Ferroelectric Distortion Research Articles

Tunable multiferroic order parameters in Sr1−xBaxMn1−yTiyO3

Responding to the rapidly increasing demand for efficient energy usage and increased speed and functionality of electronic and spintronic devices, multiferroic oxides have recently emerged as key materials capable of tackling this multifaceted challenge. In this paper, we describe the development of single-site manganese-based multiferroic perovskite materials with modest amounts of nonmagnetic Ti substituted at the magnetic Mn site in Sr1- x Ba x Mn1- y Ti y O3 (SBMTO). Significantly enhanced properties were achieved with ferroelectric-type structural transition temperatures boosted to ∼430K. Ferroelectric distortions with large spontaneous polarization values of ∼30μC/cm2, derived from a point charge model, are similar in magnitude to those of the prototypical nonmagnetic BaTiO3. Temperature dependence of the system's properties was investigated by synchrotron x-ray powder diffraction and neutron powder diffraction at ambient and high pressures. Various relationships were determined between the structural and magnetic properties, Ba and Ti contents, and T N and T C. Most importantly, our results demonstrate the large coupling between the magnetic and ferroelectric order parameters and the wide tunability of this coupling by slight variations of the material's stoichiometry.

Building ferroelectric from the bottom up: The machine learning analysis of the atomic-scale ferroelectric distortions

Recent advances in scanning transmission electron microscopy (STEM) have enabled direct visualization of the atomic structure of ferroic materials, enabling the determination of atomic column positions with approximately picometer precision. This, in turn, enabled direct mapping of ferroelectric and ferroelastic order parameter fields via the top-down approach, where the atomic coordinates are directly mapped on the mesoscopic order parameters. Here, we explore the alternative bottom-up approach, where the atomic coordinates derived from the STEM image are used to explore the extant atomic displacement patterns in the material and build the collection of the building blocks for the distorted lattice. This approach is illustrated for the La-doped BiFeO3 system.

Origin and evolution of ferroelectricity in the layered rare-earth-titanate, [formula omitted]Ti2O7, Carpy-Galy phases

We report a systematic first-principles study based on density functional theory (DFT) of the structural and ferroelectric properties of the R2Ti2O7 perovskite-related oxides with R = La, Ce, Pr, and Nd. We show that, in all cases, the ferroelectric distortion to the ground-state polar P21 structure from its parent centrosymmetric P21/m phase is driven by a single polar soft mode consisting of rotations and tilts of the TiO6 octahedra combined with displacements of the R ions. A secondary centrosymmetric distortion, which is stable in the parent structure, contributes substantially to the energy lowering of the ground state. We evaluate the trends in structure and polarization across the series as a function of R and reconcile discrepancies in reported values of polarization in the literature. Our results confirm that the family of R2Ti2O7 materials belong to the class of proper geometric ferroelectrics.

DFT Investigation of Structural and Electronic Properties of Modified PZT

Abstract Density of states and geometrical structures of modified Lead zirconate titanate are investigated using density functional theory within local density approximation. The electronic properties and bond length variation have been studied in terms of electronic structure and bonding mechanism principles respectively. Hybridization between Ti 3d - O 2p states and ferroelectric distortion have been addressed as a theoretical approach, to rule the improvement of ferroelectric properties of Lead zirconate titanate. The analysis of Ga, Tl modified Lead zirconate titanate were found to diminish the hybridization between Ti 3d - O 2p states, the relaxed behavior lead to the reversal of the known ferroelectric distortion. Y, Ho, Yb and Lu modified Lead zirconate titanate compounds have a tendency to intense the ferroelectric stability, its exhibit higher hybridization between Ti 3d - O 2p states than pure Lead zirconate titanate, also the arrangement of the ions distortions is strongly the same as the more favoured ferroelectric states of Lead zirconate titanate.

Interplay between Ca- and Ti-driven ferroelectric distortions in (Ba, Ca)TiO3 solid solutions from first-principles calculations

(Ba,Ca)(Ti,Zr)O3 solid solutions are promising lead-free piezoelectrics near their polymorphic phase boundary, which is believed to be linked to the interplay between B-site driven ferroelectricity and A-site driven ferroelectricity. Focusing on (Ba,Ca)TiO3, we support this picture from first-principles calculations. In particular, we show how steric effects related to the partial substitution of Ba by Ca largely enhance the Ca-driven ferroelectricity, already virtually allowed in the parent CaTiO3. The emergent interplay between the Ca-driven and Ti-driven mechanisms lowers the energy barrier between different polar states, which eventually results in a quasi-isotropic polarization under substitution of a small concentration of Ba by Ca. A sizeable enhancement of the piezoelectric response directly results from these features.

Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled Magnetoelectricity.

Two dimensional (2D) materials possessing ferroelectric/ferromagnetic orders and especially low-magnetic-field controlled magnetoelectricity have great promise in spintronics and multistate data storage. However, ferroelectric and magnetoelectric (ME) dipoles in the atom-thick 2D materials are difficult to be realized due to structural inversion symmetry, thermal actuation, and depolarized field. To overcome these difficulties, the monolayer structure must possess an in-plane inversion asymmetry in order to provide out-of-plane ferroelectric polarization. Herein, crystal chemistry is adopted to engineer specific atomic displacement in monolayer ReS2 to change the crystal symmetry to induce out-of-plane ferroelectric polarization at room temperature. The cationic Re vacancy in the atom-displaced ReS2 monolayer causes spin polarization of two immediate neighbor sulfur atoms to generate magnetic ordering, and the ferroelectric distortion near the Re vacancy locally tunes the ferromagnetic order thereby triggering low-magnetic-field controlled ME polarization at about 28 K. As a result, 2D ME coupling multiferroics is achieved. Our results not only reveal a design methodology to attain coexistence of ferroelectric and ferromagnetic orders in 2D materials but also provide insights into magnetoelectricity in 2D materials.

Ferroelectric Origin and Distortion Modes in Doped BiFeO <sub>3</sub> by Crystallography Approach

There are conflicts about ferroelectric origin and relationship of two distortion modes, namely, ferroelectric (FE) mode and antiferrodistortive (AFD) mode. Here two distortion modes and enhanced ferroelectric properties (Pr=89.5 μC cm-2) are observed in BiFeO3 thin films doped with small radius Mg2+ (A-site) deposited on (111) Pt/Ti/SiO2/Si substrates by a sol-gel method, relationship of two distortion modes turns from cooperative to competitive as FE mode strengthens. A new explanation of ferroelectric origin and distortion modes in doped BiFeO3 by Defect Dipoles Driven Distortions Theory (abbreviate DDD) from crystallography is reported. Meanwhile, crystal structure regulation mechanism of doped BiFeO3 thin films with substantially enhanced ferroelectric properties is also put forward. The distances of positive and negative charge center in [FeO6] octahedrons evidently increase by Mg2+ doping compared with that in Bi0.9Sm0.1Fe0.95Mn0.05O3 (BSFMO). The change of distances is contributed to law of atomic migration from move of oxygen vacancies and attraction of defect dipoles. A positive effect on ferroelectric properties of appropriate increase in oxygen vacancies is explored, which is explained by the probability of possible position that oxygen vacancies occurred and the relationship between leakage current and overlapping oxygen positions. The potential structure and more suitable doping ions are successfully predicted by crystal structure regulation mechanism. In addition, it brings a new direction for search of other structure regulation ions so as to realize perfect ferroelectric properties for practical application, which will become an important beginning of ferroelectric materials design.

Comparative study of magnetic and electronic properties of room‐temperature polar magnets ScFeO3 and InFeO3

We performed calculations based on collinear and non‐collinear spin density functional theory to investigate magnetic and electronic properties of new room‐temperature polar magnetic compounds AFeO3 (A = Sc, In). Exchange and correlation effects were simulated by standard local spin density approximation and the generalized gradient approximation. We determined that the canted G‐type antiferromagnetic (G‐AFM) structure of the Fe spin moments is the most stable magnetic structure for both compounds, in accordance with experimental facts. The calculated Fe spin magnetic moments and resulting weak ferromagnetic components perpendicular to the c‐axis also agree well with the experiment. The magnitude of the Fe spin moment does not depend on the fact if the G‐AFM structure is collinear or canted. The energy difference between these two structures is found to be much larger for InFeO3, so as the exchange interaction constant Jnn between nearest‐neighbor magnetic Fe ions. The ScFeO3 and InFeO3 electronic structures, which reproduce well their magnetic properties are presented and discussed. By comparing them with the band structures of some typical multiferroics, we conclude that different electronic mechanisms should be responsible for their ferroelectric distortion. In ScFeO3, the cause of this distortion should be attributed to the hybridization between almost empty Sc 3d and occupied O 2p states along the hexagonal c‐axis. For InFeO3, however, the stereochemical activity of the In 5s localized electrons should be the principal driving force for ferroelectric distortion.

