Integration of ferroelectric materials into novel technological applications requires low coercive field materials, and consequently, design strategies to reduce the ferroelectric switching barriers. In this first principles study, we show that biaxial strain, which has a strong effect on the ferroelectric ground states, can also be used to tune the switching barrier of hybrid improper ferroelectric Ruddlesden-Popper oxides. We identify the region of the strain -- tolerance factor phase diagram where this intrinsic barrier is suppressed, and show that it can be explained in relation to strain induced phase transitions to nonpolar phases.
We performed first principles calculations for epitaxially strained orthorhombic CaTiO$_3$. The computational results reveal the existence of a metastable ferroelectric phase at compressive strain with unexpected in-plane polarization. Symmetry analysis indicates that two distortion modes at the X point combine to induce the polarization, leading to characterization as a hybrid improper ferroelectric. Our work demonstrates a mechanism for improper ferroelectricity in simple perovskites without the need for symmetry lowering by layering or cation ordering.
We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO$_3$ ultra-thin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface, is found to be metastable in a single domain state despite the presence of a depolarizing field. We establish a connection between the result and the role of improper ferroelectric transition. Our results imply that improper ferroelectric ultrathin films can have rather unique properties that are distincti…
In this contribution to the special issue on magnetoelectrics and their applications, we focus on some single phase multiferroics theoretically predicted and/or experimentally discovered by the authors in recent years. In these materials, iron is the common core element. However, these materials are conceptually different from the mostly-studied BiFeO$_3$, since their ferroelectricity is improper. Our reviewed materials are not simply repeating one magnetoelectric mechanism, but cover multiple branches of improper ferroelectricity, including the magnetism-driven ferroelectrics, geometric ferro…
First-principles calculations are presented for the layered perovskite Ca$_3$Mn$_2$O$_7$. The results reveal a rich set of coupled structural, magnetic and polar domains in which oxygen octahedron rotations induce ferroelectricity, magnetoelectricity and weak-ferromagnetism. The key point is that the rotation distortion is a combination of two non-polar modes with different symmetries. We use the term "hybrid" improper ferroelectricity to describe this phenomenon and discuss how control over magnetism is achieved through these functional antiferrodistortive octahedron rotations.
Using first-principles calculations, we study theoretically the stable 2H hexagonal structure of BaMnO3. We show that from the stable high temperature P63/mmc structure, the compound should exhibit an improper ferroelectric structural phase transition to a P63cm ground state. Combined with its antiferromagnetic properties, 2H-BaMnO3 is therefore expected to be multiferroic at low temperature. The phase transition mechanism in BaMnO3 appears similar to what was reported in YMnO3 in spite of totally different atomic arrangement, cation sizes and Mn valence state.
Improper ferroelectrics are described by two order parameters: a primary one, driving a transition to long-range distortive, magnetic or otherwise non-electric order, and the electric polarization, which is induced by the primary order parameter as a secondary, complementary effect. Using low-temperature scanning probe microscopy, we show that improper ferroelectric domains in YMnO$_3$ can be locally switched by electric field poling. However, subsequent temperature changes restore the as-grown domain structure as determined by the primary lattice distortion. The backswitching is explained by …
Cornell University（康奈尔大学）；University of Nebraska, Lincoln（内布拉斯加大学林肯分校）；Leibniz-Institut für Kristallzüchtung（莱布尼茨晶体生长研究所）；Pacific Northwest National Laboratory（太平洋西北国家实验室）；The Pennsylvania State University（宾夕法尼亚州立大学）；University of California, Berkeley（加州大学伯克利分校）；Rice University（莱斯大学）；Kavli Institute at Cornell for Nanoscale Science（康奈尔卡弗里纳米科学研究所）；Princeton University（普林斯顿大学）
Li et al. (Science Advances, 29 January, p. eabe3068) claim the discovery of two improper ferroelectrics, dabcoHClO4 and dabcoHBF4 (dabco = 1,4-diazabicyclo[2.2.2]octane), and that these materials exhibit superior pyroelectric figures of merit. This information is misleading due to the fundamental methodological errors and false conclusions, not to mention that these ferroelectrics were reported over 20 years ago. They are proper ferroelectrics, for which the spontaneous polarization is the macroscopic order parameter. We show that the useful pyroelectric coefficients of these materials are ab…
