Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO3/SrTiO3 superlattices

Deciphering the atomic-scale structural origin for large dynamic electromechanical response in lead-free Bi0.5Na0.5TiO3-based relaxor ferroelectrics

Despite the extraordinary electromechanical properties of relaxor ferroelectrics, correlating their properties to underlying atomic-scale structures remains a decisive challenge for these "mess" systems. Here, taking the lead-free relaxor ferroelectric Bi_0.5Na_0.5TiO₃-based system as an example, we decipher the atomic-scale structure and its relationship to the polar structure evolution and large dynamic electromechanical response, using the direct atomic-scale point-by-point correlation analysis. With judicious chemical modification, we demonstrate the increased defect concentration is the main driving force for deviating polarizations with high-angle walls, leading to the increased random field. Meanwhile, the main driving force for deviating polarizations with low-angle walls changes from the anti-phase oxygen octahedral tilting to the multidirectional A-O displacement, leading to the decreased anisotropy field. Benefiting from the competitive and synergetic equilibrium of anisotropic field versus random field, the facilitated polarization rotation and extension versus facilitated domain switching are identified to be responsible for the giant electromechanical response. These observations lay a foundation for understanding the "composition-structure-property" relationships in relaxor ferroelectric systems, guiding the design of functional materials for electromechanical applications.

Rational Assembly of Two-Dimensional Perovskite Nanosheets as Building Blocks for New Ferroelectrics

Artificial materials in the form of superlattices have been studied actively in quest of new engineering methods or design rules for the development of desired functionalities, in particular high-k ferroelectricity, ferromagnetism, and high mobility electron gas. This work presents a controlled assembly strategy for fabricating atomically precise interfaces of two-dimensional (2D) homologous perovskite nanosheets (Ca₂Na_m-3Nb_mO_3m+1^-; m = 3-6) to construct artificial superlattices. The distinctive thickness of each 2D homologous perovskite nanosheets attributed to the presence of different number of NbO₆ octahedra provides an exquisite control to engineer interfacial properties for tailored design of superior high-k properties and emergence of ferroelectricity. The higher dielectric constant (ε_r = 427) and development of ferroelectricity for (Ca₂Nb₃O₁₀^-/Ca₂Na₂Nb₅O₁₆^-)₆ superlattice indicate that superlattice films with both odd number of NbO₆ octahedra possess extended polarization due to the potential effect of heterointerface and ferroelectric instabilities. Furthermore, the increased discontinuities/offsets in Ca₂Nb₃O₁₀^- and Ca₂Na₃Nb₆O₁₉^- nanosheets band alignment results in superior insulating properties (∼1 × 10^-11 A cm^-2 at 1 V) for (Ca₂Nb₃O₁₀^-/Ca₂Na₃Nb₆O₁₉^-)₆ superlattice. These findings exhibit new research opportunities for the development of novel artificial high-k dielectric/ferroelectric via precise control of interfaces at the atomic level and can be extended to the large family of 2D perovskite compounds.

Magneto-electric multiferroics: designing new materials from first-principles calculations

In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributed first-principles calculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.

Polarization Rotation in Ultrathin Ferroelectrics Tailored by Interfacial Oxygen Octahedral Coupling

Multiple polar states and giant piezoelectric responses could be driven by polarization rotation in ferroelectric films, which have potential functionalities in modern material applications. Although theoretical calculations have predicted polarization rotation in pure PbTiO₃ films without domain walls and strains, direct experiment has rarely confirmed such polar states under this condition. Here, we observed that interfacial oxygen octahedral coupling (OOC) can introduce an oxygen octahedral rotation, which induces polarization rotation in single domain PbTiO₃ films with negligible strains. We have grown ultrathin PbTiO₃ films (3.2 nm) on both SrTiO₃ and Nb:SrTiO₃ substrates and applied aberration-corrected scanning transmission electron microscopy (STEM) to study the interfacial OOC effect. Atomic mappings unit cell by unit cell demonstrate that polarization rotation occurs in PbTiO₃ films on both substrates. The distortion of oxygen octahedra in PbTiO₃ is proven by annular bright-field STEM. The critical thickness for this polarization rotation is about 4 nm (10 unit cells), above which polarization rotation disappears. First-principles calculations manifest that the interfacial OOC is responsible for the polarization rotation state. These results may shed light on further understanding the polarization behavior in ultrathin ferroelectrics and be helpful to develop relevant devices as polarization rotation is known to be closely related to superior electromechanical responses.

