Improper Ferroelectricity Research Articles

Robust resistance switching performance in a Ca3Ti2O7/La0.67Sr0.33MnO3 heterostructure induced by the interface

Ruddlesden–Popper phase oxides, such as Ca3Ti2O7, have been established as hybrid improper ferroelectrics. However, investigations into Ca3Ti2O7 have primarily concentrated on their structural and ferroelectric properties. In this study, we prepared epitaxial Ca3Ti2O7 thin films via magnetron sputtering. Conducting atomic force microscopy was employed to characterize local current variations under an applied bias voltage. Electron paramagnetic resonance measurements of the Ca3Ti2O7/La0.67Sr0.33MnO3 film were conducted to assess its defect characteristics. Interestingly, the Ca3Ti2O7/La0.67Sr0.33MnO3 stacks exhibited remarkable macroscopic resistance switching, with a resistance on/off ratio reaching 100, alongside robust retention (∼2500 s) and endurance (∼2000 cycles) features. Additionally, density functional theory calculations suggest that the resistance switching is attributable to the interface barrier of the Ca3Ti2O7/La0.67Sr0.33MnO3 interface and the efficacy of space charge limitation. This work proposes an avenue for the utilization of Ruddlesden–Popper phase Ca3Ti2O7 in various applications.

Emergence of critical thickness and multifaceted role of stacking faults in high misfit hexagonal ferrites films

Polarization in improper ferroelectrics is believed to be robust against the depolarizing field. However, this characteristic appears to be disrupted in hexagonal ferrite and manganite, leading to a controversial debate regarding the origin of the critical thickness. In this study, we design hexagonal TbFeO3 (h-TFO) films which exhibits a critical thickness exceeding 5 nm at room temperature and an exceptionally long suppression layer in thicker films. It is explained that the critical thickness effect originates from the synergistic interplay of the interface, strain, and vacancies, which is confirmed by combining scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS) and density functional theory (DFT). This explanation is equally applicable to the polarization suppression near the stacking fault (SF). Finally, we find that the SF reduces the suppression length to an extent, and contributes to changes in the topological order of the domain in h-TFO film. Our study provides a more comprehensive understanding of the origin of critical thickness and the multifaceted role of the SF in ferroelectrics.

Effects of doping with magnetic cations on the hybrid improper ferroelectricity in Sr3Sn2O7

We here report the structural, magnetic and electrical properties of Sr2.9La0.1Sn1.9M0.1O7 (M= Cr, Mn or Fe) compounds. This La/M codoping of the hybrid improper ferroelectric Sr3Sn2O7, allows introducing magnetic cations into the perovskite layers of this Ruddlesden-Popper phase. The doping induces minor structural changes, preserving the polar structure with space group A21am of the undoped compound. The perovskite tolerance factor of all doped samples is slightly lower than in Sr3Sn2O7, which preserves the tilts and rotations of SnO6 octahedra. Doped samples exhibit a paramagnetic behavior down to 5 K and obey the Curie-Weiss law above 200 K. The effective paramagnetic moments agree with the expected spin-only values of the respective magnetic M3+ cations. All samples show a ferroelectric hysteresis loop at room temperature, but the doped samples show larger coercive fields. Additionally, the doping with magnetic cations has important effects on the dielectric properties: a strong decrease of the temperature of the ferroelectric transition together with a smoothing of the anomaly in the dielectric permittivity. These results suggest that the disorder produced by the two aliovalent substitutions is detrimental for the ferroelectric properties due to point charge defects but, at the same time, the prevalence of the structural distortion (tilts and rotations) preserves the ferroelectricity in the investigated doping regime.

