Paraelectric Phase Research Articles

Microscopic picture of paraelectric perovskites from structural prototypes

We show with first-principles molecular dynamics the persistence of intrinsic $\langle111\rangle$ Ti off-centerings for BaTiO$_3$ in its cubic paraelectric phase. Intriguingly, these are inconsistent with the Pm$\bar 3$m space group often used to atomistically model this phase using density functional theory or similar methods. Therefore we deploy a systematic symmetry analysis to construct representative structural models in the form of supercells that satisfy a desired point symmetry but are built from the combination of lower-symmetry primitive cells. We define as structural prototypes the smallest of these that are both energetically and dynamically stable. Remarkably, two 40-atom prototypes can be identified for paraelectric BaTiO$_3$; these are also common to many other $AB$O$_3$ perovskites. These prototypes can offer structural models of paraelectric phases that can be used for the computational engineering of functional materials. Last, we show that the emergence of B-cation off-centerings and the primitive-cell phonon instabilities are controlled by the equilibrium volume, in turn dictated by the filler A cation.

Crystal structure, dielectric and impedance studies of a new lead free tungsten bronze ferroelectric oxide

The current manuscript describes the structure, dielectric and impedance nature of the ferroelectric ceramic Na2Ba2Y2W2Ti4Nb4O30 made by high temperature mixed oxide route. The compound formation with orthorhombic phase and surface morphology was verified by X-ray diffraction and scanning electron microscopy techniques respectively. The temperature variation of dielectric parameters (εr and tan δ) at selected frequencies of the material exhibits dual phase transitions at 323 and 573 K, respectively. The low temperature phase transition is considered as structural type in the ferroelectric phase and the high temperature (573 K) transition is related to transformation from ferroelectric phase to paraelectric phase. The room temperature polarization curve is plotted to assure the ferroelectricity in the ceramic. The frequency dependent impedance parameters at given temperature range determines the contribution of both bulk and grain boundary effects in it. The intrinsic resistance due to grains of the material follows opposite trend with temperature like semiconductors. Temperature dependent bulk resistance is used to obtain various thermistor parameters and their variation with temperature is discussed. Jonscher’s law is referred to understand the frequency dependent ac conductivity and it’s various temperature dependent parameters (A and n).

(1-x)(Na0.5Bi0.5)TiO3-x(K0.5Bi0.5)TiO3 ceramics near morphotropic phase boundary: A structural and electrical study

In this work, lead-free (1-x)(Na0.5Bi0.5)TiO3-x(K0.5Bi0.5)TiO3 ferroelectric ceramics with the compositions (x(%)= 0; 12; 16; 17; 18; 19; 20; 30 and KBT) were obtained by solid state reaction at a particularly optimized calcination temperature (1000 °C/4H). A pure perovskite structure was observed for the different compositions. Effect of KBT addition on the microstructure, dielectric, ferroelectric and electrical properties at high temperature of (1-x)NBT-xKBT ceramics are studied using SEM, impedance analyzer and electrical characterization. X-ray diffraction and Rietveld refinement analysis of all samples reveal the presence of a biphasic region in the solid-solution series with a truly rhombohedral-tetragonal symmetry observed only at (x(%) = 16–20). Hence, Raman measurements allow us to conclude that the introduction of K+ creates a local disorder in the structure with a modification of the long-range symmetry. The temperature-dependent dielectric behavior reveals particularly large anomalies in the case of (x = 16%) due to the presence of more local disorder. The ferroelectric-antiferroelectric phase transition mechanisms are discussed to explain the nature of these anomalies. The results suggest a diffuse, probably second-order phase transition from the ferroelectric to the paraelectric phase, depending on the composition of all samples.