Influence of Disorder and Anharmonic Fluctuations on the Dynamical Rashba Effect in Purely Inorganic Lead-Halide Perovskites

Doping organic metal-halide perovskites with cesium could be the best solution to stabilize highly-efficient perovskite solar cells. The understanding of the respective roles of the organic molecule, on one hand, and the inorganic lattice, on the other, is thus crucial in order to be able to optimize the physical properties of the mixed-cation structures. In particular, the study of the recombination mechanisms is thought to be one of the key challenges towards full comprehension of their working principles. Using molecular dynamics and frozen phonons, we evidence sub-picosecond anharmonic fluctuations in the fully inorganic $CsPbI_3$ perovskite. We reveal the effect of these fluctuations, combined with spin-orbit coupling, on the electronic band structure, evidencing a dynamical Rashba effect. Our study show that under certain conditions space disorder can quench the Rashba effect. As for time disorder, we evidence a dynamical Rashba effect which is similar to what was found for $MAPbI_3$ and which is still sizable despite temperature disorder, the large investigated supercell, and the absence of the organic cations' motion. We show that the spin texture associated to the Rashba splitting cannot be deemed responsible for a consistent reduction of recombination rates, although the spin mismatch between valence and conduction band increases with the ferroelectric distortion causing the Rashba splitting.

Local structure of A-atom in ABO3 perovskites studies by RMC-EXAFS

The ferroelectric distortions in perovskites were a subject of numerous investigations for a long time. However, some controversial results still exist, coming from the analysis of diffraction (X-ray, neutron or electron) data and X-ray absorption spectra. In this study, our goal was to revisit these classical materials using recently developed methods without imposing any predefined structural model. Local environment around A-type atom in ABO3 perovskites (SrTiO3, BaTiO3, EuTiO3) was studied by X-ray absorption spectroscopy (XAS) in a wide range of temperatures (20–400 K). Using reverse Monte Carlo method enhanced by evolutionary algorithm, the 3D structure was extracted from the extended X-ray absorption fine structure (EXAFS) and interpreted in terms of the radial distribution functions (RDFs). Our findings show that both diffraction and XAS data are consistent, but reflect the structure of the material from different points of view. In particular, when strong correlations in the motion of certain atoms are present, the information obtained by XAS might lead to a different from expected shape of the RDF. At the same time, the average positions of all atoms are in good agreement with those given by diffraction. This makes XAS an important technique for studying interatomic correlations and lattice dynamics.

Hybrid Improper Ferroelectricity in (Sr,Ca)3Sn2O7 and Beyond: Universal Relationship between Ferroelectric Transition Temperature and Tolerance Factor in n = 2 Ruddlesden-Popper Phases.

Hybrid improper ferroelectricity, which utilizes nonpolar but ubiquitous rotational/tilting distortions to create polarization, offers an attractive route to the discovery of new ferroelectric and multiferroic materials because its activity derives from geometric rather than electronic origins. Design approachesblending group theory and first principles can be utilized to explore the crystal symmetries of ferroelectric ground states, but in general, they do not make accurate predictions for some important parameters of ferroelectrics, such as Curie temperature ( TC). Here, we establish a predictive and quantitative relationship between TC and the Goldschmidt tolerance factor, t, by employing n = 2 Ruddlesden-Popper (RP) A3B2O7 as a prototypical example of hybrid improper ferroelectrics. The focus is placed on an RP system, (Sr1- xCa x)3Sn2O7 ( x = 0, 0.1, and 0.2), which allows for the investigation of the purely geometric (ionic size) effect on ferroelectric transitions, due to the absence of the second-order Jahn-Teller active (d0 and 6s2) cations that often lead to ferroelectric distortions through electronic mechanisms. We observe a ferroelectric-to-paraelectric transition with TC = 410 K for Sr3Sn2O7. We also find that the TC increases linearly up to 800 K upon increasing the Ca2+ content, i.e., upon decreasing the value of t. Remarkably, this linear relationship is applicable to the suite of all known A3B2O7hybrid improperferroelectrics, indicating that the TC correlates with the simple crystal chemistry descriptor, t, based on the ionic size mismatch. This study provides a predictive guideline for estimating the TC of a given material, which would complement the convergentgroup-theoretical and first-principles design approach.