Improved multiferroic properties of lanthanum ferrites through cobalt doping

LaFeO3 is a type II multiferroic material which exhibits improper ferroelectricity induced by magnetic order (antiferromagnetism). However, the antiferromagnetic order limits its applications. This study demonstrates the induction of ferromagnetism in LaFeO3 by doping with cobalt, varying the level of cobalt from 0 to 0.5 mol. Moreover, the effects of cobalt on the crystal structure, magnetic and dielectric properties are described. The XRD results show that at doping concentrations lower than 0.3 mol, LaFeO3 maintains its orthorhombic structure (Pnma), while at higher cobalt concentrations the rhombohedral (R-3c) LaCoO3 phase emerges. In addition, cobalt doping induces ferromagnetism because cobalt occupies iron positions until the solubility limit is reached (0.3 mol). All samples present high relative permittivity values, which increase with increasing cobalt content and temperature. Furthermore, the presence of cobalt results in a decrease in the dielectric dissipation factor for cobalt contents lower than 0.3 mol, due to the inhibition in the formation of vacancies. However, the presence of LaCoO3 at high cobalt contents increases the dielectric losses due to the presence of a mixtures of phases. Moreover, the electric polarization curves exhibited weak ferroelectric behavior for ferrites with cobalt contents lower than 0.3 mol, and lossy improper ferroelectric type behavior for samples with higher contents.

Hybrid improper ferroelectricity in La2Sr (Sc1−xFex)2O7 ceramics with double-layered Ruddlesden–Popper structures

Numerous hybrid improper ferroelectrics have been discovered in bulk oxides with layered perovskite structures. In contrast, the competition between the interlayer rumpling and oxygen octahedral rotation suppresses the ferroelectricity in layered perovskite material with trivalent cation at the B-site. In the present work, single-phase dense La2Sr(Sc1−xFex)2O7 ceramics with double-layered Ruddlesden–Popper structures have been prepared, and room-temperature ferroelectricity is discovered in the ceramics with x ≤ 0.10. The ferroelectric polarization and coercive field decrease with increasing content of Fe3+ cations, consistent with the decline of oxygen octahedral rotation and tilting angles. Although the linear relationship between the Curie temperature and the tolerance factor for La2Sr(Sc1−xFex)2O7 ceramics is established, the line is far away from that for A2+3B4+2O7 ceramics due to the large interlayer rumpling in the present ceramics. Although no single-phase multiferroic has been discovered in this work, an effective way to introduce magnetism into hybrid improper ferroelectric is provided.

Experimental demonstration of tunable hybrid improper ferroelectricity in double-perovskite superlattice films

Hybrid improper ferroelectricity can effectively avoid the intrinsic chemical incompatibility of electronic mechanism for multiferroics. Perovskite superlattices, as theoretically proposed hybrid improper ferroelectrics with simple structure and high technological compatibility, are conducive to device integration and miniaturization, but the experimental realization remains elusive. Here, we report a strain-driven oxygen octahedral distortion strategy for hybrid improper ferroelectricity in La2NiMnO6/La2CoMnO6 double-perovskite superlattices. The epitaxial growth mode with mixed crystalline orientations maintains a large strain transfer distance more than 90 nm in the superlattice films with lattice mismatch less than 1%. Such epitaxial strain permits sustainable long-range modulation of oxygen octahedral rotation and tilting, thereby inducing and regulating hybrid improper ferroelectricity. A robust room-temperature ferroelectricity with remnant polarization of ~ 0.16 μC cm−2 and piezoelectric coefficient of 2.0 pm V−1 is obtained, and the density functional theory calculations and Landau-Ginsburg-Devonshire theory reveal the constitutive correlations between ferroelectricity, octahedral distortions, and strain. This work addresses the gap in experimental studies of hybrid improper ferroelectricity for perovskite superlattices and provides a promising research platform and idea for designing and exploring hybrid improper ferroelectricity.

Effect of Ca2MnO4 on the magnetic and electric properties in Ca3Mn2O7 ceramics prepared with different sintering temperature

Hybrid improper ferroelectricity Ca3Mn2O7 is the promising multiferroic materials, which has attracted considerable attention due to their potential applications in areas such as electronic devices and storage media. The magnetic and electric properties of the layered perovskite Ca3Mn2O7 ceramics with different proportions of single-layer Ca2MnO4 phase synthesized by solid state reaction within adjusting the sintering temperature have been investigated. The x-ray diffraction patterns and the characteristic of magnetic transition from magnetization curves show that the ratio of Ca2MnO4 is reduced from 22.9 % of sintering at 1200 °C to 0 % of sintering at 1300 °C, which implies that the narrow suitable sintering temperature is the most factor for preparation of pure Ca3Mn2O7 ceramic samples. The ferroelectricity of all samples was investigated and the ferroelectricity decreased with the increase of Ca2MnO4 content due to the presence of non-ferroelectric Ca2MnO4 phase. The magnetoelectric coupling effect was discussed by the magnetization induced from the electric field. The results speculated that a small amount of Ca2MnO4 can improve the magnetoelectric coupling effect due to the interface effect in multiphase materials, which suggests that a new method to enhance the magnetoelectric coupling effect in Ca3Mn2O7 system.

