Improper Ferroelectric Materials Research Articles

Directed synthesis of a hybrid improper magnetoelectric multiferroic material

Preparing materials which simultaneously exhibit spontaneous magnetic and electrical polarisations is challenging as the electronic features which are typically used to stabilise each of these two polarisations in materials are contradictory. Here we show that by performing low-temperature cation-exchange reactions on a hybrid improper ferroelectric material, Li2SrTa2O7, which adopts a polar structure due to a cooperative tilting of its constituent TaO6 octahedra rather than an electronically driven atom displacement, a paramagnetic polar phase, MnSrTa2O7, can be prepared. On cooling below 43 K the Mn2+ centres in MnSrTa2O7 adopt a canted antiferromagnetic state, with a small spontaneous magnetic moment. On further cooling to 38 K there is a further transition in which the size of the ferromagnetic moment increases coincident with a decrease in magnitude of the polar distortion, consistent with a coupling between the two polarisations.

On the stability of calcium and cadmium based Ruddlesden–Popper and double perovskite structures

Ab initiostudy on stability properties and phase diagrams of Ca- and Cd-related hybrid improper ferroelectric materials with the Ruddlesden–Popper and double perovskite structures.

Enhancement in hybrid improper ferroelectricity of Ca3Ti2O7 ceramics by a two-stage sintering

Ca3Ti2O7 ceramics were prepared by a solid-state reaction process, and the influences of the sintering process on its microstructure and electric properties have been systematically studied. All the ceramic samples fabricated by different sintering processes are single polar A21am phase at room temperature, and the results indicate that the extra calcining or sintering at 1400 °C is beneficial to get the higher lattice constants. Especially the Ca3Ti2O7 ceramics fabricated by the two-stage sintering exhibit the largest rotation angle and tilt angle of the oxygen octahedron, the densest microstructure, and the largest grain size among all the samples. The correlation of crystal structure and hybrid improper ferroelectricity confirms that the amplitude of tilt and rotation of oxygen octahedron is crucial to regulate the remnant polarization of Ca3Ti2O7 ceramics. The leakage behaviors, dielectric and ferroelectric properties of Ca3Ti2O7 ceramics are dependent on the microstructure, oxygen vacancy, and grain size caused by different sintering processes, and the related mechanisms have been illuminated. The highest remnant polarization (1.319 μC/cm2) to our knowledge and the lower coercive field (78.17 kV/cm) was achieved in the ceramic sample fabricated by the two-stage sintering, and these findings have an important significance in developing excellent hybrid improper ferroelectric materials.

Mapping the structural transitions controlled by the trilinear coupling in Ca3-xSrxTi2O7

We present the results of the high-temperature neutron and x-ray diffraction experiments on the Ca3–xSrxTi2O7 (x = 0.5, 0.8, 0.85, 0.9) compounds. The ferro- to paraelectric transition in these hybrid improper ferroelectric materials arises from the so-called trilinear coupling. Depending on the strontium content, various structures and phase transitions, different from theoretical predictions, emerge. The in situ x-ray powder diffraction indicates a direct ferro- to paraelectric transition between the orthorhombic A21am and the tetragonal undistorted I4/mmm phase for x ≤ 0.6. We identified a reduction in the trilinear coupling robustness by increasing the Sr-doping level to lead to the emergence of the intermediate tetragonal P42/mnm phase and the gradual suppression of the orthorhombic phase. The observed character of the structure transitions and the Ca3–xSrxTi2O7 phase diagram are discussed in the framework of theoretical models of other related hybrid improper ferroelectric systems.

Hybrid improper ferroelectricity in B-site substituted Ca3Ti2O7: The role of tolerance factor

Hybrid improper ferroelectricity is induced by the coupling of oxygen octahedral rotation and tilting, and a promising way to create multiferroic materials with a strong magnetoelectric effect can be proposed. Although some hybrid improper ferroelectric materials with Ruddlesden-Popper structures have been discovered experimentally, the small polarizations and high coercive fields will severely limit their practical applications. To deal with this issue, an alternative way is proposed by the first-principles calculations in the present work. By calculating the energies of phases with oxygen octahedral rotation and tilting in Ca3TiMO7 (M = Si, Ge, Mn, Sn, Hf, and Zr) compounds with five B-site ordering configurations, it is found that the tolerance factor has a considerable impact on their ferroelectric properties. The energy barriers and ferroelectric polarizations increase with decreasing tolerance factor. Moreover, the individual-layer contributions to the total polarizations depend on the space group, which is also decided by the tolerance factor of the compound. Therefore, to enhance the ferroelectric polarization in B-site substituted Ca3Ti2O7, the tolerance factor should be decreased according to the calculation results of this work.

Structural transitions in hybrid improper ferroelectric Ca3Ti2O7 tuned by site-selective isovalent substitutions: A first-principles study

$\mathrm{C}{\mathrm{a}}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ is an experimentally confirmed hybrid improper ferroelectric material, in which the electric polarization is induced by a combination of the coherent $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral rotation and tilting. In this work, we investigate the tuning of ferroelectricity of $\mathrm{C}{\mathrm{a}}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ using isovalent substitutions on Ca sites. Due to the size mismatch, larger/smaller alkaline earths prefer ${A}^{\ensuremath{'}}/A$ sites, respectively, allowing the possibility for site-selective substitutions. Without extra carriers, such site-selected isovalent substitutions can significantly tune the $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral rotation and tilting, and thus change the structure and polarization. Using the first-principles calculations, our study reveals that three substituted cases (Sr, Mg, and Sr+Mg) show divergent physical behaviors. In particular, ${(\mathrm{CaTi}{\mathrm{O}}_{3})}_{2}\mathrm{SrO}$ becomes nonpolar, which can reasonably explain the suppression of polarization upon Sr substitution observed in experiment. In contrast, the polarization in ${(\mathrm{MgTi}{\mathrm{O}}_{3})}_{2}\mathrm{CaO}$ is almost doubled upon substitutions, while the estimated coercivity for ferroelectric switching does not change. The ${(\mathrm{MgTi}{\mathrm{O}}_{3})}_{2}\mathrm{SrO}$ remains polar but its structural space group changes, with moderate increased polarization and possible different ferroelectric switching paths. Our study reveals the subtle ferroelectricity in the ${A}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ family and suggests one more practical route to tune hybrid improper ferroelectricity, in addition to the strain effect.