The successful theoretical prediction and experimental demonstration of hybrid improper ferroelectricity (HIF) provides a new pathway to couple octahedral rotations, ferroelectricity, and magnetism in complex materials. To enable technological applications, a HIF with a small coercive field is desirable. We successfully grow Sr3Sn2O7 single crystals, and discover that they exhibit the smallest electric coercive field at room temperature among all known HIFs. Furthermore, we demonstate that a small external stress can repeatedly erase and re-generate ferroelastic domains. In addition, using in-…
Gadolinium molybdate is a classical example of an improper ferroelectric and ferroelastic material. It is established that the spontaneous polarization arises as a secondary effect, induced by a structural instability in the paraelectric phase, which leads to a unit cell doubling and the formation of a polar axis. However, previous X-ray diffraction studies on gadolinium molybdate have been restricted by the limited ability to include the entire 2θ range in the analysis, and thus, at atomic scale, much remains to be explored. By applying temperature dependent X-ray diffraction, we observe the …
Charged polar interfaces such as charged ferroelectric domain walls or heterostructured interfaces of ZnO/(Zn,Mg)O and LaAlO3/SrTiO3, across which the normal component of electric polarization changes suddenly, can host large two-dimensional conduction. Charged ferroelectric walls, which are energetically unfavorable in general, were found to be mysteriously abundant in hybrid improper ferroelectric (Ca,Sr)3Ti2O7 crystals. From the exploration of antiphase boundaries in bilayer-perovskites, we discover that each of four polarization-direction states is degenerated with two antiphase domains, a…
We report on multiple fundamental qualitative improvements in the growth of improper ferroelectric hexagonal YMnO$_3$ (YMO) thin films and heterostructures by pulsed laser deposition (PLD). By a combination of pre-growth substrate annealing and low-energy-fluence PLD, we obtain a two-dimensional growth mode of the YMO films on yttria-stabilized zirconia (YSZ) with ultralow roughness and devoid of misoriented nanocrystalline inclusions. By inserting a sacrificial manganite layer capped with conducting indium-tin oxide between the substrate and the final film, the latter is grown in a fully latt…
Ferroelectric domain walls are attracting broad attention as atomic-scale switches, diodes and mobile wires for next-generation nanoelectronics. Charged domain walls in improper ferroelectrics are particularly interesting as they offer multifunctional properties and an inherent stability not found in proper ferroelectrics. Here we study the energetics and structure of charged walls in improper ferroelectric YMnO$_3$, InMnO$_3$ and YGaO$_3$ by first principles calculations and phenomenological modeling. Positively and negatively charged walls are asymmetric in terms of local structure and width…
Hybrid improper ferroelectrics (HIFs) have been intensely studied over the last few years to gain understanding of their temperature induced phase transitions and ferroelectric switching pathways. Here we report a switching from first- to second-order phase transition pathway for topical HIFs $Ca_{3-x}Sr_xTi_2O_7$, which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that …
The presence of a switchable spontaneous electric polarization makes ferroelectrics ideal candidates for the use in many applications such as memory and sensors devices. Since known ferroelectrics are rather limited, finding new ferroelectric (FE) materials has become a flourishing field. One promising route is to design the so-called hybrid improper ferroelectricity. However, the previous approach based on the Landau theory is not easily adopted to the systems that are unrelated to the Pbnm perovskite structure. To this end, we develop a general design rule that is applicable to any systems. …
We study the interplay of structural and polar distortions in hexagonal YMnO3 and short-period PbTiO3/SrTiO3 superlattices by means of first-principles calculations at constrained electric displacement field D. We find that in YMnO3 the tilts of the oxygen polyhedra produce a robustly polar ground state, which persists at any choice of the electrical boundary conditions. Conversely, in PTO/STO the antiferrodistortive instabilities alone do not break inversion symmetry, and open-circuit bundary conditions restore a non-polar state. We suggest that this qualitative difference naturally provides …
The improper ferroelectricity in YMnO$_3$ and other related multiferroic hexagonal manganites are known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence across the ferroelectric transition, however, were previously not well understood. Here we reveal the evolution of the local structure with temperature in YMnO$_3$ using neutron total scattering techniques, and interpret them with the help of first-principles calculations. The results show that, at room temperature, the local and average s…
