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Lead-free ferroelectrics K1−xNaxNbO3 are promising candidates for sensors and electromechanical and cooling devices. Here, we study the thermodynamics of phase transitions and electrocaloric (EC) properties of K0.5Na0.5NbO3 single crystals subject to electric, thermal, and mechanical stimuli, from which we establish the temperature–pressure and temperature-electric field phase diagrams. The focus is on understanding the pressure effect on the ferroelectric properties of K1−xNaxNbO3, which is critical to develop chemical doping strategies to achieve targeted performances via chemical strain engineering. Under pressure-free conditions, the K0.5Na0.5NbO3 single crystal has a Curie temperature of around 673 K, which is the optimal operating temperature for the EC effect. We show that a hydrostatic pressure of around 7 GPa can reduce the optimal operating temperature to near room temperature, 300 K, at the expense of decreasing the EC temperature change ΔTEC from 5.4 K to 2.4 K when the applied electric field is 10 MV/m. The present work provides a fundamental understanding of the pressure-induced phase transitions and ferroelectric properties and can be employed to guide the future design of K1−xNaxNbO3-based materials.Lead-free ferroelectrics K1−xNaxNbO3 are promising candidates for sensors and electromechanical and cooling devices. Here, we study the thermodynamics of phase transitions and electrocaloric (EC) properties of K0.5Na0.5NbO3 single crystals subject to electric, thermal, and mechanical stimuli, from which we establish the temperature–pressure and temperature-electric field phase diagrams. The focus is on understanding the pressure effect on the ferroelectric properties of K1−xNaxNbO3, which is critical to develop chemical doping strategies to achieve targeted performances via chemical strain engineering. Under pressure-free conditions, the K0.5Na0.5NbO3 single crystal has a Curie temperature of around 673 K, which is the optimal operating temperature for the EC effect. We show that a hydrostatic pressure of around 7 GPa can reduce the optimal operating temperature to near room temperature, 300 K, at the expense of decreasing the EC temperature change ΔTEC from 5.4 K to 2.4 K when the applied electric field ...

The grain size effects on the phase coexistence and magnetostrictive response of Tb1−xDyxFe2 polycrystals near the ferromagnetic morphotropic phase boundary (MPB) are revealed through phase-field modeling. It shows that phase coexistence is a universal phenomenon in polycrystals for both ferromagnetic and ferroelectric MPB and that the range of compositions for phase coexistence increases with decreasing grain sizes. A large, reversible, and anhysteretic magnetostrictive response at low external fields is also found in the fine-grained polycrystals around the ferromagnetic MPB, which offers us a route to developing nanocrystalline magnetostrictive materials.

Potassium-sodium niobate K1-xNaxNbO3 (KNN) is one of the most promising lead-free piezoelectric materials. While there have been many studies on the microstructures and properties of KNN ceramics, the phase transitions and ferroelectric domain structures of KNN thin films are not well understood. In this work, we employ three-dimensional (3D) phase-field simulations to obtain the ferroelectric domain structures of KNN (0 ≤ x ≤ 0.5) thin films under a range of temperatures (0 K to 1300 K) and equiaxial misfit strains (–1.5% to 1.5%), based on which we establish the misfit strain-temperature phase diagrams of KNbO3 and K0.5Na0.5NbO3 thin films. We identify a wide variety of complex domain structures with coexisting ferroelectric phases, implying enhanced dielectric and piezoelectric properties. We expect this work to provide guidance for the strain engineering of domain structures and properties of KNN thin films.

It has been known that ferroelectric charged domain walls (CDWs), which break the polarization continuity, may be electrically active with an elevated conductivity. However, the bound charge at CDWs may render them energetically unstable, and thus, forming CDWs naturally and manipulating them electrically is still challenging. Here, we theoretically utilize phase-field simulations to design spontaneously generated CDWs with center-type quad-domains in a single square-shaped BiFeO3 nanoisland. It is shown that the stability of the spontaneously emerging head-to-head domain walls with center-convergent quad-domains is mainly determined by three contributions, namely, the geometrical constraint from approximately 45°-tilted bottom edges, the electric boundary condition, and the necessary screening free charges to compensate head-to-head domain walls. It is demonstrated that the center-convergent quad-domains with head-to-head CDWs can be electrically switched to the center-divergent one with tail-to-tail CDWs, providing guidance for achieving ferroelectric domain-wall-based nanodevices with low-power dissipation.