Effect of electric field on elastic properties of BaTiO3 single crystals: a micro-Brillouin scattering study

The analysis of local polar clusters formed by random fields in ferroelectrics is of technical and fundamental importance in understanding piezoelectricity. The temperature and electric field dependences of elastic properties and the ferroelectric phase transition have been investigated in (100)-oriented BaTiO3 single crystals by micro-Brillouin scattering. In the field cooling process, LA phonons are scattered by polar clusters. As a result, the LA phonon frequency increases as compared with that of the zero-field cooling process. Under the application of an external electric field along the [100] direction in the tetragonal ferroelectric phase, a complete 90° domain switching is accomplished. Under the electric field, abrupt changes in the frequency shift and FWHM of the LA phonons in the paraelectric cubic phase indicate the occurrence of a phase transition from the cubic to the tetragonal phase. An E–T phase diagram has been proposed from the field-induced phase transition.

Domain Engineering Enabled Giant Linear Electro‐Optic Effect and High Transparency in Ferroelectric KTa1−xNbxO3 Single Crystals

A giant linear electro‐optic (EO) effect and high transparency in ferroelectric potassium tantalate niobate [(), KTN] crystal is achieved via a thermally controlled domain engineering method. It involves a two‐step thermal annealing process: 1) a rapid cooling process that forms polar nano‐regions (PNRs), i.e., a cooling rate of from to where is the Curie temperature; and 2) a slow cooling process that facilitates abnormal domain growth (AGG) i.e., a cooling rate of from to . Since PNR can have a faceted boundary and high anisotropy, it can promote AGG within single crystals to realize solid‐state domain conversion macroscopically from a multi‐domain to single‐domain crystal within a slow cooling process. The resultant KTN crystal offers high transparency that is equivalent to its paraelectric phase; and a linear EO coefficient () as large as , which is five times the value of conventional KTN crystals with similar composition. This giant linear EO coefficient represents a major technical advance in EO materials and significantly reduces the driving voltage, power, and footprint of many types of EO devices.

Unraveling the Structural, Dielectric, Magnetic, and Optical Characteristics of Nanostructured La2NiMnO6 Double Perovskites.

Double perovskite La2NiMnO6 (LNMO) nanoparticles and nanorods were synthesized via a hydrothermal process, where only aqueous inorganic solvents are used to regulate the microscopic morphology of the products without using any organic template. They crystallized in a monoclinic (P21/n) double perovskite crystal structure. The LNMO nanoparticles exhibited spherical morphology with an average particle size of 260 ± 60 nm, and the LNMO nanorods had diameters of 430 ± 120 nm and length about 2.05 ± 0.65 μm. Dual chemical oxidation states of the Ni and Mn ions were confirmed in the LNMO samples by X-ray photoelectron spectroscopy. Strong frequency dispersion dielectric behavior observed in the LNMO ceramics, is attributed to the space charge polarization and the oxygen vacancy induced dielectric relaxation. A ferroelectric—paraelectric phase transition appearing near 262 K (or 260 K) in the LNMO ceramics prepared from nanoparticles (or nanorods) was identified to be a second-order phase transition. The LNMO samples are ferromagnetic at 5 K but paramagnetic at 300 K. The LNMO nanoparticles had larger saturation magnetization (MS = 6.20 μB/f.u. @ 5 K) than the LNMO nanorods (MS = 5.68 μB/f.u.) due to a lower structural disorder in the LNMO nanorods. The semiconducting nature of the nanostructured LNMO with an optical band gap of 0.99 eV was revealed by the UV–visible absorption spectra. The present results enable the nanostructured LNMO to be a promising candidate for practical spintronic devices.

Hyper‐Raman scattering study of the soft mode in relaxor ferroelectric (Ba0.8Sr0.2)0.925Bi0.05Ti0.95(Zn1/3Nb2/3)0.05O3

AbstractWe report hyper‐Raman scattering measurements of the lowest transverse optic (TO) phonon branch in the relaxor Bi0.05‐BSTZN in the paraelectric phase from 300 to 973 K. The results evidence a displacive‐like behavior from a highly damped soft mode at high temperature to an overdamped (quasi‐elastic‐like) component on cooling close to Tc. A fit with a mean field law gives a temperature T0 = 102 K significantly lower than T0 = 188 K found by dielectric measurements. Reconciling the two experiments likely deserves considering a coupled modes analysis involving a second mode at slightly higher wavenumber. This behavior is very similar to that previously observed in PMN. Considering the rather complex chemical composition of the present compound as compared with PMN, these results emphasize a universal low‐wavenumber dynamics of ferroelectric relaxors.

