Ferroelectric Transition Research Articles

Terahertz time-domain spectroscopic ellipsometry of metallic strontium titanate

The coexistence of the polar instability and metallic conduction has attracted much attention. Ferroelectric soft optic modes and carrier electrons were investigated in A- and B-site heterovalent doped strontium titanate SrTiO3 crystals by the terahertz time-domain spectroscopic ellipsometry. The observed real and imaginary parts of a complex dielectric constant were fitted by the damped harmonic oscillator model for the ferroelectric soft optic mode and the Drude-Smith model for carrier electrons. The observed soft mode frequency increases as the La and Nb content increases. It is found that the square of the ferroelectric soft mode frequency linearly changes to the carrier electron concentration. This fact indicates that the carrier electrons suppress the ferroelectric instability and the ferroelectric soft mode frequency increases as the carrier concentration increases.

Effect of Gd3+/Ti4+ heterovalent substitution on the crystal structure, morphology, optical properties, and phase transition behavior of bismuth layer-structured SrBi2Nb2O9

The double-layer Aurivillius compounds Sr1-xGdxBi2Nb2-xTixO9 (x ​= ​0, 0.2, 0.4, 0.6, 0.8, and 1), which have the potential to be developed as a ferroelectric material, have been synthesized using the molten salt method. This study aims to investigate the effect of the simultaneous substitution of Gd3+ and Ti4+ ions in SrBi2Nb2O9 on the phase formation, crystal structure, morphology, and ferroelectric properties. The single-phase product with A21am orthorhombic structure was obtained for all samples by X-ray diffraction analysis. The unit cell becomes more orthorhombic with increasing x as the degree of tetragonality increases. SEM micrographs demonstrate the plate-like grain morphology with an anisotropic growth direction, and the grain is smaller in size with increasing x. FTIR spectra indicate that cation substitution was successful, especially Ti4+ ions at the B-site. With increasing x, the ferroelectric transition temperature (Tc) increases, which has been explained in terms of a reduction in the tetragonality. The open hysteresis loops provide evidence of ferroelectric properties which is consistent with the trend of room-temperature dielectric constant. The optical bandgap value is proportional to the trend of the dielectric characteristics of the samples.

Effect of Local Structural Distortions on Antiferroelectric-Ferroelectric Phase Transition in Dilute Solid Solutions of KxNa1-xNbO3.

The fundamental principles that govern antiferroelectric (AFE)-ferroelectric (FE) transitions are not well understood for many solid solutions of perovskite compounds. For example, crystal chemical considerations based on the average Goldschmidt tolerance factor or ionic polarizability do not precisely predict the boundary between the AFE and FE phases in dilute solid solutions of alkali niobates, such as KxNa1-xNbO3 (x ≤ 0.02). Here, based on detailed structural analysis from neutron total scattering experiments, we provide insights about how the relative local distortions around the A- and B-sites of the ABO3 perovskite structure affect the AFE/FE order of the average crystallographic phases in KxNa1-xNbO3. We show that a higher (lower) ratio of B-site-centered distortions over A-site-centered distortions drives transition toward a long-range FE (AFE) phase, which is based on a competition between the long-range polarizing field of the Nb-O dipoles and the disordering effect of local distortions around the A-site. Our study provides a predictive tool for designing complex solid-solution perovskites with tunable (anti)ferroelectric polarization properties, which can be of interest for various energy-related applications such as high-density energy storage and solid-state cooling.

Enhancing the relaxor character in K0.5Bi0.5TiO3 based ternary system for energy storage applications

Lead-based ferroelectric materials dominate the electronic industry due to their possible applications in sensors, actuators, advanced storage materials, microwave devices, and MEMS. Due to the toxicity of lead, there is a need for the development of environmentally compatible alternatives. Lead-free ternary solid solutions of (1−2x)K0.5Bi0.5TiO3-xBaTiO3-xLiNbO3 (KBLN100x) with x = 0.03, 0.05, 0.07, and 0.09 are synthesized using a standard solid-state reaction method. X-ray diffraction studies confirm the pseudocubic structure of the samples. Structural, Raman, dielectric and piezoelctric studies indicate that a normal to relaxor ferroelectric transition occurs as dopant concentration increases, and it is attributed to the increase in charge fluctuation and site disorder that resulted in the reduction in correlation lengths of the inbuilt dipole moments. A recoverable energy density of 1.3 J/cm3 was achieved for the KBLN7 sample, making it an attractive candidate in the energy storage realm. A strain value of 0.17% with Smax/Emax of 218 pm/V is observed for KBLN3 sample, indicating that K0.5Bi0.5TiO3 (KBT)-based systems can be used for actuator applications on further improvement. The normal-relaxor crossover of KBT with appropriate dopants indicates that KBT-based systems are efficient for both actuator and energy storage applications on improvising and tuning as required.

