Frustration of tilts and A-site driven ferroelectricity in KNbO3-LiNbO3 alloys

Origin of giant electric-field-induced strain in faulted alkali niobate films

A large electromechanical response in ferroelectrics is highly desirable for developing high-performance sensors and actuators. Enhanced electromechanical coupling in ferroelectrics is usually obtained at morphotropic phase boundaries requiring stoichiometric control of complex compositions. Recently it was shown that giant piezoelectricity can be obtained in films with nanopillar structures. Here, we elucidate its origin in terms of atomic structure and demonstrate a different system with a greatly enhanced response. This is in non-stoichiometric potassium sodium niobate epitaxial thin films with a high density of self-assembled planar faults. A giant piezoelectric coefficient of ∼1900 picometer per volt is demonstrated at 1 kHz, which is almost double the highest ever reported effective piezoelectric response in any existing thin films. The large oxygen octahedral distortions and the coupling between the structural distortion and polarization orientation mediated by charge redistribution at the planar faults enable the giant electric-field-induced strain. Our findings demonstrate an important mechanism for realizing the unprecedentedly giant electromechanical coupling and can be extended to many other material functions by engineering lattice faults in non-stoichiometric compositions.

Maximizing the electromechanical response is crucial for developing piezoelectric devices. Here, the authors demonstrate a giant electric-field-induced strain and its origin in alkali niobate epitaxial thin films with self-assembled planar faults.

Using Feature-Assisted Machine Learning Algorithms to Boost Polarity in Lead-Free Multicomponent Niobate Alloys for High-Performance Ferroelectrics

To expand the unchartered materials space of lead-free ferroelectrics for smart digital technologies, tuning their compositional complexity via multicomponent alloying allows access to enhanced polar properties. The role of isovalent A-site in binary potassium niobate alloys, (K,A)NbO3 using first-principles calculations is investigated. Specifically, various alloy compositions of (K,A)NbO3 are considered and their mixing thermodynamics and associated polar properties are examined. To establish structure-property design rules for high-performance ferroelectrics, the sure independence screening sparsifying operator (SISSO) method is employed to extract key features to explain the A-site driven polarization in (K,A)NbO3 . Using a new metric of agreement via feature-assisted regression and classification, the SISSO model is further extended to predict A-site driven polarization in multicomponent systems as a function of alloy composition, reducing the prediction errors to less than 1%. With the machine learning model outlined in this work, a polarity-composition map is established to aid the development of new multicomponent lead-free polar oxides which can offer up to 25% boosting in A-site driven polarization and achieving more than 150% of the total polarization in pristine KNbO3 . This study offers a design-based rational route to develop lead-free multicomponent ferroelectric oxides for niche information technologies.

A Multilevel Analytical Theory for Prediction of Ferroelectric Perovskite Oxide Properties from Composition

Prediction of properties from composition is a fundamental goal of materials science that is particularly relevant for ferroelectric perovskite oxide solid solutions where compositional variation is a primary tool for material design. Design of ferroelectric oxide solid solutions has been guided by heuristics and first-principles and Landau-Ginzburg-Devonshire theoretical methods that become increasingly difficult to apply in ternary, quaternary, and quintary solid solutions. To address this problem, a multilevel model is developed for the prediction of the ferroelectric-to-paraelectric transition temperature (T_c ), coercive field (E_c ), and polarization (P) of PbTiO₃ -derived ferroelectric solid solutions from composition. The characteristics of the materials at different length scales, starting at the level of the electronic structure and chemical bonding of the constituent ions and ending at the level of collective behavior, are analytically related by using ferroelectric domain walls and cationic off-center displacements as the key links between the different levels of the model. The obtained composition-structure-property relationships provide a unified quantitatively predictive theory for understanding PbTiO₃ -derived solid solutions. Such a multilevel analytical modeling approach is likely to be generally applicable to different classes of ferroelectric perovskite oxides and to other functional properties, and to materials and properties beyond the field of ferroelectrics.

The effect of composition on phonon softening in ABO3-type perovskites: DFT modelling

The influence of the A cation on the ferroelectric instability in ABO3 perovskites, and its associated F1u IR-active phonon mode, is systematically investigated for tantalates, niobates and titanates at the hybrid density-functional theory level.

Interface and surface stabilization of the polarization in ferroelectric thin films

With an ever-increasing societal demand for energy for electronic devices and in the face of the current climate issues, the need for low-energy-consuming electronics has never been greater. Ferroelectrics are promising energy-efficient device components for digital information storage, with the functionality relying on the manipulation of their polarization in ultrathin films. Polar discontinuities at the thin film interfaces and surfaces, however, can cause loss of polarization and thus functionality. Here we show how the interface and surface influence the overall polarization of the thin film. We show that the structure of the interface and surface can be tailored toward a specific polarization direction and strength, and that great control in the engineering of ferroelectrics thin films can be achieved.

Ferroelectric perovskites present a switchable spontaneous polarization and are promising energy-efficient device components for digital information storage. Full control of the ferroelectric polarization in ultrathin films of ferroelectric perovskites needs to be achieved in order to apply this class of materials in modern devices. However, ferroelectricity itself is not well understood in this nanoscale form, where interface and surface effects become particularly relevant and where loss of net polarization is often observed. In this work, we show that the precise control of the structure of the top surface and bottom interface of the thin film is crucial toward this aim. We explore the properties of thin films of the prototypical ferroelectric lead titanate (PbTiO₃) on a metallic strontium ruthenate (SrRuO₃) buffer using a combination of computational (density functional theory) and experimental (optical second harmonic generation) methods. We find that the polarization direction and strength are influenced by chemical and electronic processes occurring at the epitaxial interface and at the surface. The polarization is particularly sensitive to adsorbates and to surface and interface defects. These results point to the possibility of controlling the polarization direction and magnitude by engineering specific interface and surface chemistries.

