BaTiO3 Single Crystals Research Articles

Effect of texture on the electrical and electrocaloric properties of 0.90Pb(Mg1/3Nb2/3)O3–0.10PbTiO3 relaxor ceramics

Functional properties of ferroelectric compositions depend strongly on the type of stable crystalline phase at a given temperature and texturing. In addition to defining a governing crystallographic axis among the grains with respect to a global reference axis, texturing in these systems is also often meant to imply the relationship between an external applied field and the polar axis. Here, we synthesize randomly oriented and ⟨001⟩pc textured 0.90Pb(Mg1/3Nb2/3)O3–0.10PbTiO3 solid solutions that fall into the relaxor category. A >95% degree of ⟨001⟩pc texturing was achieved by the use of single crystal BaTiO3 template crystallites whose volume fraction does not exceed 5% of the entire sample volume. Electrical measurements made on random and textured samples reveal the impact of texture on the hysteresis and the dielectric response. A Curie–Weiss analysis of the temperature dependent dielectric data shows the degree of relaxor behavior in random and textured samples. As similar compositions have recently been getting interest for electrothermal management applications, we compute the electrocaloric (EC) response of the random and textured samples using indirect methods followed by an estimation of the EC response with a subsequent thermodynamic analysis to shed light on the effect of texture on the observed differences between the textured and the random oriented samples. We finally compare and contrast on the desirability of texturing in these systems for use as EC components for thermal management applications.

Towards a Large-Area Freestanding Single-Crystal Ferroelectric BaTiO3 Membrane

The fabrication and transfer of freestanding single-crystal ferroelectric membranes deserve intensive investigations as to their potential applications in flexible wearable devices, such as flexible data storage devices and varied sensors in E-skin configurations. In this report, we have shown a comprehensive study approach to the acquisition of a large-area freestanding single-crystal ferroelectric BaTiO3 by the Sr3Al2O6 scarification layer method. By controlling the thickness of the BaTiO3 and Sr3Al2O6, the exposed area of the Sr3Al2O6 interlayer, and the utilization of an additional electrode La2/3Sr1/3MnO3 layer, the crack density on the freestanding BaTiO3 can be dramatically decreased from 24.53% to almost none; then, a more than 700 × 530 μm2 area high-quality freestanding BaTiO3 membrane can be achieved. Our results offer a clear and repeatable technology routine for the acquisition of a flexible large-area ferroelectric membrane, which should be instructive to other transition metal oxides as well. Our study can confidently boost flexible device fabrication based on single-crystal transition metal oxides.

Voltage-driven displacement of magnetic vortex cores

Magnetic vortex cores in polycrystalline Ni discs underwent non-volatile displacements due to voltage-driven ferroelectric domain switching in single-crystal BaTiO3. This behaviour was observed using photoemission electron microscopy to image both the ferromagnetism and ferroelectricity, while varying in-plane sample orientation. The resulting vector maps of disc magnetization match well with micromagnetic simulations, which show that the vortex core is translated by the transit of a ferroelectric domain wall, and thus the inhomogeneous strain with which it is associated. The non-volatility is attributed to pinning inside the discs. Voltage-driven displacement of magnetic vortex cores is novel, and opens the way for studying voltage-driven vortex dynamics.

Imaging microstructural dynamics and strain fields in electro-active materials in situ with dark field x-ray microscopy.

The electric-field-induced and temperature induced dynamics of domains, defects, and phases play an important role in determining the macroscopic functional response of ferroelectric and piezoelectric materials. However, distinguishing and quantifying these phenomena remains a persistent challenge that inhibits our understanding of the fundamental structure-property relationships. In situ dark field x-ray microscopy is a new experimental technique for the real space mapping of lattice strain and orientation in bulk materials. In this paper, we describe an apparatus and methodology for conducting in situ studies of thermally and electrically induced structural dynamics and demonstrate their use on ferroelectric BaTiO3 single crystals. The stable temperature and electric field apparatus enables simultaneous control of electric fields up to ≈2 kV/mm at temperatures up to 200 °C with a stability of ΔT = ±0.01 K and a ramp rate of up to 0.5 K/min. This capability facilitates studies of critical phenomena, such as phase transitions, which we observe via the microstructural change occurring during the electric-field-induced cubic to tetragonal phase transition in BaTiO3 at its Curie temperature. With such systematic control, we show how the growth of the polar phase front and its associated ferroelastic domains fall along unexpected directions and, after several cycles of electric field application, result in a non-reversible lattice strain at the electrode-crystal interface. These capabilities pave the way for new insights into the temperature and electric field dependent electromechanical transitions and the critical influence of subtle defects and interfaces.