Electrocaloric effect at special points in the composition-temperature-electric field phase diagram of barium zirconate-titanate (BZT)

The composition-temperature-electric field diagram and electrocaloric properties of BaTi1−xZrxO3 (BZT) were computed using a self-consistent phenomenological approach. By subtracting contributions from the latent heat, the adiabatic temperature change due solely to the material polarization was quantified on passing through critical end, tricritical and phase convergence points. The largest temperature change occurs at the first-order paraelectric to ferroelectric transitions where the latent heat is triggered, but the material polarization contribution is greatest at the critical temperature. The maximum electrocaloric responsivity occurs at a field 1.25 times greater than the critical field. The temperature change increases at the tricritical point due to a field-induced tetragonal to rhombohedral transformation, but the responsivity diverges due to vanishing of the critical field. The enhanced temperature change observed experimentally near the convergence point is attributed to greater polarization change in the rhombohedral phase, and to a polar glasslike state that contributes to the dipolar configurational entropy.

Pressure-induced ferroelectric transition in LiBC

Ferroelectricity is usually suppressed by pressure. Here, we establish a general phenomenological theory to describe pressure-induced ferroelectrics and then relate it to real materials. Based on first-principles calculations, we identify a pressure-induced ferroelectric transition in which the nonpolar LiBC in the ZrBeSi structure type transforms into the polar state in the LiGaGe structure type at about 108 GPa. The pressure-induced ferroelectric LiBC has a larger polarization of $1.95\phantom{\rule{0.28em}{0ex}}\mathrm{C}/{\mathrm{m}}^{2}$ and band gap of 3.27 eV at 110 GPa. A ferroelectric multiple-well energy as a function of polar distortion is found. The pressure-induced ferroelectric transition is driven by the softness of one of the ${A}_{2u}$ modes in the nonpolar LiBC above 60 GPa. We also find an enhancement of a dynamic transfer of charge along the B--C bond under pressure, which indicates that the ionic electrostatic interactions become stronger. Our work demonstrates the existence of pressure-induced ferroelectrics and thus provides opportunities to design devices with ferroelectrics working on high-pressure conditions.

Lattice dynamics and Raman spectrum of supertetragonal PbVO3

Lead vanadate PbVO3 is a polar crystal with a P4mm space group under ambient conditions. PbVO3 is isostructural with the model soft mode-driven ferroelectric PbTiO3, but it differs due to the so-called “supertetragonal” elongation of its unit cell. In this study, we investigated the lattice dynamics of PbVO3 based on Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical phonon modes were identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374, and 508 cm−1; A1 modes at 188, 429, and 874 cm−1, and B1 mode at 319 cm−1. The calculations confirmed the experimental symmetry assignment and allowed us to obtain the longitudinal optical phonon wavenumbers. In addition, we analyzed the mode eigenvectors in detail in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. Despite differences in chemistry and strain, the phonon eigenvectors were found to be highly comparable in both compounds. We investigated the position of the ferroelectric soft mode in PbVO3 compared with PbTiO3. Sizeable splitting of the B1+E modes appeared as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V–O bond frequency in PbVO3 was also addressed.

Dielectric properties of ammonium iron sulphate-dodecahydrate alum crystal

The statistical Green function theory and extended two sublattice pseudospin coupled-mode model Hamiltonian. The modified model summing with phonon anharmonic couplings, extra spin-lattice couplings, direct spin-spin coupling together with an applied electric field has been used to explain the dielectric properties of ammonium iron alum crystal. The expressions for energy shift, linewidth, ferroelectric soft mode frequency, dielectric permittivity, and dielectric loss tangent are derived. By fitted the model values of different parameters in the above theoretical expressions. The temperature dependence of soft mode frequency, dielectric permittivity, and dielectric loss properties have been numerically calculated. Our theoretical obtained results compared well agreement with experimental results reported by others.