We report the dielectric properties of improper ferroelectric h-ErMnO$_3$. From the bulk characterisation we observe a temperature and frequency range with two distinct relaxation-like features, leading to high and even 'colossal' values for the dielectric permittivity. One feature trivially originates from the formation of a Schottky barrier at the electrode-sample interface, whereas the second one relates to an internal barrier layer capacitance (BLC). The calculated volume fraction of the internal BLC (of 8 %) is in good agreement with the observed volume fraction of insulating domain walls…
Charged domain walls and boundaries in ferroelectric materials display distinct phenomena, such as an increased conductivity due to the accumulation of bound charges. Here, we report the electron microscopy observations of atomic-scale arrangements at charged domain boundaries in the hybrid improper ferroelectric Ca$_{2.46}$Sr$_{0.54}$Ti$_2$O$_7$. Like in the prototype improper ferroelectric YMnO$_3$, we find that charged domain boundaries in Ca$_{2.46}$Sr$_{0.54}$Ti$_2$O$_7$ correspond to out-of-phase boundaries, which separate adjacent domains with a fractional translational shift of the uni…
In hybrid improper ferroelectric systems, polarization arises from the onset of successive nonpolar lattice modes. In this work, measurements and modeling were performed to determine the spatial symmetries of the phases involved in the transitions to these modes. Structural and optical measurements reveal that the tilt and rotation distortions of the MnO6 or TiO6 polyhedra relative to the high symmetry phases driving ferroelectricity in the hybrid improper Ca3X2O7 system (X=Mn and Ti) condense at different temperatures. The tilt angle vanishes abruptly at T$_T$ ~ 400 K for Ca3Mn2O7 (and contin…
Standing on successful first principles predictions for new functional ferroelectric materials, a number of new ferroelectrics have been experimentally discovered. Utilizing trilinear coupling of two types of octahedron rotations, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds (Ca$_{3}$Ti$_{2}$O$_{7}$, Ca$_{3}$Mn$_{2}$O$_{7}$, and (Ca/Sr/Ba)$_{3}$(Sn/Zr/Ge)$_{2}$O$_{7}$). However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. Here we…
Ca$_3$Ti$_2$O$_7$ is an experimentally confirmed hybrid improper ferroelectric material, in which the electric polarization is induced by a combination of the coherent TiO$_6$ octahedral rotation and tilting. In this work, we investigate the tuning of ferroelectricity of Ca$_3$Ti$_2$O$_7$ using iso-valent substitutions on Ca-sites. Due to the size mismatch, larger/smaller alkaline earths prefer $A$'/$A$ sites respectively, allowing the possibility for site-selective substitutions. Without extra carriers, such site-selected iso-valent substitutions can significantly tune the TiO$_6$ octahedral …
The central goal of crystal engineering is to develop precise control over material function \emph{via} rational design of structure. A particularly successful realisation of this paradigm is the example of hybrid improper ferroelectricity in layered perovskite materials, where cation vacancies and cooperative octahedral tilts combine to break inversion symmetry. However, in the more chemically diverse and technologically relevant family of inorganic ABX$_3$ perovskites, symmetry conspires to render hybrid coupling to polar distortions impossible. In this study, we use group-theoretical analys…
Ferroic materials are well known to exhibit heterogeneity in the form of domain walls. Understanding the properties of these boundaries is crucial for controlling functionality with external stimuli and for realizing their potential for ultra-low power memory and logic devices as well as novel computing architectures. In this work, we employ synchrotron-based near-field infrared nano-spectroscopy to reveal the vibrational properties of ferroelastic (90$^\circ$ ferroelectric) domain walls in the hybrid improper ferroelectric Ca$_3$Ti$_2$O$_7$. By locally mapping the Ti-O stretching and Ti-O-Ti …
The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a high-pressure powder synchrotron X-ray diffraction experiment and $ab~initio$ calculations on the bilayered Ruddlesden-Popper phases Ca$_{3}$Mn$_{2}$O$_{7}$ and Ca$_{3}$Ti$_{2}$O$_{7}$. In both compounds we obser…
Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are introducing new directions in the 2D materials landscape. The coexistence of ferroelectricity and spin-orbit interactions play a key role in their optoelectronic properties. We perform a detailed study on a recently synthesized ferroelectric 2D-HOIP, (AMP)PbI$_4$ (AMP = 4-aminomethyl-piperidinium). The calculated polarization and Rashba parameter are in excellent agreement with experimental values. We report a striking new effect, i.e., an extraordinarily large Rashba anisotropy that is tunable by ferroelectric polarization:…
