Paraelectric Matrix Research Articles

Excellent energy storage performance of paraelectric Ba0.4Sr0.6TiO3 based ceramics through induction of polar nano-regions

Dielectric energy storage materials with congenitally high power densities and ultrafast discharge rates have been extensively studied for emergent applications. As a typical and traditional dielectric material, paraelectric Ba0.4Sr0.6TiO3 (BST) ceramic exhibits a moderate dielectric constant (εr), low dielectric loss and slightly nonlinear P–E hysteresis. However, its energy storage density (W) is extremely low because of its low maximum polarisation (Pmax) and weak breakdown strength (BDS). In this study, ferroelectric Na0.5Bi0.5TiO3 (NBT) was introduced into paraelectric BST to enhance energy storage performance. The results show that the introduction of NBT induced polar nano-regions (PNRs) in the paraelectric matrix, resulting in a slim hysteresis loop with low remnant polarisation (Pr) and high Pmax simultaneously. Furthermore, owing to a decrease in the oxygen vacancy concentration and an increase in the band gap energy, the BDS of the BST ceramic also significantly increased. As a consequence, a remarkable energy storage density (Wrec = 3.89 J/cm3) and a high energy storage efficiency (η = 83.8%) were realised in the 0.75Ba0.4Sr0.6TiO3-0.25Bi0.5Na0.5TiO3 (0.75BST–0.25NBT) ceramic under a practical electric field of 360 kV/cm. Moreover, the ceramic also exhibited an excellent current density (∼1029.7 A/cm2) and ultrahigh power density (∼128.7 MW/cm2). The attained energy storage performances indicate that the NBT-modified BST ceramics are promising materials for high energy storage capacitor applications field.

Influence of Nb5+ ions on phase transitions and polar disorder above TC in PbZrO3 studied by Raman spectroscopy

AbstractMicro‐Raman light scattering experiments on PbZrO3 (PZO) single crystal doped with Nb5+ have been investigated. Special attention was paid to the paraelectric (PE) phase in which nominally forbidden first‐order Raman spectra were detected at temperatures far above the phase transition TC. Complex Raman spectra were observed in the vicinity of three structural phase transitions. These results mainly from the coexistence of phases with different symmetries in a wide temperature range below TC. The Raman measurements have been compared with dielectric and optical observations and proved that polar nanoregions in a centrosymmetric lattice appear well above TC. It was shown that doping ABO3 perovskites with heterovalent ions like Nb5+ unbalances charge neutrality of the lattice and strongly extends the temperature range of polar regions. The investigations performed point out that in the PE matrix the interaction between electrons and lattice vibrations, as recently suggested for pure PZO, plays an important role.

Structural and dielectric characteristics of Ba3Ln3Ti5Nb5O30 (Ln = La, Nd, Sm) filled tungsten bronze ceramics

In the present work, the structural, dielectric and relaxor ferroelectric properties were investigated for Ba3Ln3Ti5Nb5O30 (Ln = La, Nd, Sm) ceramics. The filled tungsten bronze phase with space group P4/mbm was confirmed for all compositions, while a small amount of secondary phase was detected in Ba3Nd3Ti5Nb5O30 and Ba3Sm3Ti5Nb5O30. The typical relaxor ferroelectric behaviors were observed: a broad peak of dielectric constant shifting to higher temperatures and decreasing its magnitude with increasing frequency and the frequency dispersion obeying the Vogel-Fulcher relationship. The P-E (polarization-electric field) hysteresis loops were obtained for Ba3Ln3Ti5Nb5O30 (Ln = La, Nd, Sm) ceramics at low temperatures. The nanoscale ferroelectric 180° domains with strip-like shape were observed in the paraelectric matrix at room temperature, where the commensurate structural modulations were determined in the domains and incommensurate ones were determined in the matrix. The significant differences were determined between the present ceramics and Ba4Ln2Ti4Nb6O30 and Ba5LnTi3Nb7O30 because of the different distribution patterns of A1 and A2 cations.