Ferroelectricity and piezoelectricity in monolayers and nanoplatelets of SnS

The ground-state structure of monolayers and nanoplatelets of SnS with a thickness from two to five monolayers is calculated from first principles. It is shown that nanoobjects with only an odd number of monolayers are ferroelectric. The ferroelectric, piezoelectric, and elastic properties of these polar structures are calculated. The appearance of polarization in these nanoobjects is explained by an uncompensated polarization that exists in an antiferroelectric structure of bulk SnS. The mechanism of ferroelectricity, in which the ferroelectric distortion is associated with short-range ordering of lone pairs, can be regarded as a way of creating ferroelectrics with high Curie temperature.

First-principles-based strain and temperature-dependent ferroic phase diagram of SrMnO3

Perovskite structure SrMnO$_3$ is a rare example of a multiferroic material where strain-tuning and/or cation substitution could lead to coinciding magnetic and ferroelectric ordering temperatures, which would then promise strong magnetoelectric coupling effects. Here, we establish the temperature and strain dependent ferroic phase diagram of SrMnO$_3$ using first-principles-based effective Hamiltonians. All parameters of these Hamiltonians are calculated using density functional theory, i.e., no fitting to experimental data is required. Temperature dependent properties are then obtained from Monte Carlo and molecular dynamics simulations. We observe a sequence of several magnetic transitions under increasing tensile strain, with a moderate variation of the corresponding critical temperatures. In contrast, the ferroelectric Curie temperature increases strongly after its onset around 2.5\,\% strain, and indeed crosses the magnetic transition temperature just above 3\,\% strain. Our results indicate pronounced magnetoelectric coupling, manifested in dramatic changes of the magnetic ordering temperatures and different magnetic ground states as function of the ferroelectric distortion. In addition, coexisting ferroelectric and ferromagnetic order is obtained for strains above 4\,\%. Our calculated phase diagram suggests the possibility to control the magnetic properties of SrMnO$_3$ through an applied electric field, significantly altering the magnetic transition temperatures, or even inducing transitions between different magnetic states.

Atomistic simulation study of the ferroelectric and paraelectric phases of the hexagonal RMnO3 (R = Er, Tm, Yb, Lu, In and Sc) family

A set of interatomic potential parameters for modeling the hexagonal manganites h-RMnO3 (R = Er, Tm, Yb, In and Sc) was developed. The structural properties in the ferroelectric (FE) phase were studied, that is, the lattice parameters, unit cell volume and interatomic distances for all manganites. The difference between the calculated and experimental lattice parameters was less than 0.5% for all members of the h-RMnO3 family. An extensive study of the chemical and hydrostatic pressure effects for h-RMnO3 was performed via atomistic simulation and compared with the experimental results and those obtained from density functional theory. Chemical pressure produces different effects depending on the R atom of the hexagonal manganite family, and the lattice parameters decrease with an increase in the hydrostatic pressure. The effects on the ferroelectric distortion of these materials under chemical and hydrostatic pressure were analyzed, and the structural properties of the h-RMnO3 family in its paraelectric (PE) phase were determined on the basis of the interatomic potential derived for the ferroelectric phase.

Epitaxial strain dependence of band gaps in perovskite oxynitrides compared to perovskite oxides

Perovskite oxynitrides are a promising class of materials for photocatalytic water splitting due to their small band gaps and suitably aligned band edges. Recently, epitaxially strained perovskite oxynitrides started to attract interest due to the possibility to engineer the anion order and ferroelectric distortions. However, contrary to perovskite oxides there have been no studies on the band-gap evolution in perovskite oxynitrides with epitaxial strain. Here, we investigate, using density functional theory calculations, the influence of epitaxial strain on the band gaps of two different perovskite oxynitrides and compare our results with perovskite oxides. As opposed to cubic oxides where both compressive and tensile strains narrow the band gap, we find that the anion order leads to nondegenerate N $2p$ bands already at zero strain, which leads to opposite and anion order-dependent evolutions of the band gap under compressive and tensile strain. Furthermore, we find that in perovskite oxynitrides polar distortions increase the band gap by up to 1 eV for 4% strain whereas for perovskite oxides the increase is only about 0.1 eV for the same amount of strain. The reason for this difference is the larger ionic radius and smaller electronegativity of nitrogen leading to larger ferroelectric distortions and a stronger dependence of the band gap on the distortion amplitude. These results imply that ferroelectric distortions are strongly detrimental to light absorption and that their suppression, for example, above a critical temperature, should result in a marked decrease in the band gap and enhanced absorption of visible light.