Insights into the structural symmetry of YCrO3 from synchrotron X-ray diffraction

A high-resolution synchrotron X-ray diffraction study of a single-crystal YCrO3 compound was employed to obtain its crystallographic information, such as lattice parameters, atomic positions, bond lengths and angles, and local crystalline distortion size and mode. The measurements were taken at 120 K (below the antiferromagnetic transition temperature T N ≃ 141.5 K), 300 K (between T N and the ferroelectric transition temperature T C ≃ 473 K) and 500 K (above T C). Using the high intensity of synchrotron X-rays, it was possible to refine collected patterns with the previously proposed noncentrosymmetric monoclinic structural model (P1211, No. 4) and determine detailed structural parameters. Meanwhile, for a controlled study, the data were refined with the centrosymmetric orthorhombic space group (Pmnb, No. 62). The lattice constants a, b and c and the unit-cell volume increased nearly linearly upon heating. With the P1211 space group, the distributions of bond lengths and angles, as well as local distortion strengths, were observed to be more dispersed. This implies that (i) the local distortion mode of Cr2O6 at 120 K correlates with the formation of canted antiferromagnetic order by Cr1–Cr2 spin interactions, primarily via intermediate O3 and O4 ions; and (ii) the previously reported dielectric anomaly may have a microscopic origin in the strain-balanced Cr1—O3(O4) and Cr2—O5(O6) bonds as well as the local distortion modes of Cr1O6 and Cr2O6 octahedra at 300 K. Local crystalline distortion is shown to be an important factor in the formation of ferroelectric order. The comprehensive set of crystallographic information reported here allows for a complete understanding of the unique magnetic and ferroelectric properties of YCrO3.

Incommensurate Modulation and Competing Ferroelectric/Antiferroelectric Modes in Tetragonal Tungsten Bronzes

The nanoscale structure of Sr0.61Ba0.39Nb2O6, a classic uniaxial relaxor ferroelectric crystallizing with the tetragonal tungsten bronze (TTB) structure and exhibiting an incommensurate modulation, has been determined by atomistic refinements using combined data from variable-temperature neutron total scattering, extended X-ray absorption fine structure, and three-dimensional single-crystal diffuse scattering. We found the modulation to arise from the intergrowth of structural slabs featuring distinct types of octahedral rotations directed to minimize octahedral deformations. This modulation involves displacements of the A cations (Sr and Ba) but exerts no significant effect on the polar displacements of Nb. Our results demonstrated the coexistence of competing polar Γ3– and antipolar Γ2– modes for off-centering of Nb above the nominal ferroelectric transition temperature of 350 K. The nanoscale-correlated antipolar distortions, which exhibit their largest amplitudes at 425 K, are suppressed below the transition. We identified coupling between the polar off-centering of Nb on its two symmetrically distinct sites as a fundamental characteristic controlling the ferroelectric-to-relaxor crossover in TTBs. We then used it to propose a simple microscopic interpretation for the previously established empirical trend that links crystal–chemical parameters in these systems to their polar response.

Partial breakdown of translation symmetry at a structural quantum critical point associated with a ferroelectric soft mode

We report that complete suppression of a phonon-driven structural phase transition causes partial breakdown of a three-dimensional translation symmetry in a well-defined sublattice. This state is revealed for a dielectric compound, ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$, that comprises an ${\mathrm{AlO}}_{4}$ network incorporated into a hexagonal Ba(Sr) sublattice. Pair distribution function analyses and inelastic neutron scattering experiments provide clear-cut evidence of the ${\mathrm{AlO}}_{4}$ network forming a continuum of Al-O short-range correlations similar to glasses, whereas the Ba(Sr) sublattice preserves the original translational symmetry. This glassy network significantly dampens the phonon spectrum and transforms it into the broad one resembling those typically observed in glass materials.

Substitution effect of Ni2+ on the magnetism and electrical properties of Ca3CoMnO6 compound

A series of Ni2+ doped Ca3Co1-xNixMnO6 (0 ≤ x ≤ 0.15) samples were prepared for this study. Their structure, magnetism and electrical properties were investigated. The results show that the spin of Ni2+ is antiferromagnetically coupled to the neighbor Mn4+ cations. In Ni2+ substituted samples, the short-range magnetic correlation is inhibited and the long-range antiferromagnetic structure is strengthened. Due to the random distribution of Ni2+ substitution in Co2+ sites, the macroscopical ferroelectric domains of Ca3CoMnO6 are destroyed and more polar nanoregions form. With the increase of Ni2+ content, the relaxor behavior of the ferroelectric transition becomes stronger and the activation energy Ea decreases monotonously. The enhanced antiferromagnetic interaction also reinforces the dipole correlation, leading to the increased static freezing temperature Tf in Ni2+ doped samples. Magnetodielectric coupling is significantly improved due to the magnetic structural variation produced by doping of compound with Ni2+ cations. For the x = 0 sample, the maximal deviation of the dielectric permittivity at H = 7 T relative to the value at H = 0 is about 2.25 %. While for the x = 0.1 and 0.15 samples, this value is 7.5 % at H = 7 T.