Revisiting the phase transitions in BaxSr1-xTiO3 at low doping range (x≤0.1)

Ba2+ substituted SrTiO3, BaxSr1-xTiO3 (BST) at low doping range (x ≤ 0.1) is of particular interest for studying the phase transitions induced by a small amount of polar dopants in the paraelectric matrix SrTiO3. We have studied the crystal structure, dielectric and pyroelectric properties of BST (x ≤ 0.1) in this paper. It is revealed that a few of polar dopants Ba2+ in quantum paraelectric matrix SrTiO3, not only enhance the phase transitions of SrTiO3 around 18 K and 37 K, which are suppressed by the high quantum fluctuations in pure SrTiO3, but bring about “local ferroelectric symmetry breaking”. However, no structural transition related to the ferroelectric phase in the BST (x ≤ 0.1) at macroscopic scale.

Piezoelectric and elastic properties of relaxor-like PZT:Ba ceramics

The solid solutions of PbZr1-xTixO3 (PZT) are the most known and widely applicable piezoelectric ceramic materials. The influence of different dopants on PZT properties has been studied for many years. Some of such compositions have revealed a behavior typical for ferroelectric relaxors. In the case of Pb0.75Ba0.25Zr0.70Ti0.30O3 ceramics (abbreviation PBZT 25/70/30), prepared by mixed-oxide processing technique elastic, it was found that macroscopic piezoelectric activity is present far above the temperature at which structural changes and maximum of permittivity occur in unpoled samples (~ 200 °C). Anomalies of the elastic and piezoelectric properties have been observed very clearly near the temperature of 152 °C. Within the temperature range of 152–220 °C, unstable piezoelectric properties have been detected due to the existence of polar micro/nano-clusters and non-trivial elastic-electric interactions between them, through the non-polar paraelectric matrix. The origin of such interactions could be lattice instabilities, dynamic change in polar cluster sizes, and fluctuations in chemical composition. Based on the results of the research, it can be said that PZT ceramics of such composition is a relaxor-like material and it is not a ferroelectric relaxor.

Local Electrical Imaging of Tetragonal Domains and Field-Induced Ferroelectric Twin Walls in Conducting SrTiO_{3}.

We demonstrate electrical mapping of tetragonal domains and electric field-induced twin walls in SrTiO_{3} as a function of temperature and gate bias utilizing the conducting LaAlO_{3}/SrTiO_{3} interface and low-temperature scanning electron microscopy. Conducting twin walls appear below 105K, and new twin patterns are observed after thermal cycling through the transition or on electric field gating. The nature of the twin walls is confirmed by calculating their intersection angles for different substrate orientations. Numerous walls formed when a large side- or back-gate voltage is applied are identified as field-induced ferroelectric twin walls in the paraelectric tetragonal matrix. The walls persist after switching off the electric field and on thermal cycling below 105K. These observations point to a new type of ferroelectric functionality in SrTiO_{3}, which could be exploited together with magnetism and superconductivity in a multifunctional context.

Nonlinear dielectric properties of paraelectric-dielectric composites described by a 3D Finite Element Method based on Landau-Devonshire theory

The local field inhomogeneity generated by the interfaces between dissimilar phases plays a major role on the electrical properties of ferroelectric-based composites. This aspect was rarely investigated, because it requires advanced modeling tools. In the present work, a 3D Finite Element Method combined with Landau-Devonshire theory was developed for computing local fields in order to describe the role of realistic microstructures on the electrical response. Various combinations of materials and interconnectivities in paraelectric-based di-phase composites (phases randomly mixed, 0–3 connectivity or 1–3 connectivity) were analyzed and some valuable solutions for providing the most promising tunability properties are proposed. A decrease of tunability and permittivity when increasing the concentration of the dielectric component in composites with randomly distributed phases and in ones with 0–3 connectivity (paraelectric inclusions in a dielectric matrix), often reported experimentally, was obtained by this approach. For other configurations like 0–3 and 1–3 connectivity with dielectric inclusions in a paraelectric matrix, the simulations predicted comparable tunability values as in single phase paraelectrics. The best tunability properties are predicted for 1–3 connectivity with dielectric inclusions having the long axis perpendicular to the direction of the applied field, due to a favorable distribution of the local electric field within the system. Therefore, the proposed numerical method allows design composites with tailored permittivity and tunability by local field engineering.

