Paraelectric Phase Research Articles

Charged Domain Walls in BaTiO3 Crystals Emerging from Superdomain Boundaries (Adv. Electron. Mater. 6/2023)

Charged Domain Walls In article number 2300005, Petr S. Bednyakov and Jiří Hlinka observe transient domain structure with multiple superdomains containing bundles of ferroelastic subdomains during the paraelectric to the ferroelectric transformation in the crystal of barium titanate under the off-axis electric field. Operando optical microscopy reveals that after the retraction of the paraelectric phase, the superdomain boundaries convert into the desired charged domain wall species.

Impedance spectroscopy investigation and structural characterization of yttrium and lithium modified Barium Strontium Titanate composite

Structural and Dielectric characterization of lead free (Y0.5Li0.5)0.1Ba0.7Sr0.2TiO3 electro-ceramic is reported in this communication. The solid solution was synthesized using carbonates and oxides via a solid-state mixed oxide technique. Calcination was carried out at 1400 °C for 12 h and XRD measurement was carried out to confirm the crystal phase. Lattice parameter were calculated from the refined X-ray diffraction data and the crystallite size was calculated following Debye Scherrer’s equation, Williamson Hall and Haldar Wagner Method. Calcined powder was pelletized and sintered at 1400 °C for 2 h for further dielectric measurements. Dielectric characterization was carried out on the sintered sample from room temperature to 350 °C to investigate the ferroelectric to paraelectric phase transition characteristics. Activation energy (Ea) and ionic conductivity (σ̥) of the Y and Li co-modified Barium Strontium Titanate at discrete frequencies are discussed.

Effects of oxygen vacancies on electric properties of high valence ions Mo/Zr/Ti doped Bi0.85Nd0.15FeO3 ceramics with morphotropic phase boundary

The effects of oxygen vacancies on electric properties of high valence ions (Mo/Zr/Ti) doped Bi0.85Nd0.15FeO3 (marked as BNFM, BNFZ, BNFT) ceramics with morphotropic phase boundary (MPB) are investigated. MPB structures consisting of rhombohedral, intermediate and orthorhombic phases exist in all samples, which is confirmed by the XRD Rietveld refinements and TEM analyses. The BNFT has the maximum remnant polarization of 34.5 μC/cm2 at 180 kV/cm, owing to its lowest paraelectric orthorhombic phase content. XPS and impedance results reveal that the BNFT ceramic has the relatively low oxygen vacancy contents and the largest impedance value, which is beneficial for the ferroelectric test. The best comprehensive properties of BNFT open wide prospects for improving BiFeO3-based ceramics.

Towards low-temperature processing of lead-free BZT thin films for high-temperature energy storage performance

In this work, lead-free BaZr0.35Ti0.65O3 (BZT) thin films were grown on silicon by using pulsed laser deposition. The phase structures of the BZT thin films were controlled via the deposition temperature, and their effects on the breakdown strength and energy-storage performance were systematically investigated. Our results indicate that the BZT thin films deposited at a low temperature of 450 °C have a paraelectric phase and display the highest recoverable energy density of 121.2 J/cm3 and a medium energy efficiency of 88.2%. The ultrahigh energy storage achieved is mainly due to the remarkable improvement in the breakdown strength and the slimmer and slanted polarization hysteresis loop. Moreover, the BZT thin films demonstrate excellent thermal stability across a wide temperature measurement range (25–200 °C) and a superior fatigue endurance of up to 1010 switching cycles even under a high temperature measurement of 200 °C. This work provides a simple and effective way of improving the energy storage properties and stable performance of dielectric thin films for application in harsh environments.

