Hybrid Improper Ferroelectricity Research Articles

Superior hybrid improper ferroelectricity and enhanced ferromagnetism of Ca3(Ti1-xCox)2O7 ceramics through the superexchange interaction

Ca3(Ti1-xCox)2O7 ceramics were prepared by a tartaric acid sol-gel method and sintered in an oxygen atmosphere. The introduction of Co2+/Co3+ as acceptor dopants leads to the formation of more oxygen vacancies and defect dipoles in Ca3(Ti1-xCox)2O7 ceramics. Oxygen vacancy and defect dipoles lead to the transition of dielectric, leakage, and ferroelectric behaviors of Ca3(Ti1-xCox)2O7 ceramics. The coexistence of hybrid improper ferroelectricity and ferromagnetism at room temperature in Ca3(Ti1-xCox)2O7 ceramics has been successfully realized through the superexchange interaction of Co–O–Co. Ca3(Ti1-xCox)2O7 ceramics exhibit superior ferroelectricity (the remnant polarization is 3.29 μC/cm2) and enhanced ferromagnetism (the remnant magnetization reaches 6.4×10−3 emu/g). This strategy based on the introduction of transition metal ions with unfilled 3d shells at B sites is an important approach to realize novel room-temperature single-phase multiferroic materials for Ca3Ti2O7-based materials.

Hybrid Improper Ferroelectricity: A Theoretical, Computational, and Synthetic Perspective

We review the theoretical, computational, and synthetic literature on hybrid improper ferroelectricity in layered perovskite oxides. Different ferroelectric mechanisms are described and compared, and their elucidation using theory and first-principles calculations is discussed. We also highlight the connections between crystal chemistry and the physical mechanisms of ferroelectricity. The experimental literature on hybrid improper ferroelectrics is surveyed, with a particular emphasis on cation-ordered double perovskites, Ruddlesden–Popper and Dion–Jacobson phases. We discuss preparative routes for synthesizing hybrid improper ferroelectrics in all three families and the conditions under which different phases can be stabilized. Finally, we survey some synthetic opportunities for expanding the family of hybrid improper ferroelectrics.

Distortion modes and ferroelectric properties in hybrid improper ferroelectric Sr3(Sn,Zr)2O7 ceramics

In the present work, the distortion modes and ferroelectric properties of Sr3(Sn1−xZrx)2O7 (x = 0.0, 0.25, 0.5, 0.75, 1.0) ceramics with double-layered Ruddlesden–Popper structures were investigated. The amplitudes of three distortion modes for the ferroelectric phase deviated from the aristotype one were quantified using the symmetry-mode approach adopted in Rietveld refinement against x-ray diffraction patterns. The remanent polarization and the coercive field decreased with increasing the content of Zr4+ cations. The decline of polarization should attribute to the nonpolar secondary phase, while the suppression of a coercive field may connect with the decreased amplitude of the rotation mode. The Curie temperatures increased linearly with increasing x value in Sr3(Sn1−xZrx)2O7 ceramics, inducing from the decline of a tolerance factor. A typical first-order improper ferroelectric phase transition was determined using a differential scanning calorimetry measurement and dielectric characterization. From the present work, a straightforward way to quantify the amplitude of a distortion mode for the ferroelectric phase was given, and the ferroelectric properties could be effectively tuned by substituting the cations at the B site in the Sr-based hybrid improper ferroelectrics.

Hybrid improper dipolar density wave in NaLaCoWO6

Hybrid improper ferroelectricity (HIF) allows the generation of an electrical polarization in the AA${}^{\ensuremath{'}}$BB${}^{\ensuremath{'}}{\mathrm{O}}_{6}$ double perovskite materials thanks to the combination of two nonpolar octahedral distortions. Nevertheless, for selected combination of the A/A${}^{\ensuremath{'}}$ cations a nonpolar incommensurate phase is observed with average symmetry $C2/m$. Thanks to a detailed crystallographic description of the incommensurate phase, based on electron, neutron, and x-ray diffraction data, we show that the incommensurate modulation is related to an abrupt change of the out-of-phase tilting along the $a$ and $c$ axis, whereas the tilting along the $b$ axis remains constant across the structure. By using group theory and symmetry analysis we show that we observe an incommensurate analog of HIF, which induces a hybrid improper dipolar density wave in ${\mathrm{NaLaCoWO}}_{6}$. The dipolar ordering is due also in this case to a trilinear invariant involving the commensurate and incommensurate octahedra tilting.