Interface engineering for facile switching of bulk-strong polarization in Si-compatible vertical superlattices

Ferroelectric thin films incorporating different compositional layers have emerged as a promising approach for enhancing properties and performance of electronic devices. In recent years, superlattices utilizing various interactions between their constituent layers have been used to reveal unusual properties, such as improper ferroelectricity, charged domain walls, and negative capacitance in conventional ferroelectrics. Herein, we report a symmetry scheme based on the interface engineering in which the inherent cell-doubling symmetry allowed atomic distortions (phonons) in any vertically aligned superlattice activate novel interface couplings among atomic distortions of different symmetries and fundamentally improve the ferroelectric properties. In a materialized case, the ionic size difference between Hf4+ and Ce4+ in the HfO2/CeO2 (HCO) ferroelectric/paraelectric superlattice leads to these couplings. These couplings mitigate the phase boundary between polar and non-polar phases, and facilitate polarization switching with a remarkably low coercive field (Ec\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${E}_{c}$$\\end{document}) while preserving the original magnitude of the bulk HfO2 polarization and its scale-free ferroelectric characteristics. We show that the cell-doubled distortions present in any vertical superlattice have unique implications for designing low-voltage ferroelectric switching while retaining bulk-strong charge storing capacities in Si-compatible memory candidates.

Electric properties of the twelve-fold vortex structure in hexagonal manganites

Hexagonal manganites, as a functional ferroelectric (FE) material, receive considerable attention due to their improper ferroelectricity and topological vortex structures. This family exhibits three low-symmetry states accompanied by distinct vortex domain structures. In addition to the FE P63 cm and anti-FE (AFE) P-3c1 states accompanied by dual six-fold vortex structures, there is another FE P3c1 state accompanied by a twelve-fold vortex structure. The responses of FE materials to external stimuli, such as external electric fields, are the core ingredients in the physics of FEs and are significant for technological applications. Under external electric fields, the responses of FE materials are determined by special FE domain structures. The electric properties of the FE P63 cm and AFE P-3c1 states are very different. However, the electric properties of the FE P3c1 state, which only stabilizes in Ga-substituted In(Mn, Ga)O3, are unclear. The present work studies the electric properties of the FE P3c1 state. The electric-field-driven transition of the FE P3c1 state is found to follow two sequences, i.e. (1) twelve-fold P3c1 → nine-fold P3c1 + P63 cm → three-fold P63 cm, and (2) twelve-fold P3c1 → six-fold P3c1 → three-fold P63 cm. The variation of average polarization with E for the FE P3c1 state with the second transition sequence manifests as an unusual triple-hysteresis loop, different from the usual single-hysteresis loop of FE materials. The results are related to the coexistence of the FE and non-FE domain walls in the FE P3c1 state. Furthermore, it is found that the FE P3c1 state at substitution concentration 0.39 exhibits the highest dielectric response. The results advance our understanding of topological vortex structures in hexagonal manganites.

Emergence of Improper Electronic Ferroelectricity and Flat Band in Twisted Bilayer Tl2S.

Two-dimensional (2D) ferroelectrics possessing out-of-plane (OP) polarization are highly desirable for applications and fundamental physics. Here, by first-principles calculations, we reveal that large-angle interlayer twisting can efficiently stabilize an unexpected ordering of sizable electric dipoles, producing OP polarization out of the centrosymmetric ground-state structure of Tl2S, in great contrast to the recently proposed interlayer-sliding ferroelectricity. The ferroelectricity originates from a nonlinear coupling between a polar order dominantly contributed by electrons and an unstable phonon mode associated with a commensurate k point (1/3, 1/3, 0) in the two constituent monolayers, therefore indicating an improper and electronic ferroelectric nature. More interestingly, a flat band and a van Hove singularity occur in its electronic structures just below the Fermi level in the large-angle twisted bilayer Tl2S. The unusual coexistence of improper electronic ferroelectricity, a flat band, and a van Hove singularity in one 2D material offers exceptional opportunities for exploring novel physics and applications.