Effect of electric field orientation on ferroelectric phase transition and electrocaloric effect

The influence of electric field orientation on phase transitions and electrocaloric effects (ECEs) in BaTiO3 single crystals is studied by performing molecular dynamics simulation of first-principles-based effective Hamiltonian. The ECE is directly characterized via the adiabatic temperature change (ΔT) under the microcanonical ensemble. The simulation results demonstrate that different orientation relationship between the electric field and the polarization direction of ferroelectric phase leads to abundant phase structures and ECE behaviors. The electric-field-induced phase transitions produce remarkable ECE peaks/valleys, whereas the polarization rotation and/or extension without phase transition induced by the electric field produces small positive ECEs, which are insensitive to temperature and electric field direction. For the tetragonal-cubic phase transition, ECEs are always positive regardless of the applied electric field direction, and the value and width of ECE peak both increase with the reduction of the angle between electric field and crystallographic orientation. For the orthorhombic-tetragonal and rhombohedral-orthorhombic phase transitions, the sign of ECE gradually changes from negative to positive during the field direction rotates in planes between crystallographic orientations, and the coexistence of positive and negative appears when the field along non-crystallographic directions. Our simulated results shed light on a comprehensive physical understanding of the electric-field-orientation dependent ECE and offer instruction for the design of the refrigeration cycle by combining positive and negative ECEs.

Magnetic and multiferroic properties of dilute Fe-doped BaTiO3 crystals

Combining and coupling both magnetic and electric properties in one single phase multiferroic material has attracted high interest recently to enable a broad range of novel devices and applications. To evaluate one potential route toward new multiferroics, we have studied 0.5% Fe-doped BaTiO3 single crystals and measured the ferroelectric, magnetic, and multiferroic properties. X-ray absorption spectroscopy shows the presence of Fe3+, and magnetic measurements confirmed that this has a significant impact on the magnetic properties. Doping of iron introduces paramagnetism from lone iron atoms as well as what appears to be a weak ferromagnetism. Multiferroicity and magnetoelectric (ME) coupling were observed in the polarization-electric field hysteresis loops with an applied magnetic field, yet there was no direct evidence that ME coupling persists when the sample was in the defect dipole-aligned state.

On the metastable phase in BaTiO3 single crystals

An intermediate structural phase was found between the cubic and tetragonal phases during investigations of dielectric dispersion of highly perfect BaTiO3 single crystals in temperature range 121 °C → 80 °C. The phase was identified as a metastable phase, typical for the first-order phase transitions. It is suggested that the phase is a common property of BaTiO3 single crystals. The manifestation of the metastable phase is usually excluded by growth defects and defects introduced during preparation and measurements which create nucleation centers of a low-temperature phase.

Voltage-driven annihilation and creation of magnetic vortices in Ni discs.

Using photoemission electron microscopy (PEEM) to image ferromagnetism in polycrystalline Ni disks, and ferroelectricity in their single-crystal BaTiO3 substrates, we find that voltage-driven 90° ferroelectric domain switching serves to reversibly annihilate each magnetic vortex via uniaxial compressive strain, and that the orientation of the resulting bi-domain reveals the chirality of the annihilated vortex. Micromagnetic simulations reveal that only 60% of this strain is required for annihilation. Voltage control of magnetic vortices is novel, and should be energetically favourable with respect to the use of a magnetic field or an electrical current. In future, stray field from bi-domains could be exploited to read vortex chirality. Given that core polarity can already be read via stray field, our work represents a step towards four-state low-power memory applications.