Dielectric response of 0.85 Ba(Ti0.96Zr0.04)O3–0.15 Bi(Mg0.5Ti0.5)O3 relaxor ferroelectrics under electric field: evolution of PNRs

0.85 Ba(Ti0.96Zr0.04)O3–0.15 Bi(Mg0.5Ti0.5)O3 relaxor ferroelectric ceramic samples were synthesized through traditional solid state reaction method. The asymmetric single peak of (200) at 45° was revealed by XRD measurement, combined with diffuse phase transition peaks in dielectric-temperature spectrum and a slim but discernible hysteresis loop, which proved the existence of the polar nano-regions (PNRs) in paraelectric matrix of the sample. The investigation of dielectric response of the ceramic samples under electric field (changing field magnitude and field history) was employed to analysize the evolution of PNRs. Electric field dependence of dielectric constant was obtained both by small signal bias electric field measurement and differential calculation from P–E curves, they all reached the maximum value firstly at the critical electric field and then decreased with the continuous increasing of electric field, instead of decreasing directly. However, the value of dielectric constant and critical field of bias electric field measurement was hysteretic electrically to those of differential calculation from P–E curves. Both the glassy model and macro–micro theory model were employed to explain and explore the evolution of PNRs, though from different perspective.

Griffith phase-induced ferromagnetic diffuse phase transition

A ferromagnetic transition is generally regarded as a diffusionless process, exhibiting a sharp change of magnetic properties with temperature. In this paper, however, we report a ferromagnetic diffuse phase transition (FDPT) induced by Griffiths phase in La1 − xSrxMnO3 (0·06 ≤ x ≤ 0·2). Such a FDPT experiences a broad transition temperature range, resulting in temperature-controlled ferromagnetic cluster owing to the existence of Griffiths phase during the diffusion process. The transition temperature range calculated from M–T curve is as large as 146 K for the FDPT sample. Further analysis suggests that this novel FDPT has the same scenario with that in the ferroelectric relaxor, where the ferroelectric clusters are embedded in the paraelectric matrix.

An Analytical Model for Porous Polymer-Ceramic Capacitive Pressure Sensors

An analytical model for pressure sensors is constructed, predicting the capacitive response of a porous, polymer-ceramic composite under an applied pressure. Consisting of mechanical and dielectric counterparts, the iterative model is constructed in detail. The elastic modulus of the three-phase material is approximated by first considering only the polymer-ceramic composite mixture, and then incorporating porosity into the solid composite model. A new model has been developed for approximating the changing elastic modulus of porous polymers undergoing quasi-static compression, which induces the collapsing pores. Necessary material constants were obtained from experimental data published in literature. The permittivity of the paraelectric polymer matrix is modeled, accounting for piezodielectric effects imposed by external pressure and thermally induced stresses caused by substrate pinning. Similarly, the ferroelectric ceramic filler is modeled, considering changes in polarization caused by thermally induced phase transformations in the crystal structure. The final model is evaluated against experimental data, providing insight into composition and microstructure effects on the sensor response.

Temperature-dependent Raman scattering study of the defect pyrochlores RbNbWO6 and CsTaWO6

Lattice dynamics calculations and temperature-dependent Raman scattering experiments were performed on RbNbWO6 and CsTaWO6 pyrochlore oxides. The observed bands were assigned to the respective motions of atoms in the unit cell. The spectra showed the presence of additional Raman bands not allowed for by the cubic structure. We have shown that these bands appear due to both substitutional disorder in the 16c sites and displacive disorder of the A ions. Raman studies also revealed the presence of an additional 80 cm−1 band at room temperature for RbNbWO6, not observed for CsTaWO6. The presence of this band has been attributed to off-center displacement of the Nb and W ions due to structural phase transition into a tetragonal ferroelectric phase. The temperature evolution of the 80 cm−1 band intensity revealed that it disappeared at a much higher temperature (about 650 K) than the reported phase transition temperature (about 360 K). This behavior is reminiscent of chemically disordered perovskite ferroelectrics, including relaxor ferroelectrics, and was attributed to the presence of small polar regions with local tetragonal distortion embedded in the paraelectric matrix of the structure.