A thermodynamic potential for Ba1-xSrxTiO3 solid solutions is developed, and the corresponding thermodynamic properties of Ba1-xSrxTiO3 single crystals are calculated. The predicted temperature-composition phase diagram from the thermodynamic potential agrees well with the experimental measurements. Based on this potential, the energy storage performances and electrocaloric effects of Ba1-xSrxTiO3 single crystals are obtained using the phase-field method. It is found that there is an optimal Sr concentration which maximizes the discharged energy density of a Ba1-xSrxTiO3 single crystal under an applied electric field. The electrocaloric effects of Ba0.8Sr0.2TiO3, Ba0.7Sr0.3TiO3, Ba0.6Sr0.4TiO3, and Ba0.5Sr0.5TiO3 single crystals are also predicted, from which the corresponding optimal temperatures are identified.

Adding high-κ ceramic nanoparticles into polymers is a general strategy to improve the performances in energy storage. Classic effective medium theories may fail to predict the effective permittivity in polymer nanocomposites wherein the space charge effects are important. In this work, a computational model is developed to understand the space charge effects on the frequency-dependent dielectric properties including the real permittivity and the loss for polymer nanocomposites with both randomly distributed and aggregated nanoparticle fillers. It is found that the real permittivity of the SrTiO3/polyethylene (12% SrTiO3 in volume fraction) nanocomposite can be increased to as high as 60 when there is nanoparticle aggregation and the ion concentration in the bulk polymer is around 1016 cm–3. This model can be employed to quantitatively predict the frequency-dependent dielectric properties for polymer nanocomposites with arbitrary microstructures.

Understanding electric field effects on precession dynamics is crucial to the design of spin transfer torque devices for improving the performance in nano-oscillator. In this letter, the precession dynamics of a CoFeB/MgO multi-layer structured nano-oscillator under externally applied electric field is predicted using a micromagnetic simulation. It is revealed that the electric field can modify the range of oscillation spectra in single frequency mode. With the increase in electric field, there is a red-shift of the resonant frequency. When a positive electric field pulse is applied, a phase lag of the spin precession is induced, which is proportional to the pulse amplitude and duration. The present work is expected to stimulate future experimental efforts on designing devices with electric-field modulated spin transfer torque nano-oscillators.

Charge injection is a common phenomenon in heterostructures or devices containing metal-insulator interfaces under a voltage bias ranging from dielectric capacitors to electroluminescent and lasing devices. It is generally believed that charge injection only significantly increases the conductivity near the interfacial region or in capacitors with very thin dielectric layers. In this work, the impact of charge injection on bulk conductivity of a 0.5 mm thick Fe-doped SrTiO3 single crystal is investigated with a combination of experimental impedance measurements and computational modelling. It is found that the interfacial charge injection may increase the predicted bulk conductivity of a dielectric by more than one order of magnitude as a consequence of Schottky barrier height lowering.

Environment-friendly lead-free piezoelectric materials, such as the niobate perovskite compound (K1-xNaxNbO3), have attracted increasing attention over the last decade. Although extensive efforts have been made to study these materials experimentally, there have been much fewer theoretical studies on the microstructural evolution and property optimization of lead-free systems due to the lack of a comprehensive thermodynamic assessment. In this study, we first established a general procedure for constructing thermodynamic potentials for ferroelectric systems. We then developed an eighth-order Landau thermodynamic potential for K1-xNaxNbO3 (0 ≤ x ≤ 0.5) single crystals, taking into account the low temperature quantum mechanical effects. Based on this potential, we computed the spontaneous polarization and the dielectric constants for each composition as well as an overall temperature-composition phase diagram, all of which agree well with the available experimental data.