Two-dimensional ferroelectricity induced by octahedral rotation distortion in perovskite oxides

Two-dimensional (2D) ferroelectricity has attracted extensive attention since its discovery in the monolayers of van der Waals materials. Here we show that 2D ferroelectricity induced by octahedral rotation distortion is widely present in the perovskite bilayer system through first-principles calculations. The perovskite tolerance factor plays a crucial role in the lattice dynamics and ground-state structure of the perovskite monolayers and bilayers, thus providing an important indicator for screening this hybrid improper ferroelectricity. Generally, the ferroelectric switching via an orthorhombic twin state has the lowest energy barrier. Epitaxial strain can effectively tune the ferroelectric polarization and ferroelectric switching by changing the amplitude of octahedral rotation and tilt distortion. The increasing compressive strain causes a polar to nonpolar phase transition by suppressing the tilt distortion. The cooperative effect of octahedral distortion at the interface with the substrate can reduce the energy barrier of the reversing rotation mode and can even change the lowest-energy ferroelectric switching path.

Local symmetry breaking and magnetic properties of acceptor-donor codoped Ca3Mn2O7 hybrid improper ferroelectrics

Layered hybrid improper ferroelectrics are very important for multiferroic devices; however, it has been very challenging to obtain multiferroic properties at room temperature. In this study we report that acceptor-donor codoping can be used to realize ferromagnetism in ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ improper ferroelectric ceramics at room temperature. The transmission electron microscope observation and first-principles calculation revealed that the obtained ferromagnetic properties result from the ${\mathrm{MnO}}_{6}$ octahedron tilt caused by the large lattice distortion near the acceptor-donor pair. At the same time, an obvious magnetoimpedance effect was observed.

Stabilization of layered perovskite structures via strontium substitution in Ca3Ti2O7 revealed via elemental mapping

Extensive studies have been performed on layered compounds, ranging from layered cuprates to van der Waals materials with critical issues of intergrowths and stacking faults. However, such structures have been studied less because of experimental difficulty. We present characteristic defect structures of intergrowths in the Ruddlesden–Popper Ca2.46Sr0.54Ti2O7, which is known to exhibit hybrid improper ferroelectricity. Transmission electron microscopy reveals that numerous intergrowths composed of 7 and 15 layers are introduced in the ferroelectric domains. Elemental maps demonstrate that Sr ions are selectively substituted into the perovskite layers of intergrowths. Density functional theory calculations support the site-selective substitution of Sr ions, favorably located in the intergrowths. The stabilization of the Ruddlesden–Popper phase and intergrowths via Sr substitution can be explained by the ionic-radius difference between Ca and Sr ions. The study reveals detailed defect structures originating from the layered perovskite structure of Ca2.46Sr0.54Ti2O7 and shows the usefulness of elemental mapping in probing the substitution effects in oxides.

Hybrid Improper Ferroelectricity in Highly Cleavable Single Crystals of Dion‐Jacobson‐Compound CsBiNb2O7

AbstractDion‐Jacobson compound CsBiNb2O7 exhibits supposedly hybrid improper ferroelectricity (HIF), resulting from two different structural distortions. The highly‐cleavable single crystal of CsBiNb2O7 has been successfully grown and found large ferroelectric polarization as large as 30 μC cm‐2 with the coercivity of 310 kV cm‐1 from polarization‐electric field measurements on the crystals. Using piezoelectric force microscopy, a systematic study is on the ferroelectric domain structure and dynamics of CsBiNb2O7, unveiling the rich nature of its domain/domain wall evolution upon poling. The record large switchable polarization in CsBiNb2O7 among HIF compounds brings HIF closer to the real Pb‐free ferroelectric applications and the high cleavability of CsBiNb2O7 crystals provides a unique opportunity to explore ultrathin ferroelectrics with in‐plane polarization.

Epitaxial growth and polarization reversal characteristics of hybrid improper ferroelectric Ca3Ti2O7 thin films

Layered perovskite oxides A3B2O7 with Ruddlesden–Popper structures have attracted great attention due to their intriguing hybrid improper ferroelectricity. In this work, we prepared high-quality epitaxial Ca3Ti2O7 (CTO) thin films on Nb-SrTiO3 (STO) and Pt–Si substrates by pulsed laser deposition. The epitaxial relationship between the CTO thin film and Nb-STO substrate is [010]CTO//[010]Nb-STO, while the CTO film on Pt–Si is the polycrystalline growth with the preferred orientation of (010). The piezoelectric force microscopy image is attested to the stable existence of ferroelectricity in CTO films at room temperature with different growth patterns. A resistive switching behavior with at least two orders of magnitude is observed in CTO films driven by their ferroelectric polarization reversal. Moreover, a downward self-poling phenomenon is observed in CTO films on different substrates, and the flexoelectric effect induced by the strain gradient is supposed to be the main origin.

