitrogen atoms and the crystal acquires the symmetry of space group P6m2. This nonpolar crystal exhibits a barely temperature-dependent dielectric constant exceeding 1000 at ambient conditions. The dielectric response is extremely anisotropic, more than 2 orders of magnitude higher along the linear hydrogen bonded chains than in perpendicular directions. The physics underlying this effect originates from proton transfers in the NH+...N bonds, leading to disproportionation defects and formation of polar nanodomains, which, on the macroscopic scale, results in one-dimensional relaxor ferroelectricity. Such properties are unprecedented for the materials with hydrogen bonds highly polarizable due to proton disorder. The proton disordering in dabcoHI is analogous to this in H2O ice, where the hydrogen bonds remain disordered until the lowest temperature.

Mechanism of Pressure-Induced Phase Transitions, Amorphization, and Absorption-Edge Shift in Photovoltaic Methylammonium Lead Iodide.

Our single-crystal X-ray diffraction study of methylammonium lead triiodide, MAPbI3, provides the first comprehensive structural information on the tetragonal phase II in the pressure range to 0.35 GPa, on the cubic phase IV stable between 0.35 and 2.5 GPa, and on the isostructural cubic phase V observed above 2.5 GPa, which undergoes a gradual amorphization. The optical absorption study confirms that up to 0.35 GPa, the absorption edge of MAPbI3 is red-shifted, allowing an extension of spectral absorption. The transitions to phases IV and V are associated with the abrupt blue shifts of the absorption edge. The strong increase of the energy gap in phase V result in a spectacular color change of the crystal from black to red around 3.5 GPa. The optical changes have been correlated with the pressure-induced strain of the MAPbI3 inorganic framework and its frustration, triggered by methylammonium cations trapped at random orientations in the squeezed voids.

Simple guanidinium salts revisited: room-temperature ferroelectricity in hydrogen-bonded supramolecular structures.

Dielectric, calorimetric, and X-ray diffraction methods have been employed to characterize the crystals of guanidinium tetrafluoroborate and guanidinium perchlorate, both built of two-dimensional honeycomb hydrogen-bonded sheets. The room-temperature ferroelectricity of these isosymmetric complexes (space group R3m) has been evidenced by the polarization switching in an external electric field and pyroelectric effect. The analysis of structural data as a function of temperature showed that the high values of spontaneous polarization of about 8.5 μC cm(-2) originate mainly from the ionic displacements, while the exceptional thermally induced increase of polarization is related with the apparent weakening of the N-H···F/N-H···O hydrogen bonds at elevated temperatures. An excellent correlation between the donor-acceptor distance and the relative displacement of the ions in the crystal lattice along the polar direction has been found. The huge entropy change at the two-closely spaced high-temperature phase transitions in guanidinium perchlorate, together with the large crystal polarization, suggest a large electrocaloric effect, the property strongly desired for solid-state cooling applications.

Structural phase transitions in guanidinium nitrate

Abstract Transformations of NH⋯O hydrogen bonds at two structural phase transitions in guanidinium nitrate (GN) crystals have been studied by X-ray diffraction, calorimetric measurements and energy calculations of crystal cohesion forces. The symmetry of the crystal phases and the structure have been determined between 128 and 420 K; preliminary results of neutron-diffraction studies between 4 and 395 K are also reported. The structure undergoes a first-order phase transition at T 12 = 296 K, and a continuous phase transition at T 23 = 384 K. Mechanisms of the two structural phase transitions reveal the interdependence between the transformations of the NH⋯O hydrogen bonds and the crystal structures: the low-temperature phase is destabilized by electrostatic interactions between sheets of hydrogen-bonded ions leading to a rearrangement of the hydrogen-bonded aggregates at T 12 , while the increased thermal vibrations change the balance between the hydrogen bonds and electrostatic forces within the sheets, leading to dynamic reorientations of the [C(NH 2 ) 3 ] + and NO − 3 ions and to a continuous phase transition at T 23 . Unusually strong crystal strain accompanying the transition at T 12 and resulting in elongation of the samples by over 44% is described in terms of the crystal lattice transformations following a rearrangement of hydrogen-bonded supramolecules, while a negative volume change at T 12 is shown to result from the increased attractive electrostatic forces between aggregates of hydrogen-bonded ions after their rearrangement.

Thermodynamic behaviour of bistable NH+–N hydrogen bonds in monosalts of 1,4-diazabicyclo[2.2.2]octane

Abstract Proton dynamics in NH + –N hydrogen bonds is characterized by high-pressure dielectric and calorimetric studies of ferroelectric/paraelectric phase transitions in perchlorate and tetrafluoroborate mono-salts of 1,4-diazabicyclo[2.2.2]octane (DABCO), [C 6 H 13 N 2 ] + ·ClO 4 − and [C 6 H 13 N 2 ] + ·BF 4 − . The p – T phase diagrams of these ferroelectrics have been determined and described. The positive pressure dependences of T c testify to the strong coupling of the ionic dynamics with the proton disordering in the NH + –N hydrogen bonds. Close isostructurality of the crystals provides a unique opportunity for analysing the proton disordering at varied crystal environments.

Bias-field and pressure effects on the one-dimensional dielectric response in N-H(+)...N hydrogen-bonded 1,4-diazabicyclo[2.2.2]octane hydrobromide crystal.