Extraordinarily Large Electrocaloric Strength of Metal-Free Perovskites.

The availability of materials with high electrocaloric (EC) strengths is critical to enabling EC refrigeration in practical applications. Although large EC entropy changes, ΔSEC , and temperature changes, ΔTEC , have been achieved in traditional thin-film ceramics and polymer ferroelectrics, they require the application of very high electric fields and thus their EC strengths ΔSEC /ΔE and ΔTEC /ΔE are too low for practical applications. Here, a fundamental thermodynamic description is developed, and extraordinarily large EC strengths of a metal-free perovskite ferroelectric [MDABCO](NH4 )I3 (MDABCO) are predicted. The predicted EC strengths: isothermal ΔSEC /ΔE and adiabatic ΔTEC /ΔE for MDABCO are 18 J m kg-1 K-1 MV-1 and 8.06 K m MV-1 , respectively, more than three times the largest reported values in BaTiO3 single crystals. These predictions strongly suggest the metal-free ferroelectric family of materials as the best candidates among existing materials for EC applications. The present work not only presents a general approach to developing thermodynamic potential energy functions for ferroelectric materials but also suggests a family of candidate materials with potentially extremely high EC performance.

Theoretical analysis of high electromechanical coupling surface acoustic wave propagating on lead-free Na0.5Bi0.5TiO3–BaTiO3 single crystal

The propagation properties of surface acoustic wave (SAW) on the Na0.5Bi0.5TiO3–BaTiO3 (NBBT) substrate with different crystal cuts were theoretically analyzed by Finite Element Method (FEM). The calculated results, for the NBBT with the optimized electrode type and thickness, indicate the existence of Rayleigh and Sezawa modes with high electromechanical coupling factor K2 ~18% and ~6.8% respectively, which are considerably higher than that of the conventional lithium niobate (LiNbO3) single crystal for SAW applications. The research provides a comprehensive understanding of the SAW propagation characteristics of lead-free NBBT, and explores the promising applications of NBBT in wideband SAW devices.

Effect of Electric Field Orientation on Ferroelectric Phase Transition and Electrocaloric Effect

This paper demonstrates the influence of electric field orientation on the phase transition and the electrocaloric effect (ECE) in BaTiO3 single crystals by performing molecular dynamics simulation of first-principles-based effective Hamiltonian, where the ECE is directly characterized through the adiabatic temperature change (ΔT) of the microcanonical ensemble. Different orientation relationship between the electric field and the polarization direction of ferroelectric phases leads to abundant polarization states and various phase structures. If the electric field direction is collinear to the polarization direction of a ferroelectric phase, the stability of the corresponding phase will be improved, and its phase area in the phase diagram is also enlarged; whereas the noncollinear electric field has reverse impacts and distorts the lattice to a low symmetry monoclinic phase. The phase transitions produce large ECEs far beyond those caused by the domain switching. The tetragonal-cubic phase transition produces a positive ECE regardless of the electric field directions, while the orthorhombic-tetragonal and rhombohedral-orthorhombic phase transitions induce positive or negative ECE, or even their coexistence, depending on the field-induced phase transitions under different electric field directions. The signs of ECE are determined by the lattice symmetry before and after the field-induced phase transition. Our simulated results help reveal a more comprehensive physical understanding of ECE and offer an instruction for the design of the refrigeration cycle by combining positive and negative ECEs.

Hierarchically-structured large superelastic deformation in ferroelastic-ferroelectrics

Large superelastic deformation in ferroelastic-ferroelectrics (FMs) is a complex phenomenon involving multiple mechanisms operating simultaneously. Understanding how these mechanisms contribute corporately is critical to apply this useful property to the intrinsically brittle FMs, which can therefore display both excellent functional and mechanical performance. Here, we have directly observed and quantitatively analyzed in situ in a transmission electron microscope the three main mechanisms of twinning domain, phase transformation and mobile point defect contributing to extremely large superelastic deformation in single-crystal BaTiO3 (5.0% strain) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (10.1% strain). Our results reveal the hierarchical origin of large recoverable strain in “brittle” FMs.