Octahedral rotations in Ruddlesden-Popper layered oxides under pressure from first principles

The combination of reduced dimensionality and tunable structural distortions in layered perovskite oxides makes these materials ideal platforms for designing novel properties and functionalities. One example is hybrid improper ferroelectricity in $n=2$ Ruddlesden-Popper oxides, where the combination of a layered crystal structure and rotations of the metal-oxide octahedra break symmetry and induce a polarization. Precisely controlling the octahedral rotation distortions, for example by the application of hydrostatic pressure, provides a pathway to tune and optimize the properties of these materials. We combine group theoretic methods, density functional theory calculations, and Landau theory analysis to investigate how octahedral rotations respond to pressure in the hybrid improper ferroelectrics ${\mathrm{Sr}}_{3}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}, {\mathrm{Ca}}_{3}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, and ${\mathrm{Sr}}_{3}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$. We find that factors that are known to control the pressure response of $\mathrm{A}\mathrm{B}{\mathrm{O}}_{3}$ perovskites---the formal charge of the $A$- and $B$-site cations, tolerance factor, and $B$-site chemistry---also impact the pressure response of these layered perovskites. We also show that coupling between the octahedral rotation and strain order parameters plays a key role in determining the overall pressure response. Despite some similarities, we find that these layered perovskites display a distinct pressure response compared to their $\mathrm{A}\mathrm{B}{\mathrm{O}}_{3}$ perovskite analogs. By identifying trends and underlying mechanisms that control octahedral rotations in Ruddlesden-Popper oxides under pressure, this work lays the foundation for tailoring the structure and properties of these materials.

Asymmetric Character of the Ferroelectric Phase Transition and Charged Domain Walls in a Hybrid Improper Ferroelectric

AbstractIn improper ferroelectrics, the spontaneous ordering is typically driven by a structural distortion or a magnetic spin alignment. The induced electric polarization is only a secondary effect. This dependence is a rich source for unusual phenomena and ferroelectric domain configurations for proper, polarization‐driven ferroelectrics. This study focuses on the polar domain structure and the hysteretic behavior at the ferroelectric phase transition in Ca3Mn1.9Ti0.1O7 as a representative of the recently discovered hybrid improper ferroelectric class of multiferroics. Combining optical second harmonic generation and Raman spectroscopy gives access to the spontaneous structural distortion and the resulting improper electric polarization. This study shows that hybrid improper ferroelectrics contrast proper and improper ferroelectrics in several ways. Most intriguingly, adjacent ferroelectric domains favor head‐to‐head and tail‐to‐tail domain walls over charge‐neutral configurations. Furthermore, the phase transition occurs in an asymmetric fashion. The regime of phase coexistence of the nonpolar and polar phases shows a clear and abrupt upper temperature limit. In contrast, the coexistence toward low temperatures is best described as a fade‐out process, where 100‐nm‐sized islands of the nonpolar phase expand deep into the polar phase.

Optimization of sintering process and enhanced hybrid improper ferroelectricity of Ca3Ti2O7 ceramics fabricated by an acetic acid sol–gel method

Optimization of sintering process and enhanced hybrid improper ferroelectricity of Ca3Ti2O7 ceramics fabricated by an acetic acid sol–gel method

First-Principles Investigation into Hybrid Improper Ferroelectricity in Ruddlesden–Popper Perovskite Chalcogenides Sr3B2X7 (B = Ti, Zr, Hf; X = S, Se)

First-Principles Investigation into Hybrid Improper Ferroelectricity in Ruddlesden–Popper Perovskite Chalcogenides Sr₃B₂X₇ (B = Ti, Zr, Hf; X = S, Se)

Hybrid improper ferroelectricity and magnetoelectric coupling in a two-dimensional perovskite oxide