Investigation of relaxor and diffuse dielectric phase transitions of Ba1-XBixTi0.8Fe0·2O3 materials

Different doping elements have been used to enhance the dielectric properties of BaTiO3 ceramic. In this work, the effect of substitution of Ba by Bi in A site and Ti by Fe in B site on structural, dielectric and electrical properties of Ba1-xBixTi0.80Fe0·20O3 ceramics at (x = 0.00, 0.05, 0.10 and 0.15) was investigated by X-ray diffraction, Raman spectroscopy, Scanning Electron Microscopy (SEM), Mössbauer spectroscopy as well as dielectric measurements. The Rietveld refinement results revealed that the prepared compounds crystallize in both tetragonal (P4mm) and hexagonal (P63/mmc) phases for x = 0.00 and 0.05 while at x = 0.10 and 0.15, the hexagonal phase disappears and only the tetragonal phase is fitted. The Raman spectra confirmed the disappearance of hexagonal phase in benefit of tetragonal phase as the Bi3+ substitution increases. Based on Mössbauer analyses results, all the samples are in paramagnetic state at room temperature and the Fe is oxidized under Fe3+ without the presence of Fe2+ or Fe4+ ions. The dielectric measurements as function of temperature are studied and tree broad and relaxor phase transitions were detected: from rhombohedral to orthorhombic phase TR-O and to tetragonal ferroelectric phase TO-T then to cubic paraelectric phase Tm. These phase transitions were displaced to the lower temperature with increasing of Bi3+ substitution. The values of ε′r increase gradually with increasing of Bi3+contents which confirmed the enhancement of dielectric properties of BaTi0·80Fe0·20O3 by Bi substitution on Ba site. The diffuse phase transitions were described by fitting the modified Uchino relation. The Cole–Cole analyses showed that both the grain and grain boundaries resistivity values are higher for Bi3+ substituted samples which are responsible to the dielectric properties improvement.

Stochastic theory of ferroelectric domain structure formation dominated by quenched disorder

A self-consistent stochastic model of domain structure formation in a uniaxial ferroelectric, quenched from a high-temperature paraelectric phase to a low-temperature ferroelectric phase, is developed with an account of the applied electric field and the feedback effect via local depolarization fields. Both polarization and field components are considered as Gauss random variables. A system of integro-differential equations for correlation functions of all involved variables is derived and solved analytically and numerically. Phase diagram in terms of the average value and dispersion of polarization reveals different possible equilibrium states and available final single-domain and multi-domain states. The time-dependent evolution of the average polarization and dispersion discloses a bifurcation behavior and the temperature-dependent value of the electric field, deciding between the single-domain and multi-domain final states, which can be interpreted as the coercive field. Analytical and numerical results for the time-dependent correlation length and correlation functions exhibit plausible agreement with available experimental data.

Improvements of physicochemical properties of K0.5Na0.5NbO3 ceramics by addition of selected chelating agents

In this paper, K0.5Na0.5NbO3 (KNN) samples were successfully synthesized via modified Pechini technique with many different chelating agents. Physicochemical studies as differential scanning calorimetry (DSC) measurements, dielectric measurements, impedance spectroscopy and structural analyses confirmed the influence of chelating agent on phase transformation from the ferroelectric orthorhombic to tetragonal and ferroelectric tetragonal to paraelectric cubic phase. The study of the charge transfer mechanism of KNN is carried out based on small polaron hopping theory. In addition, the high dielectric constant associated with a low dielectric loss observed for KNN material obtained using combination of glycine and oxalic acid makes it a possible use in electronic devices.

Phase Transition and Energy Storage Density in Lead-Free Ferroelectric Ba1−xSrxTiO3 (x = 0.1, 0.3, and 0.7) Capacitors

Structure, phonon, and energy storage density in Sr2+-substituted lead-free ferroelectric Ba1−xSrxTiO3 (BSTx) for compositions x = 0.1, 0.3, and 0.7 were investigated using X-ray diffraction, Raman, and ferroelectric polarization measurements as a function of temperature. The samples were tetragonal for x = 0.1 with a large c/a ratio. The tetragonal anisotropy was decreased upon increasing x and transforming to cubic for x = 0.7. The changes in structural and ferroelectric properties were found to be related to the c/a ratios. The temperature-dependent phonon spectroscopy results indicated a decrease in tetragonal–cubic phase transition temperature, Tc, upon increasing x due to a reduction in the lattice anisotropy. The intensity of ~303 cm−1 E(TO2) mode decreased gradually with temperature and finally disappeared around the tetragonal ferroelectric to cubic paraelectric phase at about 100 ℃ and 40 ℃ for x = 0.1 and 0.3, respectively. A gradual reduction in the band gap Eg of BSTx with x was evident from the analysis of UV-visible absorption spectra. The energy storage density (Udis) of the ferroelectric capacitors for x = 0.7 was ~0.20 J/cm3 with an energy storage efficiency of ~88% at an applied electric field of 104.6 kV/cm. Nearly room temperature transition temperatures TC and reasonably fair energy storage density of the BSTx capacitors were found.

