Ferroelectric Phase Transition Research Articles

The effect of hydrostatic compression on the crystal structure of glycinium phosphite

The crystal structure of glycinium phosphite (GPI) was studied in the pressure range from ambient to 6.5 GPa at 293 K using single-crystal X-ray diffraction. The changes in the unit-cell volume and parameters were continuous and anisotropic. The major compression was observed normal to the direction of the spontaneous polarization that occurs in this structure during a ferroelectric phase transition on cooling, whereas the structural compression along the b axis coinciding with the 21 axis was almost zero. The effect of pressure on the hydrogen bonds linking the H2PO3 tetrahedra into zigzag chains along the c axis was different from that on the hydrogen bonds connecting the glycinium cations with the H2PO3 tetrahedra in the (b × c) plane. The discontinuous changes in the geometries of selected hydrogen bonds at about 1.21 GPa may be evidence of a phase transition, e.g. into an ordered ferroelectric phase (with ordered positions of protons). The anisotropy of compression of GPI in the ferroelectric state (at 0 K) was studied using DFT calculations taking into account dispersive van der Waals interactions. The calculations predicted negative linear compressibility along the b axis.

Dielectric, piezoelectric and electrostrictive properties of antiferroelectric lead-zirconate thin films

Dielectric, piezoelectric and electrostrictive properties of antiferroelectric lead zirconate thin films, elaborated by sol gel on alumina substrates, have been studied as a function of the driving field magnitude EAC. Measurement of the displacement shows that the strain for applied fields below the antiferroelectric-ferroelectric transition is relatively small and that its main contribution arises from the electrostrictive effect. At the same time, due to the presence of a residual ferroelectricity, the piezoelectric effect also contributes to the displacement. At higher fields, a large strain is visible which comes mainly from the antiferroelectric to ferroelectric phase transition, the electrostrictive contribution however still being present. Similar to what has been shown for ferroelectric materials, strain versus the square of polarization loops S(P2) of the studied antiferroelectric material, exhibit hysteresis character due to the 180∘ domain walls which contribute to polarization but not to strain. Simultaneous measurement of polarization and displacement enables the extraction of the electrostrictive coefficient and a value Q = 0.082 ± 0.009 m4 C−2 has been obtained. Subtraction of the pure electrostrictive contribution from the displacement curve allows evidencing that the piezoelectric activity coexists with the electrostrictive effect at low fields. Maximum values of the equivalent piezoelectric coefficients are respectively 93 ± 3 pm V−1 and 100 ± 3 pm V−1 for the positive and the negative parts of the curve for a driving field magnitude EAC = 700 kV cm−1.

The correlation between electric polarization and magnetic properties in [N(C2H5)4]2CoCl2Br2 crystal at low temperatures

The ferroelectric and magnetic properties of the potential magnetic multiferroic – [N(C2H5)4]2CoCl2Br2 solid solution are reported. This crystal was found to possess the proper ferroelectric phase transition at T1 = 249.5 K and possible magnetoelectric interactions at lower temperatures. The spontaneous polarization exists down to 1.8 K, manifesting the step like anomalies at the temperatures 223.1 K and 6.9 K. The spontaneous polarization arises below the Curie point due to ordering of the inorganic anions carrying a dipole moment. There were observed the correlations between the magnetic state of Co(II) ions and electric polarization. At the lowest temperatures, the energy barrier between the two Kramers doublets can be reduced and the doublet with MS = ±3/2 becomes the ground state at sufficiently high fields. Due to the magnetoelectric correlations, the magnetic field affects the resulting electric polarization that looks very important for possible applications in the functional electronics.

Origin of the Large Entropy Change in the Molecular Caloric and Ferroelectric Ammonium Sulfate

AbstractThe deceptively simple inorganic salt ammonium sulfate undergoes a ferroelectric phase transition associated with a very large entropy change and both electrocaloric and barocaloric functionality. While the structural origins of the electrical polarisation are now well established, those of the entropy change have been controversial for over 50 years. This question is resolved here using a combination of density‐functional theory phonon calculations with inelastic neutron scattering under variable temperature and pressure, supported by complementary total and quasielastic neutron scattering experiments. A simple model of the entropy in which each molecular ion is disordered across the mirror plane in the high symmetry phase, although widely used in the literature, proves to be untenable. Instead, the entropy arises from low‐frequency librations of ammonium ions in this phase, with harmonic terms that are very small or even negative. These results suggest that, in the search for molecular materials with functionality derived from large entropy changes, vibrational entropy arising from broad energy minima is likely to be just as important as configurational entropy arising from crystallographic disorder.