Ferroelectric behavior in the high temperature paraelectric phase in a poly(vinylidene fluoride-co-trifluoroethylene) random copolymer

In this report, ferroelectric behaviors were investigated as a function of temperature and poling frequency for a uniaxially stretched poly(vinylidene fluoride-co-trifluoroethylene) 50/50 (mol./mol.) film. A variety of ferroelectric behaviors, including normal ferroelectric, antiferroelectric-like, and paraelectric behaviors, were observed by varying temperature, poling frequency, and poling electric field. Especially, the ferroelectric (FE) to paraelectric (PE) transition and ferroelectric behaviors of the high temperature PE phase were studied in detail by electric displacement-electric field loop measurements. At a high poling frequency (e.g., 1000 Hz) and 100 °C (above the Curie temperature at 64 °C), a paraelectric behavior was obtained due to the nucleation of electric field-induced FE nanodomains inside the PE phase matrix. These FE nanodomains were highly reversible and they quickly depolarized upon removal of the poling field. At an intermediate poling frequency (e.g., 10 Hz) and 100 °C, an antiferroelectric-like behavior was observed, which could be attributed to the competition between depolarization and polarization fields upon reverse poling. Finally, at a low poling frequency (e.g., 1 Hz) and 100 °C, a normal ferroelectric behavior with rectangular hysteresis loops was seen because the small, reversible FE domains had enough time to grow into large irreversible ones. The presence of electric field-induced FE domains in the PE matrix was proved by field dependent Fourier transform infrared study. On the basis of this study, understanding of the paraelectric behavior in polar crystalline polymers will help us design new materials to meet the requirements for high energy density and low loss dielectric applications.

Bismuth-induced ferroelectric relaxor behavior in paraelectric LaAlO3

Perovskite-type dense ceramics (La0.9Bi0.1)AlO3 (LBAO) have been prepared through a solid-state reaction route. LaAlO3 is indeed a paraelectric, whereas a significant ferroelectric relaxor behavior is obtained in LBAO. The variations of ferroelectric hysteresis loops versus the applied field or frequency indicate the nature of the ferroelectric domain switching. A broad dielectric anomaly coupled with the shift in dielectric maxima toward a higher temperature with increasing frequency has been observed in LBAO, and the indicator of degree of diffuseness γ is about 1.86. Moreover, the dielectric relaxation, which follows the Vogel–Fulcher relationship with TVF = 706.9 K, f0 = 1.85 × 109 Hz, and Ea = 0.09 eV, further supports spin-glass-like characteristics. Compared with LaAlO3, the abnormal contraction of crystal cell is observed in LBAO, which does not follow the normal Vegard’s law. This can be attributed to the covalent hybridization of the Bi3+ ions with the surrounding O2- ions. The large local displacements at the off-centered Bi position lead to the formation of polarized nanoclusters in the paraelectric matrix and the reorientation of these unstable polarized nanoclusters further results in ferroelectric relaxor behavior.

Ab initio studies of the para- and antiferroelectric structures and local polarized configurations in NH4H2PO4

A study of differently polarized structures relevant to the H-bonded antiferroelectric (AFE) compound NH(4)H(2)PO(4) (ADP) is performed by first-principles calculations in the framework of the density functional theory. The calculated structures for the AFE and paraelectric (PE) phases are found in general good agreement with the available experimental data. We study the energetics and relative stability of different polarized clusters embedded in a PE matrix of ADP. We find that local ferroelectric and AFE clusters are stable and may coexist in the PE phase, which explains the coexistence of both type of microregions determined by electron spin probe measurements above the AFE-PE transition temperature. The dependency with the O-H···O bridge length of the energy barrier heights for proton transfer is studied for coordinated proton displacements along the bridges within clusters of different sizes. This dependency may have implications for the geometric isotopic effects on T(c). We analyze Mulliken orbital and bond populations which confirm the existence of a charge flow within the NH(4)(+) ion, an essential fact for the stabilization of the AFE phase over other possible polarized structures. This charge transfer is correlated with the optimization of the N-H···O bridges and with distortions of the NH(4)(+) group.

Phase Front Near the Critical End Point as the Boundary of the Polar Nanoregions in Relaxor Ferroelectrics

The static modulation structure of the phase front at the field-induced first-order transition point near the critical end point (CEP) was discussed on the basis of the Landau-type free-energy function. It was confirmed that the field E dependence of the thickness of the phase front is proportional to ( E CEP - E ) -1/2 , diverging at CEP ( E = E CEP ). It is pointed out that, in relaxor ferroelectrics, the boundary between the polar nanoregion (PNR) and the paraelectric matrix may be considered as a phase front, and that the contribution to the dielectric response of the phase front cannot be ignored.