La2SrSc2O7: A-Site Cation Disorder Induces Ferroelectricity in Ruddlesden-Popper Layered Perovskite Oxide.

Rational design of ferroelectrics in layered perovskites, like n = 2 Ruddlesden-Popper (RP) phase A3B2O7, has been achieved by the hybrid-improper ferroelectric (HIF) mechanism, in which an electric polarization is induced via a trilinear coupling to nonpolar BO6 octahedral rotation and tilt distortions around crystallographic axes. In the present work, hybrid improper ferroelectricity in n = 2 RP-type La2SrSc2O7 induced by the disordering of Sr2+/La3+ cations on the A-sites in rocksalt ([Sr/La]Rs = 25/75) and perovskite ([Sr/La]Pv = 50/50) layers is demonstrated through experimental and theoretical investigations. The ferroelectric A21am structure (a-a-c+ in Glazer notation) at room temperature and the second-order phase transition to paraelectric Amam structure (a-a-c0) at TC ∼ 600 K are determined by a combination of X-ray and neutron diffraction and optical second harmonic generation. The ferroelectric hysteresis loop measurements prove the switchable electric polarization indicative of ferroelectricity. These results represent an unprecedented example of ferroelectricity in the n = 2 RP family of Ln2AB2O7 with inequivalent Ln3+ and A2+ cations. Combining the abovementioned experimental results with the first-principles calculations, we verify the role of Sr/La distributions in regulating the interlayer rumpling, which, in addition to the structural tolerance factor, is key to controlling the structural distortions of RP phases. The stabilization of the ferroelectric, a-a-c+ distorted structure is a consequence of the disordered Sr/La distribution on the A-sites, which suppresses the rumpling-induced octahedral deformations in competition with the octahedral rotations and thus enables the concurrence of a0a0c+ rotations and a-a-c0 tilts required for the HIF mechanism. This work demonstrates the possibility of altering the crystal symmetry of RP phases through the A-site cation disorder and provides a complementary approach to the rational design of new HIF materials.

Panoply of Ni-Doping-Induced Reconstructions, Electronic Phases, and Ferroelectricity in 1T-MoS2.

The distorted phases of monolayer 1T-MoS2 have distinct electronic properties, with potential applications in optoelectronics, catalysis, and batteries. We theoretically investigate the use of Ni-doping to generate distorted 1T phases and find not only the ones usually reported but also two further phases (3 × 3 and 4 × 4), depending on the concentration and the substitutional or adatom doping site. Corresponding pristine phases are stable after removal of dopants, which might offer a potential route to experimental synthesis. We find large ferroelectric polarizations, most notably in 3 × 3 which─compared to the recently measured 1T″─has 100 times greater ferroelectric polarization, a lower energy, and a larger band gap. Doped phases include exotic multiferroic semimetals, ferromagnetic polar metals, and improper ferroelectrics with only in-plane polarization switchable. The pristine phases have unusual multiple gaps in the conduction bands with possible applications for intermediate band solar cells, transparent conductors, and nonlinear optics.

Physical mechanism of Ge doping enhanced Ruddlesden-Popper structure quasi-2D Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic hybrid improper ferroelectricity

Hybrid improper ferroelectricity with quasi-two-dimensional (quasi-2D) structure has attracted much attention recently due to its great potential in realizing strong magnetoelectric coupling and room-temperature multiferroicity in a single phase. However, recent studies show that there appears high coercive field and low remnant polarization in ceramics, which severely hinders the applications of this material. In this work, high-quality Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramics with a Ruddlesden-Popper (R-P) structure are successfully prepared, and their crystal structures and electrical properties are investigated in detail. It is found that the Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic exhibits a lower coercive field that is close to that of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> single crystal. Moreover, via a small amount of Ge doping, the polarization reaches 0.34 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> and 0.61 μC/cm<sup>2</sup> for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub>. Combining crystal lattice dynamic studies, we analyze the Raman and infrared responses of the samples, showing the information about the tilting and rotation of the oxygen octahedra in the samples. The improved ferroelectricity after doping may be attributed to the increased amplitude of the tilt mode and the reduced amplitude of rotation mode. Besides, the enhanced ferroelectric properties through Ge doping and its mechanism are further investigated by the Berry phase approach and the Born effective charge method. Furthermore, via the UV-visible spectra, the optical bandgap is determined to be 3.91 eV for Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> ceramic and 3.95 eV for Sr<sub>3</sub>Sn<sub>1.99</sub>Ge<sub>0.01</sub>O<sub>7</sub> ceramic. Using the Becke-Johnson potential combined with the local density approximation correlation, the bandgap is calculated and is found to be in close agreement with the experimental result. And the electronic excitations can be assigned to the charge transfer excitation from O 2p to Sn 5s (Ge 4s). The effects of Ge doping on the ability of Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub> to gain and lose electrons and the bonding strength of Sn-O bond are analyzed via two-dimensional charge density difference. In conclusion, this study provides insights into the synthesis method and modulation of ferroelectric properties of hybrid improper ferroelectrics Sr<sub>3</sub>Sn<sub>2</sub>O<sub>7</sub>, potentially facilitating their widespread applications in various capacitors and non-volatile memory devices.