Insight into Pyroelectricity and Phase Transitions in Ferroelectrics from Nonequilibrium Approach: The Case of PbTiO3

AbstractComputational methodology is proposed to simulate time dependent temperature variations via thermostat approach. The methodology is applied to study pyroelectricity under nonequilibrium conditions in prototypical ferroelectric PbTiO3. It is found that at room temperature the nonequilibrium effects in pyroelectricity begin manifesting above 1.0 THz and originate from the overlap with the intrinsic soft mode frequency. Therefore, the proposed computational methodology can be used to predict equilibrium pyroelectric response by choosing a frequency of temperature variation below the intrinsic polar mode frequencies for the material. Probing the dynamics of ferroelectric phase transition with the proposed methodology revealed that it takes about a nanosecond for the computational supercell to undergo phase transition. It is concluded that computational techniques capable of simulating picoseconds can be used to compute pyroelectric response from nonequilibrium dynamical approach while those that can reach nanoseconds are suitable for simulating phase transitions in ferroelectrics.

Strain-engineered divergent electrostriction in KTaO3

We investigate the electrostrictive response across a ferroelectric phase transition from first-principles calculations and show that $M$, the field-induced electrostrictive tensor, controlling the amplitude of the electric-field induced strain, can be made arbitrarily large through strain engineering. We take as a case study the epitaxial strain-induced transition from para- to ferroelectricity of ${\mathrm{KTaO}}_{3}$. We show that the magnitude of the field-induced electrostriction diverges with the permittivity at the transition, hence exhibiting giant responses through a calculation of both the $M$ and $Q$ electrostrictive tensors. We explain the origin of this giant electrostrictive response in ${\mathrm{KTaO}}_{3}$ using a microscopic decomposition of the electrostriction coefficients, and use this understanding to propose design rules for the development of future giant electrostrictors for electromechanical applications. Finally, we introduce a further means to calculate electrostriction, specific to ferroelectrics, and not yet utilized in the literature.

Synthesis and Characterization of a Displacement-Type Ferroelectric-Ferroelectric Phase Transition Compound [(NH3)(CH2)3(NH3)]2[InBr6]Br·H2O.

Ferroelectric materials have aroused the researchers' great interest due to their wide applications. Here, a displacement-type ferroelectric-ferroelectric phase transition material [(NH3)(CH2)3(NH3)]2[InBr6]Br·H2O (1) with Tc = 143 K was successfully prepared. The ferroelectric phase transition is verified by the characterization techniques such as differential scanning calorimetry, single-crystal structure elucidation, dielectric and ferroelectric measurements. The single-crystal structure elucidation reveals that the displacement and distortion of [(NH3)(CH2)3(NH3)]2+ cations lead to the phase transition from Cmc21 to Pca21. The spontaneous polarizations at 293 and 133 K are 0.15 and 0.12 μC·cm-2, respectively. We expect that this work will help in further exploration of some new ferroelectric materials.

Stable large-area monodomain in as-grown bulk ferroelectric single crystal Sn2P2S6

Driven by the minimization of total energy, the multi-domain morphology is preferred in as-grown ferroelectrics to reduce the depolarization and strain energy during the paraelectric to ferroelectric phase transition. However, the complicated multi-domain is not desirable for certain high-performance ferroelectric electro-optic devices. In this work, we achieve a reproducible and stable large-area monodomain in as-grown bulk ferroelectric single crystal [Formula: see text]. The monodomain dominates the entire single crystal, which is attributed to the internal charge carriers from the photoexcited disproportionation reaction of Sn ions. The charge carriers effectively screen the depolarization field and therefore decrease the depolarization energy and facilitate the formation of monodomain. This work offers a potential approach for engineering bulk ferroelectrics with a stable monodomain, which is desirable for the high-performance ferroelectric electro-optic devices.