The orientation dependence of the converse piezoelectric constants in epitaxial single domain piezoelectric films constrained by substrate is calculated. The calculations are supported by the experimental measuring results of longitudinal piezoelectric constants of ferroelectric tetragonal Pb(Zr0.2Ti0.8)O3 thin films with (001), (011), and (111) orientations.

Patterning thin ferroelectric films into discrete islands is an effective way to release the constraint imposed by a substrate and the unpoled nonpiezoactive surrounding film to enhance the film piezoresponse. The converse piezoresponse measured by the surface displacement of ferroelectric islands, with lateral size changing from a nanoscale to a continuous film, has been modeled using three-dimensional finite element method. The modeling has shown that piezodeformation of the islands results in a local deformation of a substrate in the vicinity of island. The deformation is larger when the substrate is softer. The deformation, together with clamping strain in the film, decreases the effective d33 of the film island. The effect of the top electrode on d33 measured by surface displacement is also modeled. The piezoresponse of different size island capacitors with PbZr0.5Ti0.5O3/SrTiO3/Si and PbZr0.2Ti0.8O3/SrTiO3 heterostructures has been calculated. The results of modeling are in good agreement with experimental data on d33 obtained by piezoresponse force microscopy.

Phase-field method micromagnetic microelastic modeling is employed to simulate the thermal domain stability and enhanced magnetostrictive responses around the ferromagnetic morphotropic phase boundary (MPB) in giant magnetostrictive Tb1−xDyxFe2 ( x≈0.27) single crystal. The simulation shows that the rhombohedral and tetragonal phases coexist in equilibrium in the vicinity of MPB region due to the balance of weak magnetocrystalline anisotropy and strong exchange, magnetostatic and ferroelastic interaction. Enhanced magnetostrictive response is found in the vicinity of MPB, which could be attributed to the low-energy rotating pathways of local magnetization vectors in the phase coexisting region.

The dielectricphase transition behavior of imprinted lead magnesium niobate–lead titanate relaxor ferroelectricthin films was mapped as a function of temperature and dc bias. To compensate for the presence of internal fields, an external electric bias was applied while measuring dielectric responses. The constructed three-dimensional dielectric maps provide insight into the dielectric behaviors of relaxor ferroelectricfilms as well as the temperature stability of the imprint. The transition temperature and diffuseness of the dielectric response correlate with crystallographic disorder resulting from strain and defects in the films grown on strontium titanate and silicon substrates; the latter was shown to induce a greater degree of disorder in the film as well as a dielectric response lower in magnitude and more diffuse in nature over the same temperature region. Strong and stable imprint was exhibited in both films and can be utilized to enhance the operational stability of piezoelectric devices through domain self-poling.

The phase diagram of a PbZr1−xTixO3 (PZT) film constrained by a much thicker substrate was studied using both thermodynamic calculations and phase-field approach. It was found that the ferroelectric transition temperature is increased with substrate constraint regardless of the nature of the constraint, i.e., tensile or compressive. The maximum increase in the transition temperature occurs near x=0.5, and the morphotropic phase boundary is shifted considerably by the substrate constraint. It is shown that the orthorhombic phase (|P1|=|P2|≠0, P3=0) that does not exist in the bulk becomes stable under a tensile constraint, and the rhombohedral phase (|P1|=|P2|=|P3|≠0) in the bulk is distorted in the constrained film, i.e., |P1|=|P2|≠0, |P3|≠0. The phase diagrams obtained by the phase-field approach indicated that the stability region for the tetragonal phase is much wider than that obtained from the thermodynamic calculations assuming a single-domain, especially under tensile substrate constraint. The discr...

We report on the out-of-plane piezoelectric response (d33), measured via piezoresponse scanning force microscopy, of submicron capacitors fabricated from epitaxial PbZrxTi1−xO3 thin films. Investigations on 1 μm2 and smaller capacitors show that the substrate-induced constraint is dramatically reduced by nanostructuring. At zero field, the experimentally measured values of d33 for clamped as well as submicron capacitors are in good agreement with the predictions from thermodynamic theory. The theory also describes very well the field dependence of the piezoresponse of clamped capacitors of key compositions on the tetragonal side of the PbZrxTi1−xO3 phase diagram as well as the behavior of submicron PbZr0.2Ti0.8O3 (hard ferroelectric) capacitors. However, the field-dependent piezoresponse of submicron capacitors in compositions closer to the morphotropic phase boundary (soft ferroelectrics) is different from the behavior predicted by the theoretical calculations.