Stability of ferroelectric phase and structural characteristics in oriented PbTiO3 ceramic coating formed by aerosol deposition method

Temperature-dependent structural variations of ceramic coatings of lead titanate PbTiO3 (PT) formed at room temperature (RT) via aerosol deposition (AD) have been investigated to evaluate the stability of the ferroelectric phase. Synchrotron radiation x-ray diffraction experiments in the as-deposited state showed a polycrystalline film oriented preferentially along the c-axis of the perovskite-type ferroelectric tetragonal structure on the quartz glass substrate. The c-axis orientation may be attributed to the anisotropic structural characteristics of the crystal with a layered structure composed of a two-dimensional (2D) covalent bonding network in the ferroelectric phase at RT. The 2D interfaces of the ceramic particles in the aerosol, which are chemically activated by collision with the substrate, are most likely to recombine during the AD process and form a dense and hard coating at RT. When the PT film was heated to the paraelectric phase with a cubic structure at 1000 K, the c-axis orientation observed in the as-deposited state disappeared upon cooling owing to the degree of freedom in selecting the polar axis associated with the cubic-tetragonal phase transition. The spontaneous lattice distortion of the AD film at RT was smaller than that of the bulk ceramic. However, the phase transition temperature (TC) was approximately 100 K higher only during the first heating process from the c-oriented as-deposited state. The AD method stabilizes the ferroelectric phase up to a higher temperature, although the spontaneous polarization is suppressed. The ferroelectric ceramic materials synthesized mainly via particle collisions are assumed to have low ferroelectricity but high TC.

In search of Pca21 phase ferroelectrics

In recent years, hafnia-based ferroelectrics have attracted enormous attention due to their capability of maintaining ferroelectricity below 10 nm thickness and excellent compatibility with microelectronics flow lines. However, the physical origin of their ferroelectricity is still not fully clear, although it is commonly attributed to a polar Pca21 orthorhombic phase. The high-temperature paraelectric phases (the tetragonal phase or the cubic phase) do not possess a soft mode at the Brillouin zone center, thus the ferroelectric distortion has to be explained in terms of trilinear coupling among three phonon modes in the tetragonal phase. It is necessary to explore new materials with possible ferroelectricity due to the polar Pca21 phase, which in turn should be very helpful in evaluating the microscopic theory for ferroelectric hafnia. In this work, based on the idea of the Materials Genome Engineering, a series of hafnia-like ferroelectrics have been found, exemplified by LaSeCl, LaSeBr, LuOF and YOF, which possess adequate spontaneous polarization values and also relatively favorable free energies for the polar phase. Their common features and individual differences are discussed in detail. In particular, a promising potential ferroelectric material, Pca21 phase LuOF, is predicted and recommended for further experimental synthesis and investigation.

Doping effects on the ferroelectric properties of wurtzite nitrides

Ferroelectric materials have been explored for a long time for easy integration with state-of-the-art semiconductor technologies. Doped wurtzite nitrides have been reported as promising candidates due to their high stability, compatibility, and scalability. We investigate doping effects on ferroelectric properties of Sc-doped AlN (AlScN) and B-doped AlN (AlBN) by first-principles methods. The energy barrier against polarization switching is observed to decrease with increasing doping concentration at low concentration ranges, which is the origin of the emerging ferroelectricity in doped AlN. Further increasing the doping concentration to a critical value, the ferroelectric wurtzite phase transforms into paraelectric phases (a rock salt phase for AlScN and a zinc blende phase for AlBN), making it invalid to decrease the coercivity by increasing the doping concentration. Furthermore, it is revealed that different nonpolar structures (a hexagonal phase for AlScN and a β-BeO phase for AlBN) appear in the ferroelectric switching pathway, generating different switching features in doped AlN. Our results give a microscopic understanding of the ferroelectricity in doped wurtzite materials and broaden the route to improve their ferroelectric properties.