Advances in magnetoelectric multiferroics

The manipulation of magnetic properties by an electric field in magnetoelectric multiferroic materials has driven significant research activity, with the goal of realizing their transformative technological potential. Here, we review progress in the fundamental understanding and design of new multiferroic materials, advances in characterization and modelling tools to describe them, and the exploration of devices and applications. Focusing on the translation of the many scientific breakthroughs into technological innovations, we identify the key open questions in the field where targeted research activities could have maximum impact in transitioning scientific discoveries into real applications.

Effect of ionic radii on the Curie temperature in Ba1-x-ySrxCayTiO3 compounds

A series of Ba_1-x-ySr_xCa_yTiO₃ compounds were prepared with varying average ionic radii and cation disorder on A-site. All samples showed typical ferroelectric behavior. A simple empirical equation correlated Curie temperature, T_C, with the values of ionic radii of A-site cations. This correlation was related to the distortion of TiO₆ octahedra observed during neutron diffraction studies. The equation was used for the selection of compounds with predetermined values of T_C. The effects of A-site ionic radii on the temperatures of phase transitions in Ba_1-x-ySr_xCa_yTiO₃ were discussed.

Interface control of emergent ferroic order in Ruddlesden-Popper Sr(n+1)Ti(n)O(3n+1)

We discovered from first principles an unusual polar state in the low n Sr(n+1)Ti(n)O(3n+1) Ruddlesden-Popper (RP) layered perovskites in which ferroelectricity is nearly degenerate with antiferroelectricity, a relatively rare form of ferroic order. We show that epitaxial strain plays a key role in tuning the "perpendicular coherence length" of the ferroelectric mode, and does not induce ferroelectricity in these low-dimensional RP materials as is well known to occur in SrTiO(3). These systems present an opportunity to manipulate the coherence length of a ferroic distortion in a controlled way, without disorder or a free surface.

Ferroelectricity of Li-doped silver niobate (Ag, Li)NbO3

Phase evolution in (Ag(1-x)Li(x))NbO(3) (ALN) solid solution was investigated by the x-ray diffraction technique, dielectric and polarization measurements. It is shown that a small substitution of Ag with Li gives rise to an orthorhombic-rhombohedral structural transformation in ABO(3)-perovskite silver niobate at room temperature. Structural refinements indicate that both the A- and B-site displacements contribute to the spontaneous polarization of the ferroelectric phase with symmetry R3c. Increasing Li-concentration enhances the ferroelectric rhombohedral distortion, resulting in an increase of the para-ferroelectric phase transition temperature and the polarization of the solid solutions.

Role of Ti antisitelike defects in SrTiO3

Through first-principles calculations, the role of Ti antisitelike defects in the electrical and optical properties of SrTiO3 is proposed. Significant Ti off-centering from the Sr site toward the [100] or [110] direction leads to switchable polar states, and attractive interactions with the O vacancy drive them to form defect pairs. In these defect configurations, localized electronic states are introduced below the conduction band minimum. Our findings on Ti antisitelike defects suggest that they are responsible for the ferroelectricity and blue light emission in nonstoichiometric SrTiO3.

Anomalous phase diagram of ferroelectric (Ba,Ca)TiO3 single crystals with giant electromechanical response

We report the anomalous phase evolution in ferroelectric single crystals Ba1-xCaxTiO3 (0.02<x<0.34), and demonstrate the significant effects of quantum fluctuation on the ferroelectric phase transition. In addition, large electromechanical responses in this class of crystals are also demonstrated. Our results indicate that an effective approach to control the ferroelectricity of perovskite oxide can be realized not only by the covalency between A site atom and oxygen but also by the substitution of A site with small ions with off-centering nature. Theoretical calculations support the idea that the off-center displacements of the smaller Ca ions in the Ba-site play an important role in the exotic natures of Ba1-xCaxTiO3.

Polar behavior in a magnetic perovskite from a-site size disorder: a density functional study

We elucidate a mechanism for obtaining polar behavior in magnetic perovskites based on A-site disorder and demonstrate this mechanism by density functional calculations for the double perovskite (La,Lu)MnNiO6 with Lu concentrations at and below 50%. We show that this material combines polar behavior and ferromagnetism. The mechanism is quite general and may be applicable to a wide range of magnetic perovskites.

Ferrodistortive instability at the (001) surface of half-metallic manganites

We present the structure of the fully relaxed (001) surface of the half-metallic manganite La0.7Sr0.3MnO3, calculated using density functional theory. Two relevant ferroelastic order parameters are identified and characterized. The known tilting of the oxygen octahedra, which is present in the bulk phase, decreases towards the surface. A ferrodistortive Mn off-centering, triggered by the surface and not reported before, decays monotonically into the bulk. This distortion affects neither the half-metallicity nor the zero-temperature magnetization, but does change the effective spin-spin interactions, and thus the temperature dependence of the magnetic properties.

Nonmonotonic TC trends in bi-based ferroelectric perovskite solid solutions

We use first-principles density functional theory calculations to investigate the strongly nonlinear compositional trends in ferroelectric BiBO3-PbTiO3 solid solutions for a variety of cations on the perovskite B site. We demonstrate that previously tabulated crystal chemical parameters (extracted from other Pb-based perovskite alloys [Grinberg et al., J. Appl. Phys. 98, 094111 (2005)]) permit accurate prediction of cation displacements in these new Bi-Pb alloys. We find that observed transition temperatures in these materials are well correlated with computed polarization magnitudes. The presented model for coupling between compositional variation and cation displacements explains the highly nonlinear and often nonmonotonic dependence of the Curie temperature (T(C)) on composition observed in these solid solutions.