Ferroelectric-paraelectric Transition Research Articles

Spectral weight reduction of two-dimensional electron gases at oxide surfaces across the ferroelectric transition

The discovery of a two-dimensional electron gas (2DEG) at the hbox {LaAlO}_3/hbox {SrTiO}_3 interface has set a new platform for all-oxide electronics which could potentially exhibit the interplay among charge, spin, orbital, superconductivity, ferromagnetism and ferroelectricity. In this work, by using angle-resolved photoemission spectroscopy and conductivity measurement, we found the reduction of 2DEGs and the changes of the conductivity nature of some ferroelectric oxides including insulating Nb-lightly-substituted hbox {KTaO}_3, hbox {BaTiO}_3 (BTO) and (Ca,Zr)-doped BTO across paraelectric-ferroelectric transition. We propose that these behaviours could be due to the increase of space-charge screening potential at the 2DEG/ferroelectric regions which is a result of the realignment of ferroelectric polarisation upon light irradiation. This finding suggests an opportunity for controlling the 2DEG at a bare oxide surface (instead of interfacial system) by using both light and ferroelectricity.

Enhanced Spontaneous Polarization by V4+ Substitution in a Lead-Free Perovskite CaMnTi2O6.

Spontaneous polarization (Ps) of novel order-disorder type lead-free ferroelectric CaMnTi2O6 was successfully enhanced by partial V4+ substitution for Ti4+. A synchrotron X-ray diffraction study revealed that the polar displacement of octahedrally coordinated (Ti, V) in CaMn(Ti1-xVx)2O6 (0 ≤ x ≤ 0.4) increases with V4+ substitution having Jahn-Teller activity owing to the d1 electronic configuration. Our magnetic study suggested the presence of antisite disorder between Ca2+ and square planar coordinated Mn2+ associated with Mn-V intermetallic charge transfer for x ≥ 0.4, resulting in decreases in spontaneous polarization and the ferroelectric-paraelectric transition temperature. This is the first report on the enhanced polarization owing to the Jahn-Teller distortion of V4+ without stereochemical Pb2+ or Bi3+.

Influence of diluted Fe3+ doping on the physical properties of BaTiO3

In this work, we present a study of the structural, optical, ferroelectric and dielectric properties of BaTiO3 doped with diluted quantities of Fe3+ (x = atomic fraction; x = 0, 0.01, 0.02, 0.04; BaFexTi1-xO3). Pure and doped BaTiO3 were prepared through mechanical milling and conventional solid-state reaction methods. The XRD analysis revealed the formation and progressive increase of a hexagonal phase with the addition of Fe3+. Furthermore, the microstructure and morphology demonstrated to be highly sensitive to the incorporation of Fe3+, observing an important refinement of the grain size. The optical band gap remained almost unchanged with the addition and increase of Fe3+. The ferroelectric performance was severely affected by the incorporation of dopant, deteriorating the remnant polarization from an initial value of 9.42 μC/cm2 for pure BaTiO3 to 0.2 μC/cm2 for the sample with the largest content of dopant. On the other hand, the Curie temperature showed a continuous reduction with the increase of dopant from 132 °C in the pure BaTiO3 to 71 °C for the sample with the largest amount of dopant. Likewise, the incorporation of Fe3+ caused a reduction of the relative permittivity and the evolution of the ferroelectric-paraelectric transition from a conventional Curie-Weiss behavior to a partial diffuse transition.