Struktur, Sifat Dielektrik dan Optik Senyawa Aurivillius (Ca0,5Ba0,5)Bi4Ti4O15 yang Disintesis dengan Teknik Lelehan Garam

Synthesis of ferroelectric material based onAurivillius phases, (Ca0.5Ba0.5)Bi4Ti4O15 (CBBT), have been done using a molten salts technique.The results of X-ray diffraction (XRD) analysis showed the characteristic peaks at 2θ = 12.93°, 17.31°, 21.64°, 23.23°, 27.71°, 30.33°, and 32.90° which was correspond to the four-layers of Aurivillius phase.Refinement structure using the Le Bail method, the sample is suitable for orthorhombic structures with the A21am space group. The orthorhombicity of the CBBT is small and nearer to tetragonal symmetry.Fourier Transform Infra-red (FTIR) spectra show distortion of TiO6 octahedral at 847 cm-1 and strain vibrations of TiO6 octahedra at 545 cm-1 due to A-O bonds.Analysis using Scanning Electron Microscopy (SEM) shows the morphology of the sample in the form of plate-like which is characteristic of Aurivillius phase.The ferroelectric to paraelectric phase transition temperature (Tc) was obtained at 635ºC.The band gap value of the CBBT compound was 3.17 eV, relatively the same with the CaBi4Ti4O15 (CBT) compound.

Observation of cation-specific critical behavior at the improper ferroelectric phase transition in Gd2(MoO4)3

Gadolinium molybdate is a classical example of an improper ferroelectric and ferroelastic material. It is established that the spontaneous polarization arises as a secondary effect, induced by a structural instability in the paraelectric phase, which leads to a unit cell doubling and the formation of a polar axis. However, previous x-ray diffraction (XRD) studies on gadolinium molybdate have been restricted by the limited ability to include a wide $2\ensuremath{\theta}$ range in the analysis, and thus, at atomic scale, much remains to be explored. By applying temperature-dependent XRD, we observe the transition from the paraelectric tetragonal phase to the orthorhombic ferroelectric phase. The ferroelastic strain is calculated based on the thermal evolution of the lattice parameters, and Rietveld refinement of the temperature-dependent data reveals that the displacement of different cations follows different critical behavior, providing insight into the structural changes that drive the improper ferroelectricity in gadolinium molybdate.

Plastic/Ferroelectric Crystals with Distorted Molecular Arrangement: Ferroelectricity in Bulk Polycrystalline Films through Lattice Reorientation

AbstractPlastic/ferroelectric crystals have attracted increasing attention as emerging functional molecular materials with unique features. In this study, a new plastic/ferroelectric crystal is developed from ionic molecules, which exhibits three solid phases: a paraelectric plastic crystal phase and two ferroelectric phases. Owing to the malleability in the plastic crystal phase, by applying pressure to the powder at a high temperature, transparent polycrystalline films are easily fabricated, exhibiting ferroelectric performance at room temperature. The crystal structures of the three solid phases are determined by single‐crystal X‐ray diffraction analysis. The arrangement of anions and cations in the ferroelectric crystal is significantly distorted from the CsCl‐type structure in the paraelectric cubic crystal. The large distortion results in distinct splitting of the diffraction peaks that are equivalent in the cubic phase, which enables the observation of electric‐field‐induced lattice reorientation using powder X‐ray diffraction measurements. These results broaden the range of compounds for which plastic/ferroelectric crystals can be developed and provide a solid understanding of the switching process in their bulk polycrystals, which will facilitate their application to a variety of device elements.

