Order-disorder Ferroelectric Phase Transition Research Articles

Dielectric ordering of water molecules arranged in a dipolar lattice

Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H2O molecules in nanosized cages formed by ions of a dielectric crystal. Arranging them in channels at a distance of ~5 Å with an interchannel separation of ~10 Å prevents the formation of hydrogen networks while electric dipole-dipole interactions remain effective. Here, we present measurements of the temperature-dependent dielectric permittivity, pyrocurrent, electric polarization and specific heat that indicate an order-disorder ferroelectric phase transition at T0 ≈ 3 K in the water dipolar lattice. Ab initio molecular dynamics and classical Monte Carlo simulations reveal that at low temperatures the water molecules form ferroelectric domains in the ab-plane that order antiferroelectrically along the channel direction. This way we achieve the long-standing goal of arranging water molecules in polar order. This is not only of high relevance in various natural systems but might open an avenue towards future applications in biocompatible nanoelectronics.

Crossover from Ferroelectric to Relaxor Behavior in Ba1-xCaxTiO3 (x = 0.17) System.

The dielectric properties of Ba1−xCaxTiO3 (x = 0.17) ceramics were studied in a wide frequency range of 20 Hz–53 GHz. Diffused ferroelectric phase transition was revealed close to 339 K in the dielectric properties of ceramics. The behaviour of distributions of relaxation times in vicinity of the ferroelectric phase transition temperature is also typical for order-disorder ferroelectric phase transition. However, at lower temperatures (below 200 K), the most probable relaxation increased according to the Arrhenius law. At lower temperatures the maximum of the imaginary part of dielectric permittivity versus temperature strongly shifted to higher temperatures when the frequency increased (from 125 K at 1.21 kHz to 300 K at 33 GHz). This behaviour was attributed to the dynamics of Ti ions. The origin of the crossover from ferroelectric to relaxor behaviour of Ba1−xCaxTiO3 (x = 0.17) ceramics is discussed in the paper.

Hydrogen-bonded and supramolecular ferroelectricity in a new hybrid (C12H25NH3)2CoCl4

A new organic-inorganic layered halide perovskite ((C12H25NH3)2CoCl4) has been synthesized and characterized by: chemical analysis, x-ray powder diffraction, differential scanning calorimetry, and differential thermal analysis thermographs. The dielectric and ferroelectric properties have been investigated. Analysis of the ac conductivity σac (ω, T) measured in the temperature interval from 110 K to 370 K and at selected frequencies in the range 1 kHz < f < 100 kHz and the results show that the Jonscher universal power law is accurately obeyed. The existence of ferroelectricity has been demonstrated by the hysteresis P-E loop. The collective data evidenced that the hybrid behaves as a proper ferroelectric rather than a relaxor one and undergoes an order-disorder ferroelectric phase transition at the Curie temperature Tc (Tc ≈ 360 K) accompanied with a structural distortion from an intercalated to non- intercalated state as the other Co and/or Zn hybrids. Since the room temperature space group of the hybrid is centrosymmetric (P21/c), the ferroelectricity should be due to the supramolecular interaction which results from the induced dipoles. The hydrogen bonding system of the type N–H….Cl plays the key role in the mechanism of supramolecular ferroelectricity.

High-spin Fe(III) Schiff based complexes with photoactive ligands. Synthesis, EPR study and magnetic properties

A series of three novel Fe(III) compounds of the formula [FeL2]X (where X = Cl−(1), PF6−(2), NO3– (3), and L is a photoactive ligand, (4)-4-(((2-(ethylamino)ethyl)imino)methyl)-3-hydroxyphenyl 4-bromobenzoate) was synthesized and studied by means of electron paramagnetic resonance (EPR) and pulsed laser irradiation. The Fe3+ ions in these compounds are in a high-spin state. A thorough analysis of the EPR data suggests that compounds 1 and 2 undergo an order–disorder ferroelectric phase transition, and below the phase transition temperature (Tc = 100 and 200 K for compounds 1 and 2, respectively) a nonzero average electric dipole moment appears. To get an insight into molecular structure of Fe3+ ions and their supramolecular organization in low-temperature (LT) and high-temperature (HT) phases of compounds 1 and 2, a series of density functional theory calculations was performed. On the basis of our findings, the LT- and HT-phase structures were proposed for these compounds. It was also shown that, whereas the chloride and hexafluorophosphate anions are able to form a network of hydrogen bonds between the [FeL2]X units (ionic pairs), which enable an electric dipole ordering in the sample, the nitrate anions, in contrast, tend to form hydrogen bonds inside the ionic pair. This conclusion is evidenced by the observed EPR spectra, which are different for compound 3 and are not indicative of the existence of an order–disorder ferroelectric phase transition. The EPR data obtained upon irradiation of compound 1 show that photoexcitation in the UV region at 5 K destroys hydrogen bonds and converts cationic complexes into ligand-to-metal charge transfer (LMCT) states, in which the iron is ferrous, and the unpaired electron is located on the salicylidene moieties. The LMCT states decay back to the ferric one, and ferric complexes further form the most stable (LT) phase structure.

A high-temperature organic–inorganic ferroelectric with outstanding switchable dielectric characteristics

A new molecular ferroelectric is discovered in an organic–inorganic hybrid compound, (C6H5NH3)2CdCl4, which undergoes a reversible order–disorder ferroelectric phase transition at 369 K.