Colossal dielectric response, relaxation mechanism and multiferroic properties of (Ba1-xSmx)(Ti1-xFex)O3 (0.0 ≤ x ≤ 0.5)

Structural, microstructural, dielectric, multiferroic characteristics and relaxation mechanisms of (Ba1-xSmx)(Ti1-xFex)O3 samples with varrying compositions (x = 0.0–0.5) are reported in the present article. Standard crystal data and the Rietveld refinement results suggest tetragonal structure for all the compositions similar to the pure BaTiO3. SEM, EDX and color mapping are used for the analysis of microstructural, compositional and elemental study. Impedance analysis is carried out using Nyquist plot. Relaxation mechanisms are analyzed from the complex permittivity Cole-Cole plot and the Bode plot. Colossal dielectric constant was observed over a wide temperature range for compositions x = 0.3, 0.4 and 0.5 due to the formation of internal barrier layer capacitors and the ferroelectric-paraelectric transitions of BaTiO3 and magnetic transitions of SmFeO3. The colossal dielectric response of these compositions makes them suitable for potential applications in microelectronic devices such as dynamic random access memory (DRAM) and on-chip capacitors.

Cascaded pyroelectric conversion: Optimizing the ferroelectric phase transition and electrical losses

The dramatic increase in the pyroelectric coefficient around the ferroelectric–paraelectric phase transition suggests high potential energy conversion efficiencies, but the reality is more complicated when thermal and electrical losses are considered. The performance of prototype mono-domain lead titanate thin films is simulated around phase transition using a phenomenological modeling approach. Thermodynamic properties are calculated using a modified Landau–Devonshire potential that is a function of temperature, applied electric field, and uniaxial tensile stress for bulk films. Significant performance enhancement near the ferroelectric–paraelectric phase transition is observed. However, increases in both the specific heat and the dielectric constant reduced the anticipated improvement. Critically, electrical losses during charging and discharging processes within the energy conversion cycle are included and calculated using the dielectric dissipation factor. Cascaded pyroelectric conversion cycles are considered where heat for each subsequent stage is provided by the previous stage, allowing for the segmentation of large temperature changes into multiple cycles. The implementation of a multi-stage or cascade approach could greatly increase the cycle efficiency over a large temperature range while utilizing lower, more realistic, electric field strengths. We show how each stage could be optimized through a secondary multi-caloric effect where the ferroelectric–paraelectric transition temperature is shifted using an applied biaxial stress. Loss tangents as low as 5% are shown to negate the benefits of cascading for high number of stages (n > 10). Using a stress-tuned optimum phase transition temperature for cascade stages provides roughly a 100% increase in thermal efficiency vs the unoptimized material for low electric field cycles.

Structural Tailing and Pyroelectric Energy Harvesting of P(VDF-TrFE) and P(VDF-TrFE-CTFE) Ferroelectric Polymer Blends.

The copolymer P(VDF-TrFE) is a normal ferroelectric because the bulky TrFE monomer improves its crystalline chain structure, while the terpolymer P(VDF-TrFE-CTFE) is a relaxor ferroelectric because the third monomer CTFE makes it amorphous. Herein, in order to induce a crystalline beta phase in the terpolymer, we blended a small amount of crystalline P(VDF-TrFE) into P(VDF-TrFE-CTFE) and investigated the effect of blending on the pyroelectric energy harvesting (PyEH) properties. The polarization-electric field hysteresis loops at different temperatures and energy densities were investigated. The PyEH energy density (ND) is compared with the electrical energy density (UE). The UE and ND at the ferroelectric–paraelectric transition temperature for the χ = 0.1 blend are reported as 3.18 and 5.04 J/cm3, respectively, which are higher than the other polymer blends. Interestingly, the ND of the χ = 0.9 blend is found to be 3.44 J/cm3 when operated at lower and higher temperatures, that is, at TL = 25 °C and TH = 40 °C, respectively, which is the highest possible energy density at the lowest possible transition temperature for the polymer blends.