Impact of non-ferroelectric phases on switching dynamics in epitaxial ferroelectric Hf0.5Zr0.5O2 films

Determining the switching speed and mechanisms in ferroelectric HfO2 is essential for applications. Switching dynamics in orthorhombic epitaxial ferroelectric Hf0.5Zr0.5O2 films with either significant or negligible presence of monoclinic paraelectric phase is characterized. Switching spectroscopy reveals that the polarization dynamics in pure orthorhombic ferroelectric phase films can be modeled by the Kolmogorov–Avrami–Ishibashi mechanism with large characteristic time (≈1 µs), which is shortened in fatigued junctions. The long switching time indicates that non-archetypical switching mechanisms occur and that ionic motion or other extrinsic contributions might be at play. Films containing a higher amount of paraelectric monoclinic phase show a shorter switching time of 69 ns, even in pristine state, for applied electric field parallel to the imprint field, enabling synaptic-like activity using fast electric stimuli. Thus, the presence of defects or paraelectric phase is found to improve the switching speed, contrary to what one can expect a priori.

Four-Phonon Scattering Effect and Two-Channel Thermal Transport in Two-Dimensional Paraelectric SnSe.

In recent years, increased attention has been paid to the study of four-phonon interactions and diffusion transport in three-dimensional (3D) thermoelectric materials because they play an essential role in understanding the thermal transport process. In this work, we study four-phonon scattering and diffusion transport in two-dimensional (2D) thermoelectric materials using the paraelectric phase of 2D SnSe as an example. The inherent soft phonon modes are treated by the self-consistent phonon theory, which considers the temperature-induced renormalization of the phonons. Based on density functional theory and the Peierls-Boltzmann transport equation for phonons, we show that the four-phonon interactions can reduce the thermal conductivity of the 2D SnSe sheet by nearly 40% due to the collapse of soft optical modes, and the contribution of diffusion transport to the total thermal conductivity accounts for 14% at a high temperature of 800 K due to the short phonon mean free path approaching the Ioffe-Regel limit, suggesting the two-channel transport in this system. The results are further confirmed by using the machine learning-assisted molecular dynamics simulations. This work provides a new insight into the physical mechanisms for thermal transport in 2D systems with strong anharmonic effects.

Interfacial Polarization Phenomena in Compressed Nanowires of SbSI.

The systematic studies of the extrinsic Maxwell–Wagner–Sillars polarization process in compressed antimony sulfoiodide (SbSI) nanowires are carried out by dielectric spectroscopy. The dielectric response is studied in temperature () K and frequency () Hz ranges. Dielectric functions commonly used for the analysis of dielectric spectra related to intrinsic polarization processes were applied in the elaboration of experimental data. It was found that the respective “semi-circles” in the Cole–Cole-type plots display a characteristic pear-like shape for the ferroelectric phase. On the other hand, the data for the paraelectric phase form symmetrical arcs. This response is effectively parametrized using the experimental Cole–Davidson and Cole–Cole functions fitted to the data obtained for the ferroelectric and paraelectric phases, respectively. It is deduced that the particular shape of spectra in the ferroelectric phase is due to spontaneous polarization, which is responsible for an asymmetric broadening of relaxation functions related to the interfacial polarization.

Enhanced energy storage properties and superior thermal stability in SNN-based tungsten bronze ceramics through substitution strategy

Tungsten bronze ceramics of composition Sr2Ag0.2Na0.8Nb5-xTaxO15 were synthesized by solid state methods to investigate the impact of Ta replacement on the structure and energy-storage properties (ESP). The study on the relationship between structure and electric properties revealed three main conclusions: (1) as the Ta5+ concentration increased, the crystal structure transformed from an orthorhombic Im2a phase to a tetragonal paraelectric P4bm phase; (2) a high recoverable energy storage density (1.44 J/cm3) and a moderate efficiency (82%) under low-electric fields was obtained in x = 0.3 sample; (3) both dielectric properties (−9.6 to 232.7 °C) and ESP (30–150 °C) exhibit an excellent thermal stability for x = 0.3 sample. In addition, a high current density (863.69 A/cm2) and a large powder density (70.21 MW/cm3) were achieved simultaneously. The current system could be a promising candidate in temperature-stable dielectric capacitors under low-fields and over a broad temperature range.