Polar behaviour induced by lithium in potassium tantalate ceramics

Polar behaviour in K1−xLixTaO3 ceramics with x = 0.02, 0.05 and 0.10, processed by the conventional solid state method, is studied by Raman spectroscopy and thermally stimulated depolarization current (TSDC) techniques between 10 and 290 K. The TO1 mode of KTaO3 is revealed to harden in the whole temperature range and to split in the low-temperature range by Li doping. One splitting is observed for x = 0.02 and two consequent splittings are detected for x = 0.05 and 0.10. The temperatures, where TO1 mode splitting occurs, are found to correspond to those of the peaks of TSDC, and hence to the onset of the electric polarization. Such behaviour provides evidence for the order–disorder ferroelectric phase transition induced in KTaO3 by lithium doping, which emerges from deformations of the cubic phase developed on cooling either in one (for x = 0.02) or two steps (for x = 0.05 and 0.10).

Critical slowing down of relaxation dynamics near the Curie temperature in the relaxor Pb(Sc0.5Nb0.5)O3

Brillouin spectroscopy of Pb(Sc0.5Nb0.5)O3 single crystals reveals an order-disorder ferroelectric phase transition occurs at 110 °C upon cooling. A softening of the longitudinal acoustic (LA) mode is observed that can be attributed to the coupling between polar nanoregions and acoustic modes. A critical slowing down of the central peak, a feature of order-disorder ferroelectric phase transitions, is observed near Tc. The similarity in temperature dependences of the two kinds of relaxation times determined from the central peak and LA mode suggests that the changes in the central peak arise from local polarization fluctuations in the polar nanoregions.

Molecular dynamics in triglycine sulphate by cold neutron spectroscopy

Molecular dynamics of the ferroelectric compound triglycine sulphate (TGS) was studied by quasi-elastic neutron scattering. From among the three glycine molecules forming the unit cell the non-planar GI molecule is responsible for the polarisation which occurs in crystal and for the order–disorder ferroelectric phase transition at T c = 49 °C. This molecule also shows a complex dynamics much faster than that performed by the other glycine molecules. The key group of this molecule is the amino group that, on one hand, seems to be responsible for the phase transition and, on the other hand, performs motional processes spread on a wide time scale. These motions have been detailed studied by means of different energy resolution windows within the quasi-elastic scattering range. We found that the dynamics of this group develops in a restricted spatial volume and consists in a faster rotational diffusion around the molecular axis of the group and a slower flipping dynamics between two symmetrical positions on both sides of a mirror plane containing the molecular axis of GI molecule.

Effect of isotope substitution and pressure on the phase transition in triglycine sulphate

A quantitative estimate of dipolar interaction energy causing the order–disorder ferroelectric phase transition in triglycine sulphate (TGS) is obtained and also the effect of small hydrostatic pressures on the dipolar energy is modelled. At high pressures of the order of 2.5 GPa a crystallographic phase transition to a non-ferroelectric phase takes place in these crystals, the spontaneous polarization goes to zero most likely because of the glycine ion GI becoming planar at high pressures. The inverse Ubbelohde effect observed in TGS is successfully explained using the corrected valence bond model. The increase in transition temperature ( T C) on deuteration is associated with the increase in value of the reorientable dipole moment of TGS. A quantitative estimate of increase is obtained using the Mecke dipole function.

Infrared dielectric response of ordered and disordered ferroelectric Pb(Sc 1/2Ta 1/2)O 3 ceramics

IR reflectivity of B-site ordered and disordered Pb(Sc 1/2Ta 1/2)O 3 ceramics was measured and evaluated between 20 and 500 K. All the IR modes were assigned and their parameters and temperature dependences determined and discussed. No softening of any phonon mode was observed near the ferroelectric transition. Therefore, the transition is of order–disorder type and Pb vibrational anharmonicity seems to be the most important cause of disorder. To our knowledge, this represents the first case of prevailingly order–disorder ferroelectric phase transition among perovskite ferroelectrics. The comparison of ordered and disordered samples shows that the strength of the weak mode near 315 cm −1 assigned to Sc–Ta vibrations can be used to estimate the degree of local order in the disordered sample. This was estimated to be 21 vol.% in good agreement with 18 vol.% estimated from X-ray diffraction.

Temperature dependence of electrically induced perturbations of nuclear quadrupole interaction

The temperature dependence of electrically induced perturbations of the nuclear quadrupole interaction is experimentally found and is theoretically investigated. A good agreement between the theory and experiment is obtained through the use of the order-disorder ferroelectric phase transition model.

Effect of hydrostatic pressure and deuteration on the order of phase transition in triglycine selenate

It is known that triglycine selenate (TGSe) is isomorphous with triglycine sulphate (TGS) and triglycine fluoroberyllate (TCFB). Each of these crystals shows an order-disorder ferroelectric phase transition of the second order.1-5 This phase transition is accompanied by the change of symmetry 2/m → 2. The behaviour of TGS and TGFB in the vicinity of the phase transition can be well described by a phenomenological theory.2-5 The situation is more complicated in the case of TGSe. The experimental discontinuity of CE at the transition temperature T, exceeds 2.5 times the value predicted by the phenomenological theory.5 Other discrepancies between theoretical and experimental results for TGSe have been found6 Besides that, in TCSe, unlike TGS and TGFB, the maximum permittivity at Tc, (εmax), sharply decreases with increasing hydrostatic pressure p but the difference between the Curie and Curie-Weiss temperatures (Tc - To) increases. A new non-pyroelectric phase with unknown symmetry appears at p > 7 kbar in TGSe.7,8 Recently it was found that the phase transition in deuterated TGSe (DTGSe) was the lst-order one.9 Such unusual behaviour of TGSe requires further examination.