Phase transition enhanced pyroelectric nanogenerators for self-powered temperature sensors

Pyroelectric materials are of interest for waste heat utilization and thermal detection. However, the low output current and inefficiency reduces their effectiveness. Here, we utilize the abrupt decrease in polarization of ferroelectric BaTiO3 materials around Curie temperature to improve the output performance of a pyroelectric nanogenerator. The variation of the polarization leads to a large change in the density of surface free charges, resulting in an increase of pyroelectric current. We have designed a temperature control and recording system to realize direct measurement of the pyroelectric output current. The pyroelectric current, power and energy conversion efficiency of the device near the Curie temperature were measured to be 15.6-fold, 18-fold and 15.8-fold higher than those acquired near room temperature. Moreover, the temperature induced current and charge density enhancement can be applied to detect temperature and temperature change. The responsivity of the self-powered temperature sensor near Tc is 120 nC/cm2∙K, which 4.8 times higher than that near room temperature (25.1 nC/cm2∙K). The results confirmed the ability to exploit a ferroelectric phase transition for pyroelectric performance enhancement.

Role of Defects and Structure Evolution across Ferroelectric Phase Transitions Studied by Quantitative Aberration-Corrected STEM

Role of Defects and Structure Evolution across Ferroelectric Phase Transitions Studied by Quantitative Aberration-Corrected STEM

Ultrahigh electric‐field‐induced strain in KNN‐based piezoelectric ceramics sintered in reducing atmosphere

AbstractWith the goals of sustainable development adopted by more and more nations, the lead‐free trend of multilayer ceramic actuators has become a widely recognized consensus. Sintering in a reducing atmosphere is a necessary approach for preparing lead‐free ceramics co‐fired with low‐cost nickel internal electrodes. Herein, (1 − x)K0.48Na0.52Nb0.96Ta0.04O3–xSr(Zr0.5Ti0.5)O3 + 0.03ZrO2 + 0.08MnO2 (KNNT‐SZT) ceramics were sintered in a reducing atmosphere using a solid‐state reaction. The Sr(Zr0.5Ti0.5)O3 (SZT) dopant decreased the Curie temperature TC and the O–T phase transition temperature. The enhanced diffuseness of the ferroelectric phase transition of KNNT‐SZT ceramics is attributed to the element doping and the locally inhomogeneous microstructure. High inverse piezoelectric coefficients d33* of 883 and 787 pm/V were obtained for the 8‐ and 9‐mol% SZT‐doped samples, respectively. These results were attributed to the synergistic effect of ferroelectric domains with different sizes. The diffused O–T phase transition significantly improved the piezoelectric temperature stability of the 9‐mol% SZT‐doped sample, whose electric‐field‐induced strain decreased by <10% from room temperature (25°C) to 115°C. The cyclic fatigue test showed that >98% of the initial d33* was retained after 106 cycles of unipolar electrical loading. The KNNT‐SZT ceramics sintered in a reducing atmosphere possessed excellent inverse piezoelectric properties and are expected to be applied in the field of multilayer ceramic piezoelectric actuators with base metal internal electrodes.

The ferroelectric phase transition in a 500 nm sized single particle of BaTiO3 tracked by coherent X-ray diffraction

The shapes and sizes of grains influence the anisotropy of crystal structures and the configuration of ferroelectric domains. In order to better understand these effects, we introduce a method to observe a ferroelectric phase transition in a single ferroelectric particle of sub-micrometer size. The phase transition was observed by cooling the sample through its Curie temperature, and studying the pattern variations of Bragg coherent X-ray diffraction from a single particle of 500 nm sized BaTiO3. A change from a single 200 peak (cubic phase) to both 200 and 002 peaks (tetragonal phase); was seen, with fringes connecting them like a bridge. The pattern from the BaTiO3 particle in the cubic phase was also imaged using Bragg coherent diffraction imaging. The apparent strain distribution caused by dislocation internal to the BaTiO3 crystal was visible in the reconstructed image.