Study of the structural, ferroelectric, dielectric, and pyroelectric properties of the K0.5Na0.5NbO3 system doped with Li+, La3+, and Ti4+

Pure K0.5Na0.5NbO3 (KNN) and KNN doped with Li+ (6% mole), La3+(1.66%, 5%, 6% mole), and Ti4+ (10% mole) were prepared by mixture of oxides using high-energy milling and conventional solid-state reaction. The effects of the dopant on the physical properties of pure KNN have been evaluated based on the structural, ferroelectric, pyroelectric, and dielectric measurements. The XRD measurements show that KNN pure sample contains a mixture of monoclinic and orthorhombic crystalline phases, with a slightly higher concentration of monoclinic phase. In contrast, all doped samples show a higher concentration of the orthorhombic phase, as well as the presence of a secondary phase (K6Nb10.8O30), also detected by Raman measurements. The samples with a higher concentration of this secondary phase, also present greater dielectric losses and lower values of remnant polarization. The dielectric measurements allowed us to detect temperatures of structural transitions (orthorhombic-tetragonal, O-T) previous to the ferroelectric-paraelectric transition (tetragonal-cubic, T-C), and also in this set of samples, a direct correlation was found between the values of remnant polarization and the corresponding pyroelectric signal response.

Antiferromagnetic and dielectric behavior in polycrystalline GdFe0.5Cr0.5O3 thin film

Single phase materials with both spontaneous electric polarization and magnetization are rare, despite remarkable efforts in developing magnetoelectric multiferroics. In this work, a single-phase polycrystalline GdFe0.5Cr0.5O3 (GFCO) thin film was spin-coated onto a platinized silicon substrate. X-ray diffraction data suggest that the film exhibits an orthorhombic perovskite structure with a Pbnm space group. No other impurity phases were detected. Magnetization measurements reveal the Néel temperature of the GFCO film to be ∼220 K and illustrate a weak ferromagnetic component at 5 K, which could be due to spin canting. Frequency dependent ferroelectric–paraelectric transition was observed around 480 K, indicating the diffuse relaxor-like behavior. The electric field dependent polarization measurements show a lossy behavior below 200 K. The electric field dependent dielectric constant (tunability) measured at 1 MHz in a wide temperature range reveals that the tunability maximizes near the observed dielectric maxima, which further confirms the ferroelectric to paraelectric transition in the present film.

Polyvinylidene difluoride-based composite: glassy dynamics and pretransitional behaviour

This paper presents results of broadband dielectric spectroscopy studies in the composite system for which particularly strong interactions between polyvinylidene difluoride (PVDF: ferroelectric polymer, TC = 453−473 K) matrix and barium strontium titanate (BST) ferroelectric micro-particles can be expected. For PVDF the super-Arrhenius (SA) dynamics, associated with segmental motions freezing at the glass temperature Tg = 235 K, is evidenced. The addition of BST particles qualitatively changes dynamics, converting the SA-type behaviour in PVDF to the clear Arrhenius one in BST/PVDF composite. The latter crossovers to the relaxor-type SA dynamics on cooling, exactly at the glass temperature of PVDF. The preliminary model explaining such unique behaviour is proposed. For the consistent portraying of the SA evolution of primary relaxation times in PVDF and BST/PVDF, the activation energy index analysis was carried out and the new equation, entropy and symmetry controlled, is introduced. Studies are accomplished by the analysis of the ferroelectric-paraelectric transition in PVDF and for the composite system. They led to the discovery of the strong pretransitional anomaly of dε∕dT, extending even to the vicinity of the room temperature, The semi-discontinuous nature of melting in PVDF and its composites, with the discontinuity metric △T ≈ 20 K is suggested.Graphical abstract

Giant electrocaloric response in compositional manipulated BaTiO3 relaxor–ferroelectric system

The compositional manipulation of the multiphase coexistence in relaxor–ferroelectrics has been reported to be an effective approach to generate the giant electrocaloric effect. In this work, we systemically investigate the temperature-dependent electrocaloric effect in BaZr0.18Ti0.82O3–BaSn0.11Ti0.89O3 relaxors, where Zr and Sn are introduced into BaTiO3 to develop the multiphase coexistence at the ferroelectric–paraelectric transition. Through the direct measurement by the heat flux sensor, a large temperature change around 4 K under an electric field of 10 MV m−1, combined with a broad temperature span (20 °C–60 °C), has been observed. For comparison, the electrocaloric properties are also indirectly deduced based on Maxwell equations and Landau–Ginzburg–Devonshire phenomenological theory, and the validity of the indirect methods in this relaxor–ferroelectric system has been discussed. This work would shed light on developing giant electrocaloric materials with a wide operating temperature range.