Vibrational spectroscopy, electrical, and thermal properties of [N (CH3)4][N(C2H5)4]CoCl4 compound

Abstract[N (CH3)4][N(C2H5)4]CoCl4 new hybrid compound was synthesized by slow evaporation at room temperature. The X‐ray powder diffraction indicates that this material has an orthorhombic structure with P212121 space group. Two endothermic peaks at T1 = 293 K and T2 = 387 K are observed by differential scanning calorimetric. The real and imaginary parts of the permittivity as function of temperature shows the ferroelectric phase below T = 288 K. The Nyquist diagrams are simulated based on the equivalent circuit chosen by the Z‐View‐software and show the contribution of the grain and electrodes effect. Raman spectroscopy study at room temperature confirms the composition of the synthesized compound. Furthermore, the ferroelectric–paraelectric phase transition T1 is demonstrated by temperature‐dependent Raman spectroscopic studies.

Synchrotron based x-ray absorption spectroscopy investigation and temperature dependent ferroelectric properties of Ni doped BaTiO3 nanostructures

In this report, the local structure geometry of BaTi1-xNixO3 (0.0 ≤ x ≤ 0.06) ceramics has been studied through synchrotron based x-ray-absorption spectroscopy, measured at the Ti K, Ba L3 and Ni K-edges at room temperature. The x-ray absorption near edge spectroscopy (XANES) at Ti K-edge confirms the TiO6 octahedron geometry of Ti atom due hybridization between Ti(3d)-O(2p) orbitals and signifies the non-centrosymmetric nature of BaTiO3 samples. However, Ti–O off-center displacement (non-centrosymmetric) is disturbed under the effect of Ni doping as a result of oxygen vacancies formation. More interestingly, XANES studies at Ni K-edge ensure successful substitution of Ni in BaTiO3 as Ni2+ ions. Extended x-ray absorption fine structure (EXAFS) data at Ni K-edge have been fitted to estimate pertinent local structural parameters of the Ni–O, Ni–Ba, Ni–Ti and Ni–Ni shells (viz. bond lengths and disorder parameters) and it also reveal that the structural disorders around the Ti sites in the doped BaTiO3 expand with Ni doping. The mixed-valence states of titanium ion, i.e., Ti4+ and Ti3+ in the doped samples were established with the x-ray photoelectron spectroscopy (XPS). Moreover, XPS divulges the creation of oxygen vacancies due to Ni doping in BaTiO3 matrix. The complementary information about the lattice vibration is analyzed through Raman studies that approve the softening of the transverse optical (TO) mode present at 515 cm−1. The temperature dependent ferroelectric studies affirm that the ferroelectricity vanishes in the doped samples due to the decrement in the off-center displacement between the Ti4+ and O2− ions in the TiO6 octahedral geometry. Differential thermal analysis (DTA) exhibits a ferroelectric tetragonal to para-electric cubic phase transition in the pristine as well as Ni doped BaTiO3 samples.

Elaboration and characterization of ferroelectric materials Sr2(1-x)Na(1+x)GdxNb5O15 synthesized by solid process

The solid solutions Sr2(1-x)Na(1+x)GdxNb5O15 (SNGN) with substitution rates of Sr2+ by Gd3+ varying between 0 and 1 were prepared and have undergone several heat treatments. The XRD spectra show that the powders calcined at 650 °C for 6 h exhibit a mixed structure consisting of orthorhombic-quadratic phases. On the other hand, calcination at 1200 °C for 12 h ensures complete crystallization of the SNGN compounds in the TTB structure of quadratic symmetry. The SEM images show grains of regular and spherical shapes. The Raman spectra of the solid SNGN solution calcined at 1200 °C (x = 0; 0.2; 0.4 and 0.6), show two broad bands attributed to the transverse deformation modes of the O-Nb-O bond and of elongation modes of the Nb-O bond of octahedra (NbO6). IR spectroscopy highlights the existence of absorption bands corresponding to the elongation and deformation vibrations of some bonds. The temperature measurements of the dielectric permittivity εr of the ceramic (SNN; x = 0) show for all the frequencies used, the presence of a narrow peak during the heating and the cooling phases with a maximum located respectively at the Curie temperature Tc ∼ 235.07 °C and Tc ∼ 224.84 °C. The dielectric losses tan (δ) values are very weak in the ferroelectric phase (T < Tc), then an increase, which reaches 2.3 for the lowest frequency, occurs in the paraelectric phase. The maximum permittivity ε'rmax undergoes a decrease as the frequency increases; this diffusive behavior can be associated to the domains walls in the micro-domains of the ferroelectric phase.