Crystal structure and ferroelectric polarization of tetragonal (Bi1/2Na1/2)TiO3–12BaTiO3

We have investigated the crystal structure and ferroelectric polarization of (Bi1/2Na1/2)TiO3–12%BaTiO3 by high-resolution neutron powder diffraction Rietveld analysis and density-functional theory calculations. It is revealed that the sample has a ferroelectric distortion in tetragonal P4mm symmetry along with a large tetragonality c/a of 1.022. The A- and B-site atoms exhibit a cooperative off-center displacement along the polar c-axis, which results in a large spontaneous polarization of 54 µC/cm2.

Vibrational spectra of multiferroics with Y- and Z-type hexaferrite structures

We investigated polarized infrared and Raman spectra of the multiferroic hexaferrites BaSrCoZnFe11AlO22 (Y-type) and (Z-type) on cooling from room temperature down to 4 K, and we compared the observed numbers of phonons with factor-group analyses of their lattice vibrations. All phonons can be explained within non-polar space groups and for Y- and Z-type hexaferrites, respectively. Thus, the vibrational spectra provide no evidence of ferroelectric order in both kinds of hexaferrites, although their multiferroic properties with anomalously strong magnetoelectric coupling were reported; consequently, the crystal structure should be non-centrosymmetric. The ferroelectric distortions and breakings of the inversion symmetry are probably very small, therefore polar phonons become very weak in the Raman spectra, and Raman modes originally active in non-polar phases are also weak in the infrared spectra. In contrast, electromagnons are both infrared- and Raman-active, as expected for polar (ferroelectric) phases. Based on the electromagnon activities in infrared and THz spectra, polar point groups of both hexaferrite systems are proposed.

Structural and electronic properties of Fe monolayer on BaTiO3(0 0 1)

The structural, electronic, and phonon properties of the BaTiO3(0 0 1) surface and the Fe/BaTiO3(0 0 1) interface have been studied within the density functional theory. Attention is paid to the lattice instabilities (soft phonon modes) that induce ferroelectric distortions in the surface and the interface. A phonon-induced monoclinic (Cm) thin-film counterpart of the low-temperature rhombohedral (R3m) ferroelectric bulk BaTiO3 phase is found. The changes in crystal structure, electronic density of states, atomic charges, and magnetic moments associated with the ferroelectric distortions are discussed comparing the results of the standard GGA and the hybrid DFT calculations. The magnetoelectric coupling at the Fe/BaTiO3(0 0 1) interface is investigated by the analysis of changes in magnetic moments on Fe and Ti atoms induced by the atomic displacements perpendicular and parallel to the surface.

Rashba–Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces

Despite the imperative importance in solar-cell efficiency, the intriguing phenomena at the interface between perovskite solar-cell and adjacent carrier transfer layers are hardly uncovered. Here we show that PbI$_2$/AI-terminated lead-iodide-perovskite (APbI$_3$; A=Cs$^+$/ methylammonium(MA)) interfaced with the charge transport medium of graphene or TiO2 exhibits the sizable/robust Rashba-Dresselhaus (RD) effect using density-functional-theory and ab initio molecular dynamics (AIMD) simulations above cubic-phase temperature. At the PbI$_2$-terminated graphene/CsPbI3(001) interface, ferroelectric distortion towards graphene facilitates an inversion breaking field. At the MAI-terminated TiO$_2$/MAPbI$_3$(001) interface, the enrooted alignment of MA$^+$ towards TiO$_2$ by short-strong hydrogen-bonding and the concomitant PbI$_3$ distortion preserve the RD interactions even above 330 K. The robust RD effect at the interface even at high temperatures, unlike in bulk, changes the direct-type band to the indirect to suppress recombination of electron and hole, thereby letting these accumulated carriers overcome the potential barrier between perovskite and charge transfer materials, which promotes the solar-cell efficiency.

Probing the diffusion behavior of polymorphic phase transition in K0.5Na0.5NbO3 ferroelectric ceramics by Eu3+ photoluminescence

By studying the temperature-induced structural transition in Eu3+-doped K0.5Na0.5NbO3 (KNN) ferroelectric ceramics, as well as the evolution of photoluminescence, dielectric, and ferroelectric properties, we find that the photoluminescence performance of the Eu3+ ions could be employed in ferroelectric ceramics as a probe to investigate the phase structure transition from the crystal symmetry view. The polymorphic orthorhombic-to-tetragonal phase transition region is found to belong to the diffusive phase transition, which is caused by the structural disorder and ferroelectric distortion in KNN host. Photoluminescence properties of the Eu3+ ions are discussed based on the Judd-Ofelt and crystallographic symmetry theories. These results show a potential strategy for studying the phase structural transitions in ferroelectric materials.