Slippery Paraelectric Transition-Metal Dichalcogenide Bilayers.

Traditional ferroelectrics undergo thermally induced phase transitions whereby their structural symmetry increases. The associated higher-symmetry structure is dubbed paraelectric. Ferroelectric transition-metal dichalcogenide bilayers have been recently shown to become paraelectric, but not much has been said of the atomistic configuration of such a phase. As discovered through numerical calculations that include molecular dynamics here, their paraelectricity can only be ascribed to a time average of ferroelectric phases with opposing intrinsic polarizations, whose switching requires macroscopically large areas to slip in unison.

Role of phonon mode in the enhancement of ferroelectric polarization in a perovskite-based eco-friendly functional material

We report here the role of component freezing of three-dimensional polar () phonon mode corresponding to the center of cubic Brillouin zone in tuning the structure-property correlations of a scientifically enriched and technologically important barium-titanate–based eco-friendly functional material (Ba0.92Ca0.08)(Zr0.05Ti0.95−x Sn x )O3; BCZTSnx synthesized via solid-state reaction method. The combined X-ray diffraction, Raman spectroscopic analysis, and temperature-dependent dielectric studies have revealed the presence of several crystallographic phase transitions with coexisting phases, viz., , as a function of Sn(x) content. These crystallographic phases, viz., P4mm, Amm2, and R3m results due to freezing of the component(s) of phonon mode (belonging to space group), with the respective order parameter directions (0,0,a), (a,a,0), and (a,a,a) leading to ferroelectric polarization along , and directions, respectively. The ceramic composition corresponding to exhibits a significant reduction in the coercive field (E c ) and an enhancement in ferroelectric polarization (P r ) in comparison to , inferred from PE loop measurements. The enhancement in ferroelectric polarization at has been attributed to the inter-ferroelectric three-phase coexistence around this composition and significantly enhanced amplitudes of ferroelectric phonon modes corresponding to orthorhombic and rhombohedral phases, calculated using the symmetry mode analysis technique. The existence of a high ferroelectric polarization and low coercive field may lead to composition as an eco-friendly candidate for ferroelectric memory devices.

Short-range magnetic order within the multiferroic erythrosiderite mineral(NH4)2FeCl5·H2O

Molecular multiferroics are receiving a large amount of interest, in part due to the ease of synthesis and their potential for tuneability. An important compound within this field is ammonium pentachloroaquaferrate(III), ${({\mathrm{NH}}_{4})}_{2}{\mathrm{FeCl}}_{5}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$, which has received a lot of attention due to it having both magnetic and ferroelectric transitions at 7.3 and 6.9 K, respectively. The magnetic structure is complex, with many different exchange pathways; however, to date there has been no consideration of magnetic interactions above the critical temperature. Our paper utilizes muon spin spectroscopy, inelastic neutron scattering and quasielastic neutron scattering (QENS) to show that, as one might expect for these complex magnetic systems, there is indeed evidence for short-range order above 7.3 K and this persists up to $\ensuremath{\sim}15$ K. We also highlight the sensitivity of the QENS technique to the magnetic transition and any localized fluctuations.

Thermodynamics and dielectric response of BaTiO3 by data-driven modeling

Modeling ferroelectric materials from first principles is one of the successes of density-functional theory and the driver of much development effort, requiring an accurate description of the electronic processes and the thermodynamic equilibrium that drive the spontaneous symmetry breaking and the emergence of macroscopic polarization. We demonstrate the development and application of an integrated machine learning model that describes on the same footing structural, energetic, and functional properties of barium titanate (BaTiO3), a prototypical ferroelectric. The model uses ab initio calculations as a reference and achieves accurate yet inexpensive predictions of energy and polarization on time and length scales that are not accessible to direct ab initio modeling. These predictions allow us to assess the microscopic mechanism of the ferroelectric transition. The presence of an order-disorder transition for the Ti off-centered states is the main driver of the ferroelectric transition, even though the coupling between symmetry breaking and cell distortions determines the presence of intermediate, partly-ordered phases. Moreover, we thoroughly probe the static and dynamical behavior of BaTiO3 across its phase diagram without the need to introduce a coarse-grained description of the ferroelectric transition. Finally, we apply the polarization model to calculate the dielectric response properties of the material in a full ab initio manner, again reproducing the correct qualitative experimental behavior.

Ultrahigh Piezoelectric Strains in PbZr1-xTixO3 Single Crystals with Controlled Ti Content Close to the Tricritical Point.