Strain-induced ferroelectricity and lattice coupling in BaSnO3 and SrSnO3

Perovskite stannates such as BaSnO₃ and SrSnO₃ exhibit promising photovoltaic properties, and hold promise for application in solar cell devices. However, the lack of ferroelectricity and the wide band gap in these materials limit their potential for photovoltaic applications. Here, by first-principles calculations, we demonstrate the realization of a primary ferroelectric polarization in non-ferroelectric BaSnO₃ and SrSnO₃ through strain engineering. In addition to the appearance of polarization, the band gaps of the materials are greatly narrowed when the paraelectric to ferroelectric phase transition takes place under compressive strain. Furthermore, an intriguing Q₂ mode triggered by lattice coupling with the polar mode is found in the stannates subjected to a sufficient tensile strain and this mode has a significant effect on the band gap, which suggests another pathway to narrow the band gap through the electric field control of the Q₂ mode. The fruitful electronic, structural, and energetic properties are discussed in detail to achieve a fundamental understanding of the strain-induced ferroelectricity, tunable band gap, and lattice couplings between the Q₂ mode and different polar/rotational distortions in the perovskite stannates.

Competition between Polar and Nonpolar Lattice Distortions in Oxide Quantum Wells: New Critical Thickness at Polar Interfaces

Two basic lattice distortions permeate the structural phase diagram of oxide perovskites: antiferrodistortive (AFD) rotations and tilts of the oxygen octahedral network and polar ferroelectric modes. With some notable exceptions, these two order parameters rarely coexist in a bulk crystal, and understanding their competition is a lively area of active research. Here we demonstrate, by using the LaAlO_{3}/SrTiO_{3} system as a test case, that quantum confinement can be a viable tool to shift the balance between AFD and polar modes and selectively stabilize one of the two phases. By combining scanning transmission electron microscopy (STEM) and first-principles-based models, we find a crossover between a bulklike LaAlO_{3} structure where AFD rotations prevail, to a strongly polar state with no AFD tilts at a thickness of approximately three unit cells; therefore, in addition to the celebrated electronic reconstruction, our work unveils a second critical thickness, related not to the electronic properties but to the structural ones. We discuss the implications of these findings, both for the specifics of the LaAlO_{3}/SrTiO_{3} system and for the general quest towards nanoscale control of material properties.

Stability of Polar Vortex Lattice in Ferroelectric Superlattices

A novel mesoscale state comprising of an ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO₃/SrTiO₃. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper and a lower bound to the length scale at which these states can be observed. We found that the critical length is related to the intrinsic domain wall width, which could serve as a simple intuitive design rule for the discovery of novel ultrafine topological structures in ferroic systems.

Epitaxial-strain-induced polar-to-nonpolar transitions in layered oxides

Epitaxial strain can induce collective phenomena and new functionalities in complex oxide thin films. Strong coupling between strain and polar lattice modes can stabilize new ferroelectric phases from nonpolar dielectrics or enhance electric polarizations and Curie temperatures. Recently, strain has also been exploited to induce novel metal-insulator transitions and magnetic reconstructions through its coupling to nonpolar modes, including rotations of BO6 transition-metal octahedra. Although large strains are thought to induce ferroelectricity, here we demonstrate a polar-to-nonpolar transition in (001) films of layered A3B2O7 hybrid-improper ferroelectrics with experimentally accessible biaxial strains. We show the origin of the transition originates from the interplay of trilinear-related lattice mode interactions active in the layered oxides, and those interactions are directly strain tunable. Our results call for a careful re-examination of the role of strain-polarization coupling in ferroelectric films with nontrivial anharmonicities and offer a route to search for new functionalities in layered oxides.

New modalities of strain-control of ferroelectric thin films

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.

Piezoelectricity and rotostriction through polar and non-polar coupled instabilities in bismuth-based piezoceramics

Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed "rotopolarization". These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi1/2Na1/2TiO3-0.25SrTiO3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.