Using first principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice- and electronic-based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An i…
We demonstrate that electromagnons can be used to directly probe the nature of a phase transition between magnetically ordered phases in an improper ferroelectric. The antiferromagnetic/paraelectric to antiferromagnetic/ferroelectric phase transition in Cu$_{1-x}$Zn$_{x}$O ($x=0, 0.05$) alloys was tracked via the electromagnon response using terahertz time-domain spectroscopy, on heating and cooling through the phase transition. The transition was found to exhibit thermal hysteresis, confirming its first-order nature, and to broaden under the influence of spin-disorder upon Zn substitution. Th…
We propose the Landau model for lock-in phase transitions in uniaxially modulated improper ferroelectric incommensurate-commensurate systems of class I. It includes Umklapp terms of third and fourth order and secondary order parameter representing the local polarization. The corresponding phase diagram has the structure of harmless staircase, with the allowed wave numbers obeying the Farey tree algorithm. Among the stable commensurate phases only those with periods equal to odd number of lattice constants have finite macroscopic polarizations. These results are in excellent agreement with expe…
Recent interests towards novel functionalities arising at domain walls of ferroic materials naturally call for a microscopic understanding. To this end, first-principles calculations have been performed in order to provide solid evidence of ferroelectricity in the twin walls of nonpolar CaTiO3 and magnetic CaMnO3. We show that ferroelectricity arises from rotation/tilting octahedral distortions --- cooperatively acting at the twin wall in both considered systems --- rather than from a proper secondary ferroelectric instability, as often believed. Our results are in excellent agreement with exp…
It is of great interest to design and make materials in which ferroelectric polarisation is coupled to other order parameters such as lattice, magnetic and electronic instabilities. Such materials will be invaluable in next-generation data storage devices. Recently, remarkable progress has been made in understanding improper ferroelectric coupling mechanisms that arise from lattice and magnetic instabilities. However, although theoretically predicted, a compact lattice coupling between electronic and ferroelectric (polar) instabilities has yet to be realised. Here we report detailed crystallog…
High-temperature electronic materials are in constant demand as the required operational range for various industries increases. Here we design $(A,A^\prime)B_2$O$_6$ perovskite oxides with [111] ``rock salt" $A$-site cation order and predict them to be potential high-temperature piezoelectric materials. By selecting bulk perovskites which have a tendency towards only out-of-phase $B$O$_6$ rotations, we avoid possible staggered ferroelectric to paraelectric phase transitions while also retaining non-centrosymmetric crystal structures necessary for ferro- and piezoelectricity. Using density fun…
There is great interest in hybrid organic-inorganic materials such as metal-organic frameworks (MOFs). The compounds [C(NH$_{2}$)$_{3}$]M(HCOO)$_{3}$, where M=Cu$^{2+}$ or Cr$^{2+}$ are Jahn-Teller (JT) active ions, are MOF with perovskite topology which crystallizes in polar space group Pna2$_{1}$. In inorganic compounds, octahedral tilting and Jahn-Teller structural distortions are usually non-polar distortions. However, in this MOF cooperative interactions between the antiferro-distortive distortions of the framework and the C(NH2)$_{3}$ organic cation via hydrogen bonding breaks the invers…
The charged domain walls in ferroelectric materials exhibit intriguing physical properties. We examine herein the charged-domain-wall structures in Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$ using transmission electron microscopy. When viewed along the [001] axis, the wavy charged domain walls are observed over a wide range ($>$5 $μ$m). In contrast, short charged-domain-wall fragments (from 10 to 200 nm long) occur because they are intercepted and truncated by the conventional 180$^°$ domain walls. These results reveal the unusual charged domain structures in Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$ and will be useful fo…
The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroic…
We propose the phenomenological description of ferroelectric disordering in ferroelectric-semiconductors caused by charged impurities with the charge density random fluctuations. The material improper conductivity is proportional to the averaged impurity charge density. For high enough conductivity free carriers screen fluctuations in the absence of an external field. When external electric field is applied, inner field fluctuations and dynamical induction fluctuations appear in the inhomogeneously polarized system "charged defect + screening cloud". We show that the macroscopic state of ferro…