Effect of phase transition on electrocaloric effect in Indium substituted BaTiO3 ceramics

In the present work the dense Ba(Ti1-xInx)O3-x/2 ceramics were prepared through a standard solid-state sintering method. Although only a tetragonal phase is detected by the X-ray diffraction in these ceramics, the coexistence of multiphase is observed by the selected area electron diffraction in two substituted ceramics. Moreover, the diffused phase transition is induced by the substitution, and this can be linked to the coexistence of multiphase in these ceramics. The maximal pyroelectric coefficient of Ba(Ti0.99In0.01)O2.995 ceramics is obviously larger than those of the other ceramics, which occurs beside the ferroelectric-ferroelectric transition. While the maximal pyroelectric coefficients of other ceramics locate beside the paraelectric-ferroelectric transition. Ba(Ti0.99In0.01)O2.995 ceramics show the largest adiabatic temperature change of 0.54 K and isothermal entropy change of 0.94 J/kg·K under an electric field of 30 kV/cm among all ceramics, and the largest electrocaloric effect strengths of 0.22 × 10−6 K m/V and 0.39 × 10−6 J m/kg·K·V under an electric field of 10 kV/cm are also obtained in this ceramic. For individual composition, the electrocaloric strength displays different tendencies, which can be linked to the diffusivity of the phase transition. From the present work, the relationship between electrocaloric effect and ferroelectric phase transition is revealed.

Enhanced Electrostrictive Properties and Thermal Stability in Zn-Modified 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 Ceramics

A 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (PMNT) relaxor ferroelectric ceramic is a good dielectric and electrostrictive actuator material. Nevertheless, important engineering problems of further enhancing its electrostrictive effect and increasing its thermal stability must be solved to promote its applications. In this work, the Zn-doped PMNT (PMNT/xZn2+) ceramics were prepared by the niobite precursor synthesis method. The transition of the PMNT/xZn2+ ceramics from relaxor ferroelectric to normal ferroelectric was realized, increasing the residual polarization (Pr), saturation polarization (Pm), the ferroelectric-paraelectric transition temperature at emax (Tm) and electric field induced strain (S) from x = 0.0 (Pr = 5.3294 μC/cm2, Pm = 26.6690 μC/cm2, Tm = 45°C, S = 1.08‰) to x = 8.0 (Pr = 19.9005 μC/cm2, Pm = 30.3718 μC/cm2, Tm = 70°C, S = 1.42‰). In addition, the upper temperature limit of the materials with a large electrostrictive effect (greater than 1‰) was extended from 40°C to 80°C. The main reason for these improved properties is that the magnesium ions were replaced by the highly reactive zinc ions at the B-site of the ferroelectric perovskite structure during the sintering and formed a certain amount of component segregation that increased the size of the polar nanoregions/domains and improved the polarizability of the materials.

Order–disorder transition in the ferroelectric LiTaO3

The temperature dependences of the damping constant and the relaxation time are calculated by using the Raman frequencies of a* and the lowest A1 (TO) phonons in the ferroelectric phase close to the ferroelectric-paraelectric transition in LiTaO3 (TC = 963 K). Both calculations are performed by considering the frequency as an order parameter for the pseudospin–phonon (PS) and the energy fluctuation (EF) models using the observed data from the literature. Values of the activation energies of this crystal are also deduced by using both models in this crystal. Our results show that the PS and EF models can describe the observed behavior adequately for the order–disorder transition in LiTaO3.