Direct Conversion of Heat to Electricity Using First-Order Phase Transformations in Ferroelectrics

The discovery of alternate methods of producing energy that do not contribute to global climate change is a compelling problem of our time. This study develops a means of generating electricity from small temperature differences by exploiting a first-order ferroelectric phase transformation. The authors model the thermodynamics of energy conversion using the Landau free energy, and experimentally demonstrate the direct conversion of heat to electricity using single-crystal BaTiO${}_{3}$. A key role is played by the influence of electric field on transformation temperature, which has the effect of opening up a mixed-phase region in the temperature-entropy diagram.

Paraelectric Elastic Properties of Reduced and Heterovalently Doped BaTiO3 Single Crystals Studied by Using Brillouin Spectroscopy

The effects of rhodium (Rh) doping on the elastic properties and phase transition behaviors of reduced BaTiO3 single crystals were investigated in the paraelectric phase by using Brillouin spectroscopy. The Rh doping induced changes in the elastic stiffness coefficients and the elastic anisotropy parameter. The degree of softening of (C11 − C12)/2 became more substantial with Rh doping, indicating that the polarization fluctuations were enhanced by heterovalent Rh3+ doping. This was supported by the much larger temperature range over which the nonzero birefringence was observed. Free charge carriers introduced by Rh doping and oxygen vacancies in the reduced BaTiO3 and their interactions with the lattice seem to be the origin of these anomalous changes in the elastic properties.

Elastic Anisotropy and Slowness Curves of BaTiO3 Single Crystals in the Paraelectric Phase Studied by Brillouin Spectroscopy

Elastic anisotropy and slowness curves of barium titanate (BaTiO3) single crystal in the paraelectric phase were investigated by using Brillouin spectroscopy. The directional dependence of sound velocities of the longitudinal and the transverse acoustic waves in the (001) plane showed a four-fold symmetry typical in the cubic phase. Especially, one of the two transverse modes exhibited a substantial softening along the directions. The sound velocity of this mode decreased significantly as temperature decreased toward the transition temperature, which corresponded to the elastic stiffness coefficient (C11 – C12)/2. This result suggests the tetragonal instability caused by the precursor polar clusters in the paraelectric phase of BaTiO3.

Elastic anomalies associated with domain switching in BaTiO3 single crystals under in situ electrical cycling

The elastic response of BaTiO3 single crystals during electric field cycling at room temperature has been studied using in situ Resonant Ultrasound Spectroscopy (RUS), which allows monitoring of both the elastic and anelastic changes caused by ferroelectric polarization switching. We find that the first ferroelectric switching of a virgin single crystal is dominated by ferroelastic 90° switching. In subsequent ferroelectric switching, ferroelastic switching is reduced by domain pinning and by the predominance of 180° ferroelectric domains, as confirmed by polarized light microscopy. RUS under in situ electric field therefore demonstrates to be an effective technique for the investigation of electromechanical coupling in ferroelectrics.

Ultraviolet emission from YAlO3:Gd3+ thin film electroluminescent devices fabricated on perovskite-type oxide substrates

Inorganic ultraviolet-electroluminescent (UV-EL) devices with a Gd3+-doped YAlO3 thin film emitting layer were fabricated by radio frequency sputtering. Three kinds of perovskite-type oxide substrates, namely a SrTiO3 single crystal (STO-SC), BaTiO3 single crystal (BTO-SC), and BaTiO3 polycrystalline disk (BTO-PC), were used for the devices. The influence of the substrates on the UV-EL properties of YAlO3:Gd3+ was investigated. It was found that EL threshold voltage, emitting locations, and EL intensity have changed depending on the substrates. The UV-EL attributed to the 6PJ-8S7/2 transitions in Gd3+ was observed in the BTO-PC substrate device with applied voltage of approximately over 200 V and in the STO-SC and BTO-SC substrate devices with that of approximately over 500 V. The STO-SC and BTO-SC devices showed EL only near the edges of a back electrode, whereas the BTO-PC device showed surface EL over the whole electrode. It was considered the local EL at the electrode edges in the SC substrate devices was caused by a concentrated electric field in the vicinity of the electrodes. The BTO-PC device showed not only the lowest EL threshold voltage but also the most intense EL in the three devices. Because the BTO-PC substrate has a high dielectric constant and appropriate surface roughness, which results in a high and uniform electric field over the back electrode, the YAlO3:Gd3+ emitting layer showed intense surface UV-EL with lower applied voltage.