Ferroelectric domain walls represent multifunctional 2D-elements with great potential for novel device paradigms at the nanoscale. Improper ferroelectrics display particularly promising types of domain walls, which, due to their unique robustness, are the ideal template for imposing specific electronic behavior. Chemical doping, for instance, induces p- or n-type characteristics and electric fields reversibly switch between resistive and conductive domain-wall states. Here, we demonstrate diode-like conversion of alternating-current (AC) into direct-current (DC) output based on neutral 180$^{\…
Transition metal oxides hold great potential for the development of new device paradigms because of the field-tunable functionalities driven by their strong electronic correlations, combined with their earth abundance and environmental friendliness. Recently, the interfaces between transition-metal oxides have revealed striking phenomena such as insulator-metal transitions, magnetism, magnetoresistance, and superconductivity. Such oxide interfaces are usually produced by sophisticated layer-by-layer growth techniques, which can yield high quality, epitaxial interfaces with almost monolayer con…
We use a combination of symmetry arguments and first-principles calculations to explore the connection between structural distortions and ferroelectricity in the perovskite family of materials. We explain the role of octahedral rotations in suppressing ferroelectricity in these materials and show that, as the tolerance factor decreases, rotations alone cannot fully suppress ferroelectricity. Our results show that it is cation displacements (`hidden' in Glazer notation) that accompany the rotations, rather than the rotations themselves, that play the decisive role in suppressing ferroelectricit…
We report the relationship between epitaxial strain and the crystallographic orientation of the in-phase rotation axis and A-site displacements in Pbnm-type perovskite films. Synchrotron diffraction measurements of EuFeO3 films under strain states ranging from 2% compressive to 0.9% tensile on cubic or rhombohedral substrates exhibit a combination of a-a+c- and a+a-c- rotational patterns. We compare the EuFeO3 behavior with previously reported experimental and theoretical work on strained Pbnm-type films on non-orthorhombic substrates, as well as additional measurements from LaGaO3, LaFeO3, an…
Switchable Polarization in an A-site Deficient Perovskite through Vacancy and Cation Engineering
Kyoto University（京都大学）；Nantes Université（南特大学）；The University of Tokyo（东京大学）；Japan Fine Ceramics Center（日本先进陶瓷中心）；Kyushu University（九州大学）；Pennsylvania State University（宾夕法尼亚州立大学）
We present a detailed study of the structural, electronic, magnetic and ferroelectric properties of prototypical \textit{proper} and \textit{improper} multiferroic (MF) systems such as BiFeO$_{3}$ and orthorhombic HoMnO$_{3}$, respectively, within density functional theory (DFT) and using the Heyd-Scuseria-Ernzerhof hybrid functional (HSE). By comparing our results with available experimental data as well as with state-of-the-art GW calculations, we show that the HSE formalism is able to account well for the relevant properties of these compounds and it emerges as an accurate tool for predicti…
We have studied the structural phase transition of multiferroic YMnO$_3$ from first principles. Using group-theoretical analysis and first-principles density functional calculations of the total energy and phonons, we perform a systematic study of the energy surface around the prototypic phase. We find a single instability at the zone-boundary which couples strongly to the polarization. This coupling is the mechanism that allows multiferroicity in this class of materials. Our results imply that YMnO$_3$ is an improper ferroelectric. We suggest further experiments to clarify this point.
Hybrid Improper Ferroelectricity (HIF) allows the generation of an electrical polarization in the AA'BB'O$_6$ double perovskite materials thanks to the combination of two non-polar octahedral distortions. Nevertheless, for selected combination of the A/A' cations a non-polar 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 w…
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 orthorho…
In this work we analyze the finite-size effects on the structural properties and on the polarization of the rhombohedral phase of HfO$_2$ subjected to a biaxial compressive strain. We show how the presence of surface charges affects the polarization, leading to a strong reduction with respect to its bulk value. This reduction can be ascribed to two mechanisms: the coupling between compressive strain and the phase-transition order parameter; the changes in the ferroelectric distortion. We give two alternative explanations of this phenomenon: from an atomistic point of view, analyzing the evolut…