Correlations between elastic, calorimetric, and polar properties of ferroelectric PbSc0.5Ta0.5O3 (PST)

Calorimetric, elastic, and polar properties of ferrolectric lead scandium tantalate PbSc0.5Ta0.5O3 with 65% cation order have been investigated in the vicinity of the paraelectric-ferroelectric transition at Ttrans=295 K. Comparison of temperature dependencies of the excess specific heat and elastic properties indicates that both anomalies stem from thermal fluctuations of order parameters in three dimensions. These fluctuations are consistent with the tweed microstructure. This transition is driven by several coupled thermodynamic order parameters, as evidenced by a strongly nonlinear scaling of the excess entropy with the squared ferroelectric polarization.

Ferroelectricity in SrTiO3 epitaxial thin films via Sr-vacancy-induced tetragonality

SrTiO3 is one of the most widely studied perovskite oxides that exhibit quantum paraelectric behavior. Being at the border of the paraelectric-ferroelectric transition, SrTiO3 is known to become ferroelectric at a finite temperature by exploiting various approaches including epitaxial strain, oxygen isotope exchange, and dimensionality. In this study, owing to the state-of-the-art pulsed laser epitaxy, a selective engineering of elemental vacancies is proposed for the emergence and systematic control of the ferroelectricity in SrTiO3 thin films. Therein, Sr vacancy plays an essential role in inducing the cubic-to-tetragonal transition, resulting in the inversion symmetry breaking necessary for the switchable electric polarization. Furthermore, the tetragonality is enhanced with the increase in the Sr vacancies, eventually strengthening the ferroelectricity. Our research summarizes the tetragonality-induced ferroelectricity in SrTiO3 and provides facile growth control of the behavior.

Vibrational studies, dielectric, and electrical conductivity in (Ba0.95Ca0.05)0.1(Ti0.8Sn0.2)0.1Na0.9Nb0.9O3 ferroelectric ceramic

(Ba0.95Ca0.05)0.1(Ti0.8Sn0.2)0.1Na 0.9Nb0.9O3 (BCNTSNO3) ferroelectric ceramic was synthesized by traditional solid-state method. Room temperature structural analysis, using X-ray diffraction data, confirmed the prepared ceramic purity with the existence of morphotropic phase boundary (MPB) which appears from tetragonal to orthorhombic symmetry with P4mm and Pmm2 space group, respectively. Temperature dependence of the dielectric measurement exhibited a ferroelectric–paraelectric transition at 540 K. Raman spectra were studied in the range of 50–1000 cm−1 as a function of temperature of 293 K–573 K. A detailed analysis of the frequency and half-width versus temperature introduces huge changes which are associated to the ferroelectric transitions originating from the internal vibrational modes of NbO6 octahedron. Electrical conductivity was carried out, using impedance spectroscopy, in the frequency and temperature range, 1 Hz–1 MHz and 500–600 K, respectively. Both impedance and modulus analysis exhibit the contribution of grain and grain boundary to the sample electrical response. To explain the impedance results, an equivalent circuit was proposed. The temperature dependence of the alternating current conductivity (σac) and characteristic relaxation time (τ) confirmed the observed ferroelectric phase transitions in the dielectric study. Moreover, temperature dependence of frequency exponent (s) is investigated to explain the conduction mechanism in the different regions. It was attributed to the correlated barrier-hopping model (CBH) in region (I) and (II).

Magnetic and tunable dielectric properties of DyCrO3 thin films

DyCrO3 (DCO) belongs to the family of rare-earth chromites, some members of which exhibit magnetoelectric multiferroic properties. This work characterizes the dielectric tunability in DCO, which is typically, for conventional ferroelectrics, found to maximize near the ferroelectric to paraelectric transition. Two films of DCO were fabricated via a hybrid solution synthesis route on different substrates: one on single crystalline (001) LaAlO3 (LAO) and the other on (001) LAO coated with a conducting LaNiO3 (LNO) layer. The synthesis method resulted in films with both out-of-plane and in-plane texture in the DCO. The lattice mismatch strain between film and substrate caused a slight increase in the Neel temperature of the DCO film on LAO. Transmission electron microscopy images of the DCO film on the LNO/LAO revealed the epitaxial relationship between these phases. The temperature dependence of the relative permittivity showed diffused transition and a frequency dispersion similar to what is observed in typical relaxor ferroelectrics. Isothermal dielectric constant versus dc electric field measurements showed an electric field tunable relative permittivity (i.e., tunability). This tunability reached a broad plateau near the Neel temperature. Such maxima in dielectric tunability are typically observed near a ferroelectric–paraelectric transition, and its proximity to the magnetic ordering temperature in DCO indicates magnetically driven ferroelectricity in this material.