Intensive investigations of PbZr1-xTixO3 (PZT) materials with the ABO3 perovskite structure are connected with their extraordinary piezoelectric properties. Especially well known are PZT ceramics at the Morphotropic Phase Boundary (MPB), with x~0.48, whose applications are the most numerous among ferroelectrics. These piezoelectric properties are often obtained by doping with various ions at the B sites. Interestingly, we have found similar properties for undoped PZT single crystals with low Ti content, for which we have confirmed the existence of the tricritical point near x~0.06. For a PbZr0.95 ± 0.01Ti0.05 ∓ 0.01O3 crystal, we describe the ultrahigh strain, dielectric, optical and piezoelectric properties. We interpret the ultrahigh strain observed in the region of the antiferroelectric–ferroelectric transition as an inverse piezoelectric effect generated by the coexistence of domains of different symmetries. The complex domain coexistence was confirmed by determining optical indicatrix orientations in domains. The piezoelectric coefficient in this region reached an extremely high value of 5000 pm/V. We also verified that the properties of the PZT single crystals from the region near the tricritical point are incredibly susceptible to a slight deviation in the Ti content.

Significant study of BaTiO3 as a cathode for magnesium battery applications

The attempts to tailor the physical properties of barium titanate (BTO) to qualify it as a promising cathode for metal-ion batteries are interesting strategies. Herein, the paper attempts to modify the structure of BTO via electrochemical insertion of Mg ion within the framework of BTO. Introduction of Mg2+ ions within the framework of BTO drives the characteristic ferroelectric transition from 388K to 393K; furthermore, heat treatment of BTO beyond ferroelectric transition temperature Tc redistributes Mg ions within the framework of BTO, which results in downshifting of the Tc near the room temperature. XRD results confirm magnesiation and heat treatment of the magnesized BTO stimulates structure transformation from tetragonal to a cubic phase. The synthesized liquid Mg-electrolyte has exhibited an excellent LSV and Mg-platting/stripping. The assembled Mg/BTO cells with magnesized BTO cathode has relatively high initial specific capacity of 179.7 mAhg−1, compared with the pristine BTO of 72.5 mAhg−1.

Effects of Sm-substitution on dielectric, ferroelectric, and piezoelectric properties of 0.36(Bi1-xSmx)ScO3-0.64PbTiO3 ceramics

Dielectric, ferroelectric, and piezoelectric properties of 0.36(Bi1-xSmx)ScO3-0.64PbTiO3 (BSPT-xSm) ceramics were investigated to assess effects of Sm-substitution on 0.36BiScO3-0.64PbTiO3 for high temperature piezoelectric device application. Optimal sintering was achieved at 1200°C when the BSPT-xSm ceramics were fully densified and crystallized with a perovskite structure without any secondary phase. The substitution of Bi3+ with Sm resulted in degradation of rhombohedral side in BSPT-xSm ceramics having morphotropic phase boundary. In addition, variations of grain size and ferroelectric behavior after Sm-substitution were insignificant. However, dielectric constant (εT33/ε0) was significantly enhanced with an increasing of amount of Sm to 5%. Although a slight decrease of relative density in case of x exceeding 3% led to deterioration of piezoelectric values of d33, kp, and d33*, the BSPT-3%Sm ceramic exhibited excellent values of d33 of 628 pC/N, kp of 62.4%, and d33* of 718 pm/V at 4.5 kV/mm, along with a high ferroelectric transition temperature of 421°C. The highly increased diffusion coefficient of 1.909 also implies that the Sm-substitution contributed to relaxor-like ferroelectric behavior of BSPT ceramics.

Terahertz lattice and charge dynamics in ferroelectric semiconductor SnxPb1−xTe

The symmetry breaking induced by the ferroelectric transition often triggers the emergence of topological electronic states such as Weyl fermions in polar metals/semimetals. Such strong coupling between the lattice deformation and electronic states is therefore essentially important for the control of versatile topological phases. Here, we study the terahertz lattice and charge dynamics in ferroelectric semiconductor SnxPb1-xTe thin films hosting versatile topological phases by means of the terahertz time-domain spectroscopy. With lowering the temperature, the resonant frequency of transverse optical (TO) phonon shows the significant softening and upturn. This temperature anomaly of lattice dynamics directly indicates the displacive-type ferroelectric transition. The resulting phase diagram suggests the enhancement of ferroelectricity in the films possibly due to compressive strain compared with the bulk crystals. The low-energy TO phonon induces the large DC and terahertz dielectric constant even in metallic state. Furthermore, we find that the Born effective charge of phonon mode is enhanced at around the compositions showing the band gap closing associated with the topological transition.