Pressure-driven ferroelectric phase transition for the Pnma-CsPbBr3: Mechanical and dynamical stability study

Using the first-principles method, a new ferroelectric phase transition from the Pnma structure to the P212121 polar structure is predicted under a hydrostatic pressure above 3.6 ​GPa for CsPbBr3 compound. This phase transition results from the condensation of the polar atomic displacements of the silent soft mode Au at the center of the Brillouin zone. The off-center displacement of Pb is dominated relative to that of Cs and Br atoms in this soft mode. Besides, the structural, elastic, dynamic, and electronic properties of both structures are reported in the present work. The Pnma structure shows mechanical stability under pressure and a large resistance to unidirectional compression along y-axis comparing to the shear deformation. The application of pressure decreases the ductility and enhances the stiffness of the compound.

Suppressing lone-pair expression endows room-temperature cubic structure and high thermoelectric performance in GeTe-based materials

The stereochemical expression of ns2 lone-pair electrons in specific Ⅳ-Ⅵ group semiconductors is pivotal to lattice instability, which induces ferroelectric phase transition. In the thermoelectric device working temperature range, the distinct change of thermal expansion coefficient, however, is introduced by phase transition and then severely restrains material's critical applications. Herein, taking the example of thermoelectric GeTe, the hidden role of manipulating lone pair expression to control the crystal structure is theoretically unraveled. Moreover, it is experimentally demonstrated that LiBiTe2-alloyed GeTe possess a room-temperature cubic structure and competitive thermoelectric performance. Density functional theory calculations reveal that reducing the charge orientation caused by the stereochemical activity of cation lone-pair electrons in GeTe is an effective strategy to achieve room-temperature cubic structure. LiBiTe2 is found to be a novel alloying agent to realize crystal structure manipulating, carrier concentration optimization, multi-band convergence, and lattice softening. Benefiting from the optimized electrical and thermal transport properties, an ultrahigh ZTave of 1.22 within 300–773 K in cubic (GeTe)0.80(LiBiTe2)0.20 is achieved. This work provides a new insight into manipulating lone pair expression to control crystal structure, and promotes the development and application of cubic GeTe-based materials.

The Influence of the Structural Features of Molecules on Spontaneous Polarization Value in Induced Ferroelectric Liquid Crystal Systems

The occurrence of the large value of spontaneous polarization is established in LC system whose composition`s molecular structure does not satisfy the obligatory conditions for the occurrence of spontaneous polarization that are imposed by theory on a molecular structure. The essential role of short range intermolecular forces in the process of ferroelectric phase transition in the studied LC systems is shown. It was found that for {CC}, {DD} and {AD} interactions the most beneficial molecules` packing in terms of energy is the antiparallel one . In the case of {AA} and {AC} interactions both parallel and are equally beneficial in terms of energy. The possibility of formation of molecular clusters–dimers is shown. It is concluded that these are stabilized, in particular, by dipole–dipole interaction of molecules. A dimer model for increasing the value of spontaneous polarization is proposed.

Effects of vanadium doping on the charge ordering and low-temperature spin-glass phase in Pr0.45Ca0.55MnO3

We report structural, magnetic, and dielectric properties of the Pr0.45Ca0.55Mn1-xVxO3 (x = 0.05 and 0.1) polycrystalline compounds, prepared by solid state reaction method. Pnma space group and orthorhombic crystal structure of the compounds are confirmed by the Rietveld refinement of X-ray diffraction data. Magnetic study reveals a multiple-phase-separated metastable magnetic behavior with charge ordered phase followed by a spin-glass state at the low-temperature region. For a single change of vanadium content from x = 0.05–0.1, the charge ordering temperature decreases from 245 K to 239 K, the antiferromagnetic ordering temperature decreases from 131 K to 127 K, and the ferromagnetic cluster-glass ordering temperature increases from 41 K to 44 K. This indicates the weakening of charge ordering strength and development of ferromagnetic components in the antiferromagnetic background, which is also confirmed by isothermal hysteresis. Colossal dielectric constant is observed in both samples. In the measured temperature range, an implication of a ferroelectric phase transition around 253 K is observed in Pr0.45Ca0.55Mn0.90V0.10O3 compound. The calculated results indicate that the low-temperature relaxation follows variable range hopping behavior, whereas the high-temperature one takes place according to the Arrhenius model. The fitting parameters ascribe the low-temperature relaxation process to the hopping of polaron charge carriers at localized sites, whereas the high-temperature relaxation is related to Maxwell-Wagner relaxation, caused by blocking of charge carriers at grain boundaries.