Фазовые превращения в ниобате бария-стронция SBN-50 в интервале температур от 80 до 700 K по данным спектроскопии КРС

The dynamics lattice of the Sr0.5Ba0.5Nb2O6 (SBN-50) ceramic was studied using Raman spectroscopy in the temperature range of 80-700 K with a step of 10 K. The analysis of the Raman spectra in the range ν = 50-1000 cm-1 revealed features in the temperature dependence behavior of the optical modes frequencies associated with the displacement of Ba, Sr and Nb atoms from their average positions in the crystal lattice, which is associated with a macroscopic phase transition from the paraelectric to the ferroelectric phase in the vicinity of 390 K. It is shown that anomalies at T~ 565 K on the temperature dependences of the frequency and line half-width corresponding to the oscillations of the NbO6 octahedron can be caused by the presence of polar nanoregions in the paraelectric phase in SBN-50. When the sample was cooled to a temperature of 80 K, the behavior was recorded in the Raman spectra, indicating the occurrence of a structural phase transition in the ferroelectric phase in the vicinity of 190 K. The reasons for the revealed patterns are discussed.

A weak manifestation of the field effect in metal-dielectric-semiconductor structures with a ferroelectric insulating layer Ba-=SUB=-x-=/SUB=-Sr-=SUB=-1-x-=/SUB=-TiO-=SUB=-3-=/SUB=-

High-frequency measurements of the capacitance and conductivity of Ni-Ba0.8Sr0.2TiO_3-Pt and Ni-Ba0.8Sr0.2TiO_3-Si objects with a ferroelectric thickness of 120 nm in the paraelectric phase were carried out. It is shown that in the entire range of external voltages, the electric field practically does not penetrate Si. The conclusion made earlier about the reasons for the weak manifestation of the field effect is confirmed --- the polarization of the ferroelectric layer is almost completely shielded by the charges of electronic traps at the Ba0.8Sr0.2TiO_3-Si contact. It is noted that a sharp decrease due to passivation of the activity of surface traps will allow implementing transistors based on metal-BST-Si structures with a working surface channel of non-basic charge carriers and will ensure the construction of high-quality FeRAM non-volatile memory cells. Keywords: metal-dielectric-semiconductor-structures, metal-dielectric-metal-structures, ferroelectric films of the composition Ba0.8Sr0.2TiO3, high-frequency impedance.

A weak manifestation of the field effect in metal-dielectric-semiconductor structures with a ferroelectric insulating layer Ba-=SUB=-x-=/SUB=-Sr-=SUB=-1-x-=/SUB=-TiO-=SUB=-3-=/SUB=-

High-frequency measurements of the capacitance and conductivity of Ni-Ba0.8Sr0.2TiO_3-Pt and Ni-Ba0.8Sr0.2TiO_3-Si objects with a ferroelectric thickness of 120 nm in the paraelectric phase were carried out. It is shown that in the entire range of external voltages, the electric field practically does not penetrate Si. The conclusion made earlier about the reasons for the weak manifestation of the field effect is confirmed --- the polarization of the ferroelectric layer is almost completely shielded by the charges of electronic traps at the Ba0.8Sr0.2TiO_3-Si contact. It is noted that a sharp decrease due to passivation of the activity of surface traps will allow implementing transistors based on metal-BST-Si structures with a working surface channel of non-basic charge carriers and will ensure the construction of high-quality FeRAM non-volatile memory cells. Keywords: metal-dielectric-semiconductor-structures, metal-dielectric-metal-structures, ferroelectric films of the composition Ba0.8Sr0.2TiO3, high-frequency impedance.