Quantum paraelectricity and structural phase transitions in strontium titanate beyond density functional theory

We demonstrate an approach for calculating temperature-dependent quantum and anharmonic effects with beyond density-functional theory accuracy. By combining machine-learned potentials and the stochastic self-consistent harmonic approximation, we investigate the cubic to tetragonal transition in strontium titanate and show that the paraelectric phase is stabilized by anharmonic quantum fluctuations. We find that a quantitative understanding of the quantum paraelectric behavior requires a higher-level treatment of electronic correlation effects via the random phase approximation. This approach enables detailed studies of emergent properties in strongly anharmonic materials beyond density-functional theory.

Dielectric response of a ferroelectric nematic liquid crystalline phase in thin cells

ABSTRACT We studied the dielectric properties of a polar nematic phase (NF) sandwiched between two gold or ITO electrodes, serving as a cell surface. In bulk, NF is expected to exhibit a Goldstone mode (phason), because polarisation can uniformly rotate with no energy cost. However, because the coupling between the direction of nematic director and polarisation is finite, and the confinement, even in the absence of the aligning surface layer, induces some energy cost for a reorientation of polarisation, the relaxation frequency of the phason mode is measured in a kHz regime. The phason mode is easily quenched by a bias electric field, which enables fluctuations in the magnitude of polarisation to be followed in both the ferroelectric and paraelectric nematic phases. This amplitude (soft) mode is also influenced by boundary conditions. A theory describing the phase and amplitude fluctuations in the NF phase shows that the free energy of the system and, consequently, the dielectric response are dominated by polarisation-related terms with the flexoelectricity being relevant only at a very weak surface anchoring. Contributions due to the nematic elastic terms are always negligible. The model relates the observed low-frequency mode to the director fluctuations weakly coupled to polarisation fluctuations.

Morphotropic phase boundary, polymorphic phases and enhanced electrostrain/piezoelectricity in Ag1-xKxNbO3 solid-solution ceramics

Ag1−xKxNbO3(AKNx (x ≤ 0.12) ceramics were prepared to understand the relationship of structure-properties driven by compositions and temperatures. The results suggested that this binary system possessed a morphotropic phase boundary (MPB) consisted of ferrielectric and ferroelectric phases with iso-symmetry at room temperature, in which domains switching together with electric-field-induced irreversible phase transition achieved a much higher electrostrain (Smax = 0.4%) than other compositions. But this MPB was destroyed after poling, leading to inferior piezoelectricity. A phase diagram was drawn after analyzing in situ XRD and dielectric data, where an almost vertical ferrielectric/antiferroelectric ↔ polymorphic ferroelectric MPB line starting from a triple point was proposed. As temperature increased, the piezoelectricity significant enhanced near ferroelectric orthorhombic ↔ monoclinic phase boundary, while the highest piezoelectricity was achieved near the monoclinic ↔ paraelectric phase boundary with d33 = 200 pC/N. The enhanced piezoelectricity is intimately related to the ferroelectric monoclinic possessing Pm symmetry.

The Doping Effect on the Intrinsic Ferroelectricity in Hafnium Oxide-Based Nano-Ferroelectric Devices.

Hafnium oxide (HfO2)-based ferroelectric tunnel junctions (FTJs) have been extensively evaluated for high-speed and low-power memory applications. Herein, we investigated the influence of Al content in HfAlO thin films on the ferroelectric characteristics of HfAlO-based FTJs. Among HfAlO devices with different Hf/Al ratios (20:1, 34:1, and 50:1), the HfAlO device with Hf/Al ratio of 34:1 exhibited the highest remanent polarization and excellent memory characteristics and, thereby, the best ferroelectricity among the investigated devices. Furthermore, first-principal analyses verified that HfAlO thin films with Hf/Al ratio of 34:1 promoted the formation of the orthorhombic phase against the paraelectric phase as well as alumina impurities and, thus, enhanced the ferroelectricity of the device, providing theoretical support for supporting experimental results. The findings of this study provide insights for developing HfAlO-based FTJs for next-generation in-memory computing applications.

Lead-Free Hybrid Organic-Inorganic Ferroelectric: (3,3-Difluoropyrrolidinium)2ZnCl4·H2O.