Elastic and dielectric responses of VDF70/Tr30 near ferroelectric phase transition

The infra-low frequency elastic modulus and the complex dielectric permittivity of vinylidene fluoride trifluoroethylene VDF70/Tr30, are studied within the temperature range of 20 ÷ 140 °C. The experimental results are used to determine the temperature dependence of order parameter η near the Curie temperature (T c). It is revealed that spontaneous polarization P s and spontaneous strain xs are linearly coupled. The dielectric dispersion, due to the relaxation dynamics of polar microregions in the vicinity of the T c, is found. The average relaxation time follows the Arrhenius law with activation energy depending on temperature.

Dielectric properties of composites based on triglycine sulfate and nanocrystalline cellulose

Dielectric properties of mixed composites based on TGS and nanocrystalline cellulose (1-x)TGS-xnCell composites with x = 0.2; 0.7, and 0.9 (x is the mass fraction of the cellulose) were studied within temperature range of 20 ÷ 120 °C. A smearing of the ferroelectric phase transition and an increase in its temperature in TGS crystallites included in the composite were found. An analysis of the experimental results for the 0.3TGS-0.7nCell composite showed that the diffuse ferroelectric phase transition ended abruptly in the vicinity of the temperature T m ≈ 100 °C due to a structural rearrangement in TGS particles.

Strain and electric field control of magnetic and electrical transport properties in a magnetoelastically coupled Fe3O4/BaTiO3 (001) heterostructure

We present a study of the control of electric field induced strain on the magnetic and electrical transport properties in a magneto-elastically coupled artificial multiferroic Fe3O4/BaTiO3 heterostructure. In this Fe3O4/BaTiO3 heterostructure, the Fe3O4 thin film is epitaxially grown in the form of bilateral domains, analogous to a-c stripe domains of the underlying BaTiO3(001) substrate. By in-situ electric field dependent magnetization measurements, we demonstrate the extrinsic control of the magnetic anisotropy and the characteristic Verwey metal-insulator transition of the epitaxial Fe3O4 thin film in a wide temperature range between 20-300 K, via strain mediated converse magnetoelectric coupling. In addition, we observe strain induced modulations in the magnetic and electrical transport properties of the Fe3O4 thin film across the thermally driven intrinsic ferroelectric and structural phase transitions of the BaTiO3 substrate. In-situ electric field dependent Raman measurements reveal that the electric field does not significantly modify the anti-phase boundary defects in the Fe3O4 thin film once it is thermodynamically stable after deposition and that the modification of the magnetic properties is mainly caused by strain induced lattice distortions and magnetic anisotropy. These results provide a framework to realize electrical control of the magnetization in a classical highly correlated transition metal oxide.

Dielectric magnetochiral anisotropy

The combination of chirality and magnetism has steadily grown over the last decennia into an area of intense research. Magnetochiral anisotropy, chirality-induced spin-selectivity and helimagnetism are the most prominent phenomena resulting from this combination, touching different systems like topological (semi-)metals and insulators, quantum magnets, type II multiferroics and enantio-selective synthesis. As an extension to this area, we argue, based on symmetry arguments, that magnetochiral anisotropy will manifest itself in the displacement current in chiral dielectrics in a magnetic field. We confirm this conjecture by the experimental observation of very strong dielectric magnetochiral anisotropy near the ferroelectric phase transitions of triglycine sulfate and Rochelle salt, two of the oldest and most investigated chiral ferroelectrics. This effect opens a new playground in the study and characterisation of all chiral dielectrics. With our discovery, magnetochiral anisotropy now covers the (di)electrical properties of all condensed matter, from insulators to superconductors.