Enhanced polarization by the coherent heterophase interface between polar and non-polar phases

A piezoelectric composite containing the ferroelectric polar (Bi(Na0.8K0.2)0.5TiO3: f-BNKT) and the non-polar (0.94Bi(Na0.75K0.25)0.5TiO3-0.06BiAlO3: BNKT-BA) phases exhibits synergetic properties which combine the beneficial aspects of each phase, i.e., the high saturated polarization (Ps) of the polar phase and the low coercive field (Ec) of the non-polar phase. To understand the origin of such a fruitful outcome from this type of polar/non-polar heterophase structure, comprehensive studies are conducted, including transmission electron microscopy (TEM) and finite element method (FEM) analyses. The TEM results show that the polar/non-polar composite has a core/shell structure in which the polar phase (core) is surrounded by a non-polar phase (shell). In situ electrical biasing TEM experiments visualize that the ferroelectric domains in the polar core are aligned even under an electric field of ∼1 kV mm(-1), which is much lower than its intrinsic coercive field (∼3 kV mm(-1)). From the FEM analyses, we can find that the enhanced polarization of the polar phase is promoted by an additional internal field at the phase boundary which originates from the preferential polarization of the relaxor-like non-polar phase. From the present study, we conclude that the coherent interface between polar and non-polar phases is a key factor for understanding the enhanced piezoelectric properties of the composite.

Single crystal functional oxides on silicon

Single-crystalline thin films of complex oxides show a rich variety of functional properties such as ferroelectricity, piezoelectricity, ferro and antiferromagnetism and so on that have the potential for completely new electronic applications. Direct synthesis of such oxides on silicon remains challenging because of the fundamental crystal chemistry and mechanical incompatibility of dissimilar interfaces. Here we report integration of thin (down to one unit cell) single crystalline, complex oxide films onto silicon substrates, by epitaxial transfer at room temperature. In a field-effect transistor using a transferred lead zirconate titanate layer as the gate insulator, we demonstrate direct reversible control of the semiconductor channel charge with polarization state. These results represent the realization of long pursued but yet to be demonstrated single-crystal functional oxides on-demand on silicon.

Synthesis of single-crystal complex-oxide films directly on silicon is difficult due to differing interfacial chemistry. Here, the authors demonstrate room-temperature integration of single-crystal lead zirconate titanate on to silicon to act as a gate insulator in a field-effect transistor.

Structural phase transition and spin reorientation of LaFeO3 films under epitaxial strain

Structural phase transition and spin reorientation of orthoferrites LaFeO₃ epitaxially grown along the pseudocubic (001) direction are investigated based on first-principles calculation.

Interplay of coupling between strain and rotation in ferroelectric SrZrO3/SrTiO3 superlattices

The combination of oxygen octahedral rotation and epitaxial strain provides a unique opportunity to tune the ferroelectric properties of perovskite superlattices. Here, through first-principles calculations, we demonstrate that the oxygen octahedral rotation predominates the ground state and ferroelectric properties of SrZrO3/SrTiO3 superlattices. The predicted ground state combines the ferroelectric distortion and antiferrodistortive modes simultaneously. The structure-strain phase diagrams of the superlattices are calculated with and without octahedral rotations, which elucidate the interplay of coupling between epitaxial strain and octahedral rotation. It is found that the presence of octahedral rotation not only lowers the energy but also changes the sequence of phase transition from c-r-aa to c-r, in which the coupling of rotation and strain induces an out-of-plane polarization that transforms aa-phase into r-phase.

Polarization Rotation in Ferroelectric Tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 Superlattices