Structural, morphological, spectroscopic, and dielectric properties of SmFe0.5Cr0.5O3

Abstract The perovskite SmFe0.5Cr0.5O3was synthesized by conventional solid state reaction (SSR) method, based on stoichiometric mixing of Sm2O3, Fe2O3 and Cr2O3 oxides. The X-ray powder diffraction data showed that the SmFe0.5Cr0.5O3perovskite crystallizes in an orthorhombic distorted perovskite structure, while the homogeneity of the sample was affirmed basing on scanning electron microscope (SEM) images. The infrared spectra highlighted a set of absorption peaks corresponding to the stretching and bending vibrations of bands binding respectively iron and chromium with oxygen. Dielectric properties of SmFe0.5Cr0.5O3 were investigated in a wide frequency and temperature range.The evolution of ɛ’ and tan(δ) with frequency illustrated the relaxation phenomenon, while their evolution as a function of temperature revealed a ferroelectric-paraelectric transition occurring in the material. It was also observed that the compound becomes conductor at high temperatures (T > 620K). The temperature dependence of the ionic conductivity was also discussed and the activation energies were calculated. The fitted impedance spectroscopy data revealed that both grains and grain boundaries contribute in the electric conductivity of the SmFe0.5Cr0.5O3 perovskite

Multiple polarization mechanisms across the ferroelectric phase transition of the tetragonal tungsten-bronze Sr0.35Ba0.69Nb2O6.04

The broadband dielectric response of the uniaxial ferroelectric strontium barium niobate with 35% Sr has been studied from 1 kHz to 30 THz along the polar $c$ axis using high-frequency and low-frequency dielectric spectroscopies, time-domain terahertz spectroscopy, and far-infrared reflectivity, in the wide temperature interval 20--600 K. The mechanisms that contribute to the ferroelectric phase transition and to the dielectric anomaly in this material are the softening of an anharmonic excitation in the THz range related to cation hopping and the slowing down of a relaxation in the GHz range, which saturates below ${T}_{\mathrm{C}}$ near 1 MHz. In the ferroelectric phase, a relaxation, related to domain-wall dynamics, appears in the sub-GHz range and hardens on cooling up to 10 GHz. The ferroelectric-paraelectric transition, investigated by electron diffraction, has been assessed to the appearance of a supplementary mirror plane perpendicular to the polar axis.

Experimental study of the effect of yttrium on the structural, thermal, and magnetic properties of BiFeO3

In this paper, structural, thermal, and magnetic properties of Bi1−xYxFeO3 (BYFO, x = 0, 0.05, 0.1, 0.15, 0.2, and 0.25) nanoceramics have been studied. We have synthesized BYFO nanoceramics using solid-state reaction method followed by sintering. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and microstructural analysis showed that all as-prepared samples have a single-phase perovskite structure. Differential thermal analysis (DTA) indicated that TC (ferroelectric–paraelectric transition) is decreased with the addition of yttrium into the BFO structure. Magnetic properties of the samples were examined by vibrating sample magnetometer (VSM). We have observed the enhancement of magnetization by increasing yttrium content. Y-substitution decreased remanent magnetization (Mr) and increased the coercivity (Hc). A noticeable change in the magnetic hysteresis loop of Bi0.75Y0.25FeO3 can be triggered by an applied magnetic field and it confirmed the enhancement of magnetic properties.