Optimized hybrid improper ferroelectricity of (Ca1-xNdx)3Ti2O7 ceramics based on soft doping effects

Low remnant polarization and high coercive field are the main factors limiting the practical application of Ca3Ti2O7-based hybrid improper ferroelectric ceramics. Based on sol-gel method and soft doping effect, (Ca1-xNdx)3Ti2O7 (x=0.003) ceramics with higher remnant polarization of ∼3.53 μC/cm2 and lower coercive field of ∼91 kV/cm were obtained. More interestingly, thanks to the large distortion amplitude of TiO6 octahedron and effective suppression of defects such as oxygen vacancies caused by Nd3+ as a donor dopant, the remnant polarization of (Ca1-xNdx)3Ti2O7 (x = 0.003 and 0.006) ceramics measured at 10 Hz using the DHM mode were as high as 7.33 μC/cm2 and 5.85 μC/cm2. These findings prove that the soft doping at A sites of Ca3Ti2O7 material is a simple and effective strategy to enhance its hybrid improper ferroelectricity.

The novel electrical properties in quasi‐2D hybrid improper ferroelectric Sr3Sn2O7 ceramic and the structural modulation

AbstractThe quasi‐two‐dimensional Ruddlesden–Popper material, Sr3Sn2O7 with hybrid improper ferroelectricity, has gained significant scientific interest owing to its unique ferroelectric phase transition mechanism. However, its electrical behaviors near the phase transition temperature necessitate further investigation. In this study, high‐quality Sr3Sn2O7 ceramic was successfully prepared, and its temperature‐dependent electrical properties were systematically studied. The phase transition from ferroelectric to paraelectric of Sr3Sn2O7 ceramic near 160°C was evidenced by the varying electrical properties dependence of temperature. It is found that the space‐charge‐limited bulk conduction with deep traps is identified as dominant conduction mechanism for Sr3Sn2O7 ceramic that has high resistivity. An abnormal increasing trend of coercive field with temperature was observed and is discussed with trap‐filling process and octahedral rotation distortion. Interestingly, Sr3Sn2O7 ceramic exhibits negative piezoelectric effect and negative electrostriction responded to biased electric field, which is dramatically influenced by ferroelectric polarization. Overall, this study serves as a reference for exploring the electrical properties of other hybrid improper ferroelectrics, particularly near their Curie temperature.

Switching of Hybrid Improper Ferroelectricity in Oxide Double Perovskites

Switching of Hybrid Improper Ferroelectricity in Oxide Double Perovskites

Geometrical Designing of a Soft‐Layered Halide Perovskite Improper Photoferroelectric for Boosted Photostriction

AbstractHalide perovskite photoferroelectrics, materials possessing both photosensitive and ferroelectric properties, have motivated tremendous attention in the past decade owing to their distinguishing bulk photovoltaic effect (BPVE) and cross‐coupled functionalities. Despite the great achievements, the BPVE coupled functionalities are still limited by the geometric origin contradictory of internal order parameters. Herein, by exploiting a geometrical design approach, a soft layered halide perovskite improper photoferroelectric (2FEA)2(EA)2Pb3Br10 (1, where 2FEA is 2,2‐difluoroethylamonium and EA is ethylammonium) has been developed. Crystallographic investigations reveal that the incorporation of large‐size EA cations significantly induces a nonpolar lattice distortion of the inorganic semiconducting framework, which gives rise to a striking improper ferroelectricity. In the soft crystal lattices, driven by the subtle competition and coupling between photoinduced carriers and inorganic lattice distortions, single crystals of 1 display a giant cross‐coupled photostriction with a considerable photostrictive coefficient up to 4.89 × 10−6 m2 W−1, which is larger than the traditional semiconductors and ferroelectrics. This work offers an attractive route to the design of new improper photoferroelectric materials and promotes their further applications in photoelectronic devices.