Shell Model Study of Local and Global Energy Barriers in KDP

We perform a study of the energetics of KH2PO4 (KDP) by using a shell model (SM) which was constructed by adjusting the interaction parameters to ab initio calculations, and was fitted to reproduce phonons, polarization-inversion energies and structural properties. We calculate the energy profiles by performing global displacements and local distortions following the ferroelectric (FE) mode pattern in clusters of different sizes embedded in a paraelectric (PE) phase matrix. These properties are expected to be relevant to the PE-FE phase transition. The obtained SM results are compared to corresponding ab initio (AI) data. The global instabilities are found in good agreement for both KDP and DKDP. We also find qualitative good agreement in the KDP structure and even quantitative agreement in the expanded DKDP structure for the local distortions. The SM results reproduce well different trends like increasing instabilities as the cluster sizes grows, as the heavier atoms are included, and as the volume is increased, in accordance with the corresponding data from AI calculations.

Dynamics of Li+ dipoles at very low concentration in quantum paraelectric potassium tantalate

The dynamics of Li+ dipoles distributed in the quantum paraelectric matrix of potassium tantalate is investigated via linear and nonlinear dielectric response. It is found that the highly diluted solid solution K0.995Li0.005TaO3, KLT(005), reveals nearly ideal Debye-type dipolar relaxation with conventional frequency dispersion at intermediate temperatures, 30<T<60 K. At lower temperatures a drastic increase in the linear dielectric susceptibility is measured as predicted by computer simulations Su et al. [J. Appl. Phys. 90, 6345 (2001)]. Being accompanied by a negative divergence of the third-order nonlinear susceptibility, a transition into a chiral glass state as T→Tg≈6 K is conjectured.

Dielectric and soft-mode behaviors ofAgTaO3

The synthesis difficulty on ${\text{AgTaO}}_{3}$ ceramic has been surmounted by the high oxygen-pressure sintering. In this study, the synthesized sample was investigated by both dielectric and micro-Raman measurements in detail. As no dielectric anomaly was observed between 2.2 and 694 K, the previously proposed ferroelectric phase transition from $R3c$ to $R\overline{3}c$ at 170 K upon heating is not supported by our study whereas the quantum paraelectric ground state with $R\overline{3}c$ symmetry is more consistent with our experimental findings. Also, it is observed that the phase transition from this rhombohedral ground state to the higher-temperature monoclinic phase occurs at 667 K is purely soft-mode driven. Taking account the observation of additional Raman modes, at least up to 360 K, the existence of symmetry-breaking regions in the quantum paraelectric matrix is also discussed.

Coexistence of ferroelectric and antiferroelectric microregions in the paraelectric phase of NH4H2PO4 (ADP)

Ammonium dihydrogen phosphate NH4H2PO4 (ADP) is an antiferroelectric (AFE) compound belonging to the KDP-type family of hydrogen-bonded ferroelectrics. Recent ab initio results have shown that the optimization of the N–H–O bridges in ADP leads to the stabilization of the AFE state over a FE one. However, electron spin probe measurements have suggested that microregions of both phases may coexist above the critical antiferroelectric–paraelectric transition temperature. We have performed first principles studies of the energetics and relative stability of different AFE and FE defects embedded in a paraelectric (PE) matrix of ADP. Our analysis indicates that FE and AFE clusters are stable and may coexist in the PE phase, thus confirming the above suggestion.

Space charge contribution to the apparent enhancement of polarization in ferroelectric bilayers and multilayers

The authors study the effect of interfacial space charge in ferroelectric multilayers through a thermodynamic formalism. Using an electrostatic approach that takes into account the presence of trapped charges between the layers, the authors show that considerable modification in electrical properties can be exhibited, giving rise to polarization enhancement depending on the type of dopants and configuration of the layers. Space charge effect becomes more pronounced in structures with smaller ferroelectric layer fractions. It is shown that built-in polarization due to space charge can stabilize a switchable ferroelectric polarization at small fractions of the ferroelectric layer in a paraelectric matrix.