In-situ electric field induced nanoscale BO6 octahedral tilting in lead-free Fe-doped 0.95(Na1/2Bi1/2)TiO3–0.05BaTiO3 single crystal

The phase transition process of pure and 0.5 at% Fe-doped 0.95(Na1/2Bi1/2)TiO3–0.05BaTiO3 (NBBT5) single crystals under in-situ electric-field was investigated using the Raman spectroscopy. The pure NBBT5 exhibited R3c (a−a−a− octahedral tilting) symmetry when subjected to an external electric field (E) up to 70 kV/cm. While after the Fe-doping, the crystal underwent a transition from P4bm (a0a0c+ octahedral tilting) to P4mm (a0a0c0 octahedral tilting) phase under E of 30–40 kV/cm. The results indicated that the octahedral tilting in the Fe-NBBT5 was substantially suppressed by the electric field, which was suggested to contribute to the enhanced piezoelectric response compared to the pure one.

Material influence in newly proposed ferroelectric energy harvesters

Recently, a novel method for mechanical energy harvesting has been proposed, which is based on stress-induced polarization switching in ferroelectric materials. Compared with the traditional piezoelectric energy harvesters, a huge improvement in the output energy has already been theoretically demonstrated. In this article, the influence of different materials on the energy-harvesting performance associated with this new strategy is further studied. The state-of-the-art phase-field model is adopted to investigate the nonlinear hysteretic energy-harvesting process in two nanoscale ferroelectric energy harvesters, which are respectively based on two typical ferroelectric materials—single-crystal BaTiO3 and PbTiO3. In both cases, the effects of the bias voltage and bias resistance are carefully investigated and the optimum values are obtained. Later, the energy-harvesting process and energy flow details in both harvesters working at the optimum conditions are presented and carefully compared in the context of real applications. Furthermore, the energy-harvesting performance of a BaTiO3-based nanoscale piezoelectric energy harvester with equivalent material size is additionally simulated with the finite element method and compared with the corresponding results of the ferroelectric energy harvesters, where obvious advantages associated with the new strategy are demonstrated.

Forward and Backward Acoustic Waves in Crystals with High Piezoactivity and Dielectric Permittivity

Results are presented from studying the characteristics of acoustic waves of different types in plates of ceramic materials with high piezoactivity and dielectric permittivity (e.g., 0.95(Na0.5Bi0.5)TiO3–0.05BaTiO3, (K,Na)(Nb,Ta)O3, and Ba(Zr0.2Ti0.8)O3–50(Ba0.7Ca0.3)TiO3). The dependences of the phase velocities and electromechanical coupling coefficients of these waves on parameter hf (where h is the thickness of a plate, and f is the frequency of a wave) are calculated for directions XY and YX of propagation. It is found that the investigated materials are characterized by frequency ranges of the existence of backward acoustic waves whose phase and group velocities are oriented in different directions. The existence of extremely broad frequency ranges that depend very weakly on a change in the electrical boundary conditions on the surface of a plate is established for waves with negative group velocity in the case of a 0.95(Na0.5Bi0.5)TiO3–0.05BaTiO3 single crystal. It is shown that the electromechanical coupling coefficient of acoustic waves in the investigated materials is greatest for an SH1-wave in YX Ba(Zr0.2Ti0.8)O3–50(Ba0.7Ca0.3)TiO3, and can be as high as 7‒30% in the range hf = 1330–2000 m/s.