The optical and ferroelectric properties of Fe-doped hybrid improper ferroelectricity Ca3Ti2O7 via experimental and density functional theory study

The magnetoelectric multiferroic materials with ferroelectric and magnetic orders have great attributed attention in the field of magnetoelectric memories and magnetic resonance devices. Here, we systematically investigated the optical and multiferroic properties of Fe-doped hybrid improper ferroelectricity Ca3Ti2O7 prepared by sol-gel method. The distortion of TiO6 octahedron changes with the content of oxygen vacancy introduced by Fe doping, thus realizing the adjustment of remnant ferroelectric polarization of the samples. The slight blue shift of the Raman band at 667 cm−1 reflected the introduces of oxygen vacancies by Fe doping. The optical bandgaps of the doped samples were reduced after doping, agreeing with the results of the first-principles calculations. With the introduction of Fe3+, the magnetism increases gradually. The first-principles calculation shows that magnetism origins from the Fe-3d and their surrounding O-2p orbitals. The Fe-doping Ca3Ti2O7 materials realizing the introduction of magnetic properties while regulating its ferroelectric properties are expected to be applied in magnetoelectric memories, optoelectronic devices spintronics and magnetic response devices.

In situ investigation of the ferroelectric phase transition in improper ferroelectric YMnO3 thin films by electron energy loss spectroscopy

The crystal and electronic structure of $\mathrm{YMn}{\mathrm{O}}_{3}$ thin films at the paraelectric-ferroelectric phase transition are monitored by scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) during in situ heating experiments. At the phase transition of a 14-unit-cell-thick film at $\ensuremath{\sim}325\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$, we observe a clear transition from the polar $P{6}_{3}cm$ to the centrosymmetric $P{6}_{3}/mmc$ space group, manifested by the disappearance of the unit-cell tripling. Further, we reveal significant changes in the EELS data across the phase transition, and interpret them with the help of real-space multiple-scattering calculations. Upon crossing the Curie temperature, striking changes in the O-$K$ near-edge structure are identified, which are attributed to a modification in the Y $4d--\mathrm{O} 2p$ hybridization. In particular, a weakening of the Y $4d{}_{z}{}^{2}--\mathrm{O} 2{p}_{z}$ hybridization is detected for the paraelectric phase compared to the ferroelectric phase. Upon cooling below the Curie temperature, the ferroelectric phase is recovered and a stronger hybridization along the $c$-axis polarization direction is restored, stabilizing the zone-boundary ${\mathrm{K}}_{3}$ phonon mode. Our findings demonstrate the high sensitivity of the O-$K$ near-edge structure to structural distortions in $\mathrm{YMn}{\mathrm{O}}_{3}$ polymorphs and lay the groundwork for future studies on polymorphic phase transformations in improper ferroelectrics whose ferroelectricity emerges through coupling to a nonpolar lattice distortion.

Hybrid Improper Ferroelectricity in Columnar (NaY)MnMnTi4 O12.

We show that cation ordering on A site columns, oppositely displaced via coupling to B site octahedral tilts, results in a polar phase of the columnar perovskite (NaY)MnMnTi4 O12 . This scheme is similar to hybrid improper ferroelectricity found in layered perovskites, and can be considered a realisation of hybrid improper ferroelectricity in columnar perovskites. The cation ordering is controlled by annealing temperature and when present it also polarises the local dipoles associated with pseudo-Jahn-Teller active Mn2+ ions to establish an additional ferroelectric order out of an otherwise disordered dipolar glass. Below TN ≈12 K, Mn2+ spins order, making the columnar perovskites rare systems in which ordered electric and magnetic dipoles may reside on the same transition metal sublattice.