Tearing down the barriers to domain wall electronics

Using dielectric analysis, the switching dynamics of an improper ferroelectric material, hexagonal erbium manganite, has been characterized for the first time.

Soft tilt and rotational modes in the hybrid improper ferroelectric Ca3Mn2O7

Raman spectroscopy is employed to probe directly the soft rotation and tilting modes, which are two primary order parameters predicted in the hybrid improper ferroelectric material Ca$_3$Mn$_2$O$_7$. We observe a giant softening of the 107-cm$^{-1}$ octahedron tilting mode by 26~cm$^{-1}$, on heating through the structural transition from a ferroelectric to paraelectric orthorhombic phase. This is contrasted by a small softening of the 150-cm$^{-1}$ rotational mode by 6~cm$^{-1}$. In the intermediate phase, the competing soft modes with different symmetries coexist, bringing about many-faceted anomalies in spin excitations and lattice vibrations. Our work demonstrates that the soft rotation and tilt patterns, relying on a phase-transition path, are a key factor in determining ferroelectric, magnetic, and lattice properties of Ca$_3$Mn$_2$O$_7$.

B23-O-08Microstructures in improper ferroelectric compounds revealed by electron microscopy

In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [1]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [2]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr)3Ti2O7, whichresults from the combination of two or more non-ferroelectric structural order parameters [3]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl2O4 with the hexagonal structure [4]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2a ×2a×c. In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported. [1] A. P. Levanyuk et al., Sov. Phys. Usp., 17 (1974) 199-214. [2] T. Choi, et al., Nature Mater. 9 (2010) 253-258. [3] Y. S. Oh et al., Nature Mater. 14 (2015) 407-413. [4] S. Mori et al., Ferroelectrics 464 (2014) 116-121. Fig.1 Microstructure in the ferroelectric phase of BaAl2O4. B23-O-07 doi:10.1093/jmicro/dfv165

B23-O-07Multivariate Statistical Analysis of EMCD Spectra measured at the Fe/MgO Interface

In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [1]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [2]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr)3Ti2O7, whichresults from the combination of two or more non-ferroelectric structural order parameters [3]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl2O4 with the hexagonal structure [4]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2a ×2a×c. In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported. [1] A. P. Levanyuk et al., Sov. Phys. Usp., 17 (1974) 199-214. [2] T. Choi, et al., Nature Mater. 9 (2010) 253-258. [3] Y. S. Oh et al., Nature Mater. 14 (2015) 407-413. [4] S. Mori et al., Ferroelectrics 464 (2014) 116-121. Fig.1 Microstructure in the ferroelectric phase of BaAl2O4. B23-O-07 doi:10.1093/jmicro/dfv165

Intrinsic space charge layers and field enhancement in ferroelectric nanojunctions

Conducting characteristics of topological defects in ferroelectric materials, such as charged domain walls, engendered a broad interest on their scientific merit and the possibility of novel applications utilizing domain engineering. At the same time, the problem of electron transport in ferroelectrics still remains full of unanswered questions and becomes yet more relevant over the growing interest in ferroelectric semiconductors and new improper ferroelectric materials. We have employed self-consistent phase-field modeling to investigate the physical properties of a local metal-ferroelectric (Pb(Zr0.2Ti0.8)O3) junction in applied electric field. We revealed an up to 10-fold local enhancement of electric field realized by large polarization gradient and over-polarization effects due to inherent non-linear dielectric properties of Pb(Zr0.2Ti0.8)O3. The effect is independent of bias polarity and maintains its strength prior, during and after ferroelectric switching. The observed field enhancement can be considered on similar grounds as increased doping level, giving rise to reduced switching bias and threshold voltages for charge injection, electrochemical and photoelectrochemical processes.

Structural phase transition in ferroelectric NH4103

Abstract The structural phase transformation between the ferroelectric and paraelectric form of NH4IO, near 83°C is dominated by tilt motions of IO6-octahedra within a perovskite-like framework. Simultaneously, small displacements of NH4 along the polar axis give rise to a switchable spontaneous polarization at low temperatures. As the transition mechanism is governed by the tilting of octahedra and not by the appearance of ferroelectricity, NH4IO3 is characterised as an improper ferroelectric material. Although the space group of the high-temperature β-phase is polar, the polarization moment is zero. Raman-active thermal soft modes appear in the ferroelectric phase. The phase transition is first order with anomalies of dielectric, piezoelectric and elastic constants.

Incommensurate-Commensurate Phase Transitions in Ferroelectric Substances

A general approach is presented for the incommensurate states and incommensurate (IC)-commensurate (C) transitions in ferroelectrics. An emphasis is put on the determination of the free energy as a function of the IC wave vector as well as on the role played by harmonics. The Levanyuk-Sannikov model for an improper ferroelectric material (NH 4 ) 2 BeF 4 is studied in details from this point of view, and the IC-C transition of this model is shown to be of second order. Some possible extensions of the model are suggested to have a first-order IC-C transition.