Unlike van der Waals materials, perovskite oxide may undergo structural reconstruction when reduced to two dimensions. Therefore, it is still an open question whether some of the ferroelectric mechanisms and magnetoelectric coupling effects found in perovskite bulks can be maintained at the two-dimensional (2D) limit. Here we demonstrate that the ferroelectricity induced by octahedral rotation and the magnetoelectric coupling mechanism dependent on Dzyaloshinskii-Moriya (DM) interaction exist in a proposed perovskite bilayer. Although the octahedral rotation distortion of the ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ bilayer is reconstructed with respect to its layered bulk phase, tensile strain can induce the same octahedral rotation type as its bulk phase, resulting in the emergence of ferroelectric phase. We show that the modulation of the DM vector by octahedral rotation distortion provides a coupling between polarization and the induced magnetization. In ferroelectric switching, the polarization is reversed by a ${180}^{\ensuremath{\circ}}$ rotation of the tilt axis within the $ab$ plane, which also results in a reversal of magnetization. This work is expected to provide a guidance for realizing electric-field control of magnetization direction in 2D perovskite oxides.

Strong trilinear coupling of phonon instabilities drives the avalanche-like hybrid improper ferroelectric transition in SrBi2Nb2O9

Improper and hybrid-improper ferroelectrics (FEs) host spontaneous polarization ($P$) owing to the anharmonic coupling between polar and nonpolar modes. In general, the unstable zone-boundary nonpolar mode is a primary order parameter that couples to the stable zone-center polar mode to induce $P$. However, in certain FEs, such as the Aurivillius family, the polar mode is also unstable. In such cases, the strength of coupling between polar and nonpolar modes governs whether FE phase transition is a single or double step. Here we investigate a single-step avalanche-like transition in ${\mathrm{SrBi}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$, an Aurivillius family compound, using inelastic neutron scattering and Raman spectroscopy combined with group theory and first-principles simulations. We find that a strong trilinear coupling constant $\ensuremath{\lambda}$ between the two nonpolar and one polar instabilities enables their single-step condensation at the FE transition temperature ${T}_{FE}$ and overcomes the effect of the positive bi-quadratic coupling that works against it. The ratio of $\ensuremath{\lambda}$ to the leading-order term in the energy invariant polynomial is two to six times larger than in iso-structural ${\mathrm{SrBi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$, which undergoes a double-step transition. We identify and track polar and nonpolar instabilities and find their temperature dependence consistent with the theoretical prediction based on the strong trilinear coupling mechanism. From the sign ($\ifmmode\pm\else\textpm\fi{}$) of bi-quadratic coupling constants in the energy invariant polynomial, we further rule out the triggering mechanism to drive the FE transition. Moreover, we observe little change in phonons across ${T}_{FE}$ despite a first-order transition. However, a significant phonon broadening occurs even at 300 K, which we rationalize based on a large anharmonic vibrational amplitude of O atoms in the ${\mathrm{NbO}}_{6}$ octahedra.

Enhanced hybrid improper ferroelectricity in Fe/Nb cosubstituted Ca3Mn2O7 ceramics

AbstractThe prototypical Ruddlesden‐Popper compound Ca3Mn2O7 has been predicted to possess hybrid improper ferroelectricity, where the polarization is induced by the condensation of two oxygen octahedral distortion modes. Nevertheless, it is a big challenge to switch the polarization at room temperature in Ca3Mn2O7 since the presence of intermediate nonpolar Acaa phase generally leads to the complex domain morphology. Here, the effects of Fe/Nb cosubstitution on hybrid improper ferroelectricity in Ca3Mn2O7 are reported, and easy polarization switching at room temperature is achieved in Ca3[Mn0.5(Fe0.5Nb0.5)0.5]2O7. The ferroelectric phase transition occurs directly from I4/mmm to A21am at a temperature far above room temperature without intermediate nonpolar Acaa phase. The distinct transition pathway forms the alternating 180°‐type ferroelectric domains rather than the irregular 90°‐type ferroelastic domains stacked along [001], resulting in easy polarization switching at room temperature. Moreover the enhanced ferroelectric polarization (Pr ~2.0 μC/cm2) is obtained due to the increased anti‐ferrodistortive displacements of Ca cations at A‐site, arising from the larger amplitudes of oxygen octahedral distortions. Chemical pressure is emphasized here for the tunability of phase transition, domain morphology, and ferroelectric characteristics, and it provides a useful approach for designing and creating high‐performance improper ferroelectrics.