Hybrid organic-inorganic ferroelectrics (HOIFs) have a wide range of applications in the optoelectronic field in terms of rich optoelectronic properties. Particularly, lead-free HOIFs have attracted extensive attention due to their environmental friendliness, low heavy metal toxicity, and low synthesis cost. However, there are few reports about Zn-based HOIFs due to their uncontrollable ferroelectric synthesis and other reasons. Here, we designed and synthesized a zinc-based zero-dimensional (3,3-difluoropyrrolidine)2ZnCl4·H2O (DFZC) single crystal, which undergoes a phase transition from ferroelectric to paraelectric phase (space group from Pna21 to Pnma) at 295.5 K/288.9 K during the heating/cooling process. The systematic study shows that the ferroelectric phase transition is a displacive type. The ferroelectric hysteresis loop of DFZC was obtained by the double-wave method and the Sawyer-Tower method, which has a spontaneous polarization (Ps) of ∼0.4 μC/cm2. This work reveals the strategy to design new zinc-based lead-free HOIFs for potential applications in optoelectronic fields.

Coexistence of ferroelectric and paraelectric KNO3 phases in carbon nanotubes

The DTA signal was studied in comparison with X-ray phase analysis of potassium nitrate particles in carbon nanotubes. It is shown that, for potassium nitrate particles in nanotubes, the temperature range of the existence of the KNO3 ferroelectric phase expands up to 300 K and the ferroelectric and low-temperature paraelectric phases coexist.

Lattice dynamics and dielectric characteristics of c-oriented SBN-50 thin films grown on a Pt(111)/Al2O3(0001) substrate

The lattice dynamics and dielectric properties of Ba0.5Sr0.5Nb2O6 ferroelectric thin films grown by high-frequency RF-deposition in an oxygen atmosphere on a Pt(111)/Al2O3 substrate (c - cut) were studied. It has been established that in the film, in comparison with the bulk material, the transition temperature from the ferroelectric to the paraelectric phase increases. Dielectric spectroscopy data have shown that the Ba0.5Sr0.5Nb2O6/Pt/Al2O3 heterostructure exhibits the behavior of a ferroelectric relaxor. The detected Burns temperature, Tb, is 180-190 °C.

Electrical topological defects induced by terahertz laser pulses

Recent experimental evidence and theoretical simulations have shown that electrical polarization can be tuned and even deterministically controlled in ferroelectrics by applying laser pulses having a frequency that is higher than that of the main polarization (soft) mode. Here, by using ab initio computations, we further discover that such pulses can also create nontrivial topological polar phases in bulk ferroelectric materials. Such a phenomenon is found to originate from nonthermal quenching process in which the applied intense ultrashort laser pulse first squeezes the ferroelectric phase into a cubic-like paraelectric phase at a moderate temperature and then to a renucleation process akin to the Kibble-Zurek mechanism creating nontrivial topological polar defects in the bulk ferroelectric ground-state phase. Our work therefore presents a novel and optical mean of creating topological defects in ferroelectrics.

Controlling ferroelectric properties in Y-doped HfO2 thin films by precise introduction of oxygen vacancies

Thin-film ferroelectric doped hafnia has emerged as a promising candidate for non-volatile computer memory devices due to its CMOS compatibility. The ferroelectricity in thin-film HfO2 is defined by the polar orthorhombic phase, whose stabilization depends on various parameters, such as doping species, stress, thickness, crystallization annealing temperature, etc. The concentration of oxygen vacancies is yet another parameter affecting the stabilization of the ferroelectric phase in HfO2 thin films. Here, we report on the effect of oxygen vacancies introduced in Y-doped HfO2 (HYO) films during reactive pulsed laser deposition on their ferroelectric properties, which we systematically study by correlating structural and electrical properties. Among different techniques, near-edge x-ray absorption fine structure analysis is successfully employed to distinguish between structurally similar ferroelectric orthorhombic and paraelectric tetragonal phases. It is shown that oxygen vacancies introduced at a certain concentration in HYO films can be used as a tool to control the phase composition as well as to decrease the formation energy (crystallization temperature) of the ferroelectric phase. Based on these results, we demonstrate a back-end-of-line compatible ferroelectric HYO capacitor device with competitive functional properties.