In ferroelectric thin films, controlling the orientation of the polarization is a key element to controlling their physical properties. We use laboratory and synchrotron X-ray diffraction to investigate ferroelectric bicolor PbTiO3/PbZr0.2Ti0.8O3 and tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 superlattices and to study the role of the SrTiO3 layers on the domain structure. In the tricolor superlattices, we demonstrate the existence of 180° ferroelectric stripe nanodomains, induced by the depolarization field produced by the SrTiO3 layers. Each ultrathin SrTiO3 layer modifies the electrostatic boundary conditions between the ferroelectric layers compared to the corresponding bicolor structures, leading to the suppression of the a/c polydomain states. Combined with the electrostatic effect, the tensile strain induced by PbZr0.2Ti0.8O3 in the PbTiO3 layers leads to polarization rotation in the system as evidenced by grazing incidence X-ray measurements. This polarization rotation is associated with the monoclinic Mc phase as revealed by the splitting of the (HHL) and (H0L) reciprocal lattice points. This work demonstrates that the tricolor paraelectric/ferroelectric superlattices constitute a tunable system to investigate the concomitant effects of strains and depolarizing fields. Our studies provide a pathway to stabilize a monoclinic symmetry in ferroelectric layers, which is of particular interest for the enhancement of the piezoelectric properties.

Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments

Recent theoretical and experimental studies showing how polar structures or ferroelectricity arise in layered perovskites are highlighted.

Atomic-scale mapping of dipole frustration at 90° charged domain walls in ferroelectric PbTiO3 films

The atomic-scale structural and electric parameters of the 90° domain-walls in tetragonal ferroelectrics are of technological importance for exploring the ferroelectric switching behaviors and various domain-wall-related novel functions. We have grown epitaxial PbTiO3/SrTiO3 multilayer films in which the electric dipoles at 90° domain-walls of ferroelectric PbTiO3 are characterized by means of aberration-corrected scanning transmission electron microscopy. Besides the well-accepted head-to-tail 90° uncharged domain-walls, we have identified not only head-to-head positively charged but also tail-to-tail negatively charged domain-walls. The widths, polarization distributions, and strains across these charged domain-walls are mapped quantitatively at atomic scale, where remarkable difference between these domain-walls is presented. This study is expected to provide fundamental information for understanding numerous novel domain-wall phenomena in ferroelectrics.

Interplay of octahedral tilts and polar order in BiFeO3 films

Heterointerface stabilization of a distinct nonpolar BiFeO3 phase occurs simultaneously with changes in octahedral tilts. The resulting phase arises via suppression of polarization by a structural order parameter and can thus be identified as anti-ferroelectric (Fe displacements - bottom panel). The phase is metastable and can be switched into a polar ferroelectric state (top panel) under an applied electric bias.

Emergent superstructural dynamic order due to competing antiferroelectric and antiferrodistortive instabilities in bulk EuTiO3

Microscopic structural instabilities of EuTiO3 single crystals were investigated by synchrotron x-ray diffraction. Antiferrodistortive (AFD) oxygen octahedron rotational order was observed alongside Ti derived antiferroelectric distortions. The competition between the two instabilities is reconciled through a cooperatively modulated structure allowing both to coexist. The combination of electric and magnetic fields increases the population of the modulated AFD order, illustrating how the origin of the large magnetoelectric coupling derives from the dynamic equilibrium between AFD and polar instabilities.

Engineering polarization rotation in a ferroelectric superlattice

A key property that drives research in ferroelectric perovskite oxides is their strong piezoelectric response in which an electric field is induced by an applied strain, and vice versa for the converse piezoelectric effect. We have achieved an experimental enhancement of the piezoelectric response and dielectric tunability in artificially layered epitaxial PbTiO(3)/CaTiO(3) superlattices through an engineered rotation of the polarization direction. As the relative layer thicknesses within the superlattice were changed from sample to sample we found evidence for polarization rotation in multiple x-ray diffraction measurements. Associated changes in functional properties were seen in electrical measurements and piezoforce microscopy. The results demonstrate a new approach to inducing polarization rotation under ambient conditions in an artificially layered thin film.

Electrostatic coupling and local structural distortions at interfaces in ferroelectric/paraelectric superlattices

The performance of ferroelectric devices is intimately entwined with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, ordered nanodomains arise naturally in response to the presence of a depolarizing field and give rise to highly inhomogeneous polarization and structural profiles. Ferroelectric superlattices offer a unique way of engineering the desired nanodomain structure by modifying the strength of the electrostatic interactions between different ferroelectric layers. Through a combination of X-ray diffraction, transmission electron microscopy, and first-principles calculations, the electrostatic coupling between ferroelectric layers is studied, revealing the existence of interfacial layers of reduced tetragonality attributed to inhomogeneous strain and polarization profiles associated with the domain structure.