Hybrid improper ferroelectricity in A-site cation ordered Li2La2Ti3O10 ceramic with triple-layer Ruddlesden–Popper structure

Hybrid improper ferroelectricity has been extensively studied in double-layer Ruddlesden–Popper oxides in recent years. Although the hybrid improper ferroelectricity could be created among triple-layer Ruddlesden–Popper oxides with an ordered A-site cation predicted by the first-principles calculations, no experimental result has been reported yet. In the present work, the room-temperature ferroelectricity has been observed in Li2La2Ti3O10 ceramics with an A-site cation ordered triple-layer Ruddlesden–Popper structure. The polar phase P21ab has been determined by combining the first-principles calculation and the powder diffraction analysis at room temperature. The hybrid improper ferroelectricity was induced by the triple-coupled irreps including the A-site cation ordering. The variable temperature differential scanning calorimetry measurements and dielectric responses indicate no evidence of phase transition over the temperature range of 200–1080 K. The present work sheds light on designing the hybrid improper ferroelectrics in A-site ordered triple-layer Ruddlesden–Popper compounds.

First-principle investigation of hybrid improper ferroelectricity of <i>n</i> = 2 Ruddlesden-Popper Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf)

Recently, perovskite ferroelectric photovoltaic materials have been studied extensively. Traditional photovoltaic device usually uses the internal electric field formed by PN junction to realize the separation of photogenerated carriers to form the photovoltaic effect, while ferroelectric material, due to the existence of spontaneous polarization, can spontaneously realize the separation of photogenerated electrons and holes without the formation of PN junction, presenting the ferroelectric photovoltaic effect. Chalcogenide perovskite with suitable band gap and visible light absorption is expected to be a new generation of ferroelectric photovoltaic materials. However, its application is limited due to the lack of ferroelectric properties. Hybrid improper ferroelectricity (HIF) in layered perovskites has opened a new way for developing the new ferroelectrics. In contrast to the proper ferroelectricity in which the polarization is the main order parameter as the driving force, the improper ferroelectricity possesses the ferroelectric polarization that becomes a secondary order parameter induced by other orders. In this work, we study the ground state, electronic structure and hybrid improper ferroelectricity of <i>n</i> = 2 Ruddlesden-Popper (RP) Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) based on the first principles. The total energy calculations and phonon spectrum analysis show that the ground state of Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (B = Zr, Hf ) is of <i>A</i>2<sub>1</sub><i>am</i> polar phase. The hybrid improper ferroelectricity originates from the coupling between two rotation modes of <i>B</i>Se<sub>6</sub> octahedron. Electronic structure calculations show that Sr<sub>3</sub>Zr<sub>2</sub>Se<sub>7</sub> and Sr<sub>3</sub>Hf<sub>2</sub>Se<sub>7</sub> are semiconductors with direct band-gaps, which are around 1.56 eV and 1.84 eV, respectively. The ferroelectric polarization values calculated by the Berry phase method are around 12.75 μC/cm<sup>2</sup> and 9.69 μC/cm<sup>2</sup>, respectively. The contribution of each atomic layer to the ferroelectric polarization is investigated when the Born effective charge method is used. The results show that the polarization of Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) mainly comes from the Sr-Se atomic layers. To sum up, Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) show strong ferroelectric polarization and good visible light absorption characteristics and they are expected to be candidates of a new generation of ferroelectric photovoltaic materials.

Spontaneous electric polarization and electric field gradient in hybrid improper ferroelectrics: insights and correlations

Ab initio study on correlation of electric polarization and electric field gradient in hybrid improper ferroelectrics based on Ruddlesden–Popper phases and double perovskite structures.

Hybrid improper antiferroelectricity—New insights for novel device concepts

AbstractAntiferroelectrics have been studied for decades, with most research focused on PbZrO3 or related compounds obtained through chemical substitution. Although there are several important antiferroelectrics found in AVO4 (A=Dy, Bi), orthorhombic ABC semiconductors (e.g., MgSrSi) and hydrogen-bonded antiferroelectric materials, experimentally demonstrated antiferroelectrics are far less common. Furthermore, antiferroelectrics have potential applications in energy storage and for strain and force generators. In recent years, hybrid improper ferroelectrics have been intensively studied, along which the hybrid improper antiferroelectric phase was proposed and demonstrated in (001) Ruddlesden−Popper A3B2O7 thin films from first-principles calculations. Later, the hybrid improper antiferroelectric phase was discovered experimentally in several Ruddlesden−Popper perovskites in bulk. Across the hybrid improper ferroelectric-antiferroelectric phase transition, several interesting phenomena were also predicted. In this snapshot review, we describe recent progress in hybrid improper antiferroelectricity.