A phase-field model for predicting the coherent microstructure evolution in constrained thin films is developed. It employs an analytical elastic solution derived for a constrained film with arbitrary eigenstrain distributions. The domain structure evolution during a cubic→tetragonal proper ferroelectric phase transition is studied. It is shown that the model is able to simultaneously predict the effects of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, domain shapes, and their temporal evolution.

While electric field induced ferroelectric switching has been extensively studied and broadly utilized, pure mechanical switching via flexoelectric effect has recently opened up an alternative method for domain writing due to their highly localized, electrically erasable and electric damage free characteristics. Thus far, few studies have been made on the coupling effect of electro-mechanical switching in ferroelectric materials, likely due to the experimental difficulty in the accurate definition of the tip-surface contact area and in the identification of mechanical contribution from electrical effect. Here, we employed self-consistent phase-field modeling to investigate the bi-polar switching behavior of (001) oriented Pb(Zr0.2Ti0.8)O3 thin film under concurrent electric and strain field created via a piezoresponse force microscope probe. By separating the effects from electric field, homogeneous strain and strain gradient, we revealed that the homogeneous strain suppresses the spontaneous polarization and accordingly increases the coercive field, and the strain gradient favors unipolar switching and inhibit it in the reverse direction, thus causing lateral offset of the hysteresis loop. The uncertainty of flexoelectric coefficients and the influence of flexocoupling coefficients on switching have also been discussed. Our study could necessitate further understanding of the electric, piezoelectric, and flexoelectric contribution to the switching behavior in nanoscale ferroelectric oxides.

The switching behavior of a (BaTiO3)8/(SrTiO3)4 superlattice grown on a SrTiO3 substrate was simulated utilizing the phase field method. To investigate the effect of the mechanical constraint of the substrate on switching, three types of superlattice/substrate interface mechanical relaxation conditions were considered: (1) fully commensurate, (2) partially relaxed, and (3) fully relaxed. Our simulation results demonstrate that the hysteresis loops under the three types of constraints are very different. The interfacial coherency dramatically affects the coercive field and remanent polarization of the superlattices. The mechanism underlying the hysteresis loop variation with interfacial coherency was investigated by analyzing the ferroelectric domain configuration and its evolution during the switching process. The simulated hysteresis loop of the fully relaxed superlattice exhibits a shape that is potentially relevant to the application of ferroelectrics for energy storage materials.

We employ phase-field modeling to explore the elastic properties of artificially created 1-D domain walls in (001)p-oriented BiFeO3 thin films, composed of a junction of the four polarization variants, all with the same out-of-plane polarization. It was found that these junctions exhibit peculiarly high electroelastic fields induced by the neighboring ferroelastic/ferroelectric domains. The vortex core exhibits a volume expansion, while the anti-vortex core is more compressive. Possible ways to control the electroelastic field, such as varying material constant and applying transverse electric field, are also discussed.

Conducting characteristics of topological defects in ferroelectric materials, such as charged domain walls, engendered a broad interest on their scientific merit and the possibility of novel applications utilizing domain engineering. At the same time, the problem of electron transport in ferroelectrics still remains full of unanswered questions and becomes yet more relevant over the growing interest in ferroelectric semiconductors and new improper ferroelectric materials. We have employed self-consistent phase-field modeling to investigate the physical properties of a local metal-ferroelectric (Pb(Zr0.2Ti0.8)O3) junction in applied electric field. We revealed an up to 10-fold local enhancement of electric field realized by large polarization gradient and over-polarization effects due to inherent non-linear dielectric properties of Pb(Zr0.2Ti0.8)O3. The effect is independent of bias polarity and maintains its strength prior, during and after ferroelectric switching. The observed field enhancement can be considered on similar grounds as increased doping level, giving rise to reduced switching bias and threshold voltages for charge injection, electrochemical and photoelectrochemical processes.