Unusual dielectric properties of 1,4-diazabicyclo[2.2.2]octane hydrobromide [C(6)H(13)N(2)](+).Br(-) (dabcoHBr) have been investigated at ambient and hydrostatic pressures and at biasing dc electric field. The crystal exhibits a huge dielectric constant along the hydrogen-bonded chains, exceeding 1500, while in the perpendicular direction it behaves as a typical nonpolar dielectric. Though the dynamics of protons in the N-H(+)...N hydrogen bonds is essential for these properties, of key importance are weak protonic correlations leading to the formation of short-range ordered regions. The complex dielectric response of dabcoHBr is due to several contributions involving dipolar fluctuation within the polar nanoregions, fluctuations of boundaries, and excitation of solitonic kinks propagating along the chains as a result of coherent proton transfers. A relatively low dc biasing electric field distinctly modifies the dielectric response, making it reminiscent of ferroelectric relaxors. Profound changes are also induced by hydrostatic pressure, which counteracts the proton correlations and the short-range polar order formation. At elevated pressures, the hexagonal structure of dabcoHBr undergoes a phase transition, associated with a loss of the unusual dielectric properties. This is due to the breaking of the N-H(+)...N hydrogen bonds, which destroys the one-dimensional topology of the polycationic chains and results in formation of the phase built of hydrogen-bonded ionic pairs. The phase diagram, illustrating the phase boundary between the high- and low-dielectric constant phases of dabcoHBr, is presented.

Investigation of phase instabilities in guanidinium halogenoplumbates(II)

Abstract Four new compounds based on guanidinium and halogenoplumbate(II) ions, [C(NH 2 ) 3 ] 2 PbCl 4 , [C(NH 2 ) 3 ] 2 PbBr 4 , [C(NH 2 ) 3 ] 2 PbI 4 and C(NH 2 ) 3 PbI 3 , have been studied by differential thermal analysis and X-ray powder diffraction. In the chloride, four thermal anomalies have been detected at 345, 383, 412–419 and 439 K, indicating the possibility of four first-order phase transitions; the estimated transition entropies suggest the existence of phase disordering above 439 K. It was established that the phase sequences occurring on heating and on cooling the substance are different. The bromide compound undergoes two successive first-order phase transitions at 406–415 and 426 K, both associated with a large transition entropy indicating disordering of the intermediate- and high-temperature phases. In the tetraiodide, a continuous phase transition to the disordered phase was found at 307 K, while the triiodide undergoes two first-order phase transitions at 255 and 432 K, respectively. Both phase transitions in C(NH 2 ) 3 PbI 3 are of the order-disorder type, the high-temperature phase exhibits metastable properties. The spectroscopic changes observed at the high-temperature phase transitions in [C(NH 2 ) 3 ] 2 PbBr 4 and C(NH 2 ) 3 PbI 3 indicate that the mechanism of these transformations is connected both with a disordering process and a distortion of the anionic sublattices.

Origin of spontaneous polarization and reconstructive phase transition in guanidinium iodide

Structural determinations as a function of temperature allowed us to model the polar properties of guanidinium iodide, C(NH)2I, in its hexagonal P63mc-symmetric phase. These results have been compared with the experimental data of the pyroelectric measurements. It has been shown that the crystal lattice polarization, arising from the ionic displacements, increases with temperature and abruptly vanishes at the first-order phase transition to the high-temperature phase of monoclinic space group P21/m. Thus, this transition non-typically lowers the crystal symmetry when temperature is increased. Moreover, the polar–non-polar symmetry change, accompanied by the crystal polarization loss, does not occur as a ferroelectric–paraelectric phase transition. This is rather uncommon, but can be rationalized by the reconstructive character of the transition and by the structural hindrances for the polarization reversal in the low-temperature phase. The reconstructive nature of the structural changes and the 3% decrease in the crystal volume can explain a large temperature hysteresis of the transition point, a strong sensitivity to the thermal history of the sample, and the metastable features of the high-temperature phase.

Molecular interactions in crystalline dibromomethane and diiodomethane, and the stabilities of their high-pressure and low-temperature phases

Dibromomethane, CH2Br2, and diiodomethane, CH2I2, have been in situ pressure-crystallized in a diamond-anvil cell and their structures determined by single-crystal X-ray diffraction at 0.61 and 0.16 GPa, respectively. The pressure-frozen CH2Br2 crystal is isostructural with its C2/c phase obtained by cooling. CH2I2 is known to form several phases at low temperature, one of which is isostructural with CH2Br2. However, pressure freezing leads to the polar Fmm2 phase. The formation of the polar CH2I2 structure at 0.16 GPa has been rationalized by the electrostatic and anisotropic van der Waals interactions of the I atoms. No ferroelectric behaviour of the Fmm2 polar phase II of CH2I2 has been determined. The diffraction, calorimetric and dielectric constant studies reveal considerable temperature hysteresis of transformations between the CH2I2 phases, as well as metastable regions strongly dependent on the sample shape and history.

Low-temperature and high-pressure phase transitions in ferroelectric dabcoHBF4

NH--N hydrogen bonded ferroelectric dabcoHBF4, [C6H13N2]+BF4−, has been studied by dielectric spectroscopy, differential thermal analysis and pyroelectric charge measurement investigations in the low-temperature range between 12 and 300 K, and at elevated hydrostatic pressures up to 1 GPa. The p–T phase diagram has been determined and described. In addition to the known first-order phase transitions at 378 and 153 K, two new structural transitions of continuous type have been revealed, i.e., the transition at 37 K and the pressure-induced one occurring above the triple point situated at 80 MPa and 165 K. Both of these transitions have essentially displacive character and a mechanism related to distortions of the hydrogen bonded polycationic chains, preserving polar properties of the crystal. The possible order–disorder contribution to the transition at 153 K between the ferroelectric phases II and III is discussed in relation to the conformational properties of the dabco cation.