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Abstract Noncollinear ferroic materials are sought after as testbeds to explore the intimate connections between topology and symmetry, which result in electronic, optical, and magnetic functionalities not observed in collinear ferroic materials. For example, ferroaxial materials have rotational structural distortions that break mirror symmetry and induce chirality. When ferroaxial order is coupled with ferroelectricity arising from a broken inversion symmetry, it offers the prospect of electric‐field‐control of the ferroaxial distortions and opens up new tunable functionalities. However, chiral multiferroics, especially ones stable at room temperature, are rare. A strain‐stabilized, room‐temperature chiral multiferroic phase in single crystals of BaTiS₃is reported here. Using first‐principles calculations, the stabilization of this multiferroic phase havingP6₃space group for biaxial tensile strains exceeding 1.5% applied on the basalab‐plane of the room temperatureP6₃cmphase of BaTiS₃is predicted. The chiral multiferroic phase is characterized by rotational distortions of TiS₆octahedra around the longc‐axis and polar displacement of Ti atoms along thec‐axis. The ferroaxial and ferroelectric distortions and their domains inP6₃‐BaTiS₃are directly resolved using atomic resolution scanning transmission electron microscopy. Landau‐based phenomenological modeling predicts a strong coupling between the ferroelectric and the ferroaxial order makingP6₃‐BaTiS₃an attractive test bed for achieving electric‐field‐control of chirality. 

Abstract The switching characteristics of ferroelectrics and multiferroics are influenced by the interaction of topological defects with domain walls. We report on the pinning of polarization due to antiphase boundaries in thin films of the multiferroic hexagonal YbFeO₃. We have directly resolved the atomic structure of a sharp antiphase boundary (APB) in YbFeO₃thin films using a combination of aberration-corrected scanning transmission electron microscopy (STEM) and total energy calculations based on density-functional theory (DFT). We find the presence of a layer of FeO₆octahedra at the APB that bridges the adjacent domains. STEM imaging shows a reversal in the direction of polarization on moving across the APB, which DFT calculations confirm is structural in nature as the polarization reversal reduces the distortion of the FeO₆octahedral layer at the APB. Such APBs in hexagonal perovskites are expected to serve as domain-wall pinning sites and hinder ferroelectric switching of the domains. 

Abstract Atomically thin, few‐layered membranes of oxides show unique physical and chemical properties compared to their bulk forms. Manganese oxide (Mn₃O₄) membranes are exfoliated from the naturally occurring mineral Hausmannite and used to make flexible, high‐performance nanogenerators (NGs). An enhanced power density in the membrane NG is observed with the best‐performing device showing a power density of 7.99 mW m⁻²compared to 1.04 µW m⁻²in bulk Mn₃O₄. A sensitivity of 108 mV kPa⁻¹for applied forces <10 N in the membrane NG is observed. The improved performance of these NGs is attributed to enhanced flexoelectric response in a few layers of Mn₃O₄. Using first‐principles calculations, the flexoelectric coefficients of monolayer and bilayer Mn₃O₄are found to be 50–100 times larger than other 2D transition metal dichalcogenides (TMDCs). Using a model based on classical beam theory, an increasing activation of the bending mode with decreasing thickness of the oxide membranes is observed, which in turn leads to a large flexoelectric response. As a proof‐of‐concept, flexible NGs using exfoliated Mn₃O₄membranes are made and used in self‐powered paper‐based devices. This research paves the way for the exploration of few‐layered membranes of other centrosymmetric oxides for application as energy harvesters. 

Abstract It is shown that structural disorder—in the form of anisotropic, picoscale atomic displacements—modulates the refractive index tensor and results in the giant optical anisotropy observed in BaTiS₃, a quasi‐1D hexagonal chalcogenide. Single‐crystal X‐ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS₆chains along thec‐axis, and threefold degenerate Ti displacements in thea–bplane.^47/49Ti solid‐state NMR provides additional evidence for those Ti displacements in the form of a three‐horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. Scanning transmission electron microscopy is used to directly observe the globally disordered Tia–bplane displacements and find them to be ordered locally over a few unit cells. First‐principles calculations show that the Tia–bplane displacements selectively reduce the refractive index along theab‐plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS₃, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity. 

Abstract Materials with large birefringence (Δn, wherenis the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light–matter coupling, such as hyperbolic phonon polaritons. Layered 2D materials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure—Sr_9/8TiS₃—is transparent and positive‐uniaxial, with extraordinary indexn_e= 4.5 and ordinary indexn_o= 2.4 in the mid‐ to far‐infrared. The excess Sr, compared to stoichiometric SrTiS₃, results in the formation of TiS₆trigonal‐prismatic units that break the chains of face‐sharing TiS₆octahedra in SrTiS₃into periodic blocks of five TiS₆octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary indexn_eand result in record birefringence (Δn> 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive‐index modulation. 

Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy (Δn = 2.1) was reported in a quasi-one-dimensional chalcogenide, Sr9/8TiS3. Compared to SrTiS3, the excess Sr in Sr9/8TiS3 leads to periodic structural modulations and introduces additional electrons, which undergo charge ordering on select Ti atoms to form a highly polarizable cloud oriented along the c-axis, hence resulting in the colossal optical anisotropy. Here, further enhancement of the colossal optical anisotropy to Δn = 2.5 in Sr8/7TiS3 is reported through control over the periodicity of the atomic-scale modulations. The role of structural modulations in tuning the optical properties in a series of SrxTiS3 compounds with x = [1, 9/8, 8/7, 6/5, 5/4, 4/3, 3/2] is investigated using density-functional-theory (DFT) calculations. The structural modulations arise from various stacking sequences of face-sharing TiS6 octahedra and twist-distorted trigonal prisms and are found to be thermodynamically stable for 1 < x < 1.5. As x increases, an indirect-to-direct band gap transition is predicted for x ≥ 8/7 along with an increased occupancy of Ti-dz2 states. Together, these two factors result in a theoretically predicted maximum birefringence of Δn = 2.5 for Sr8/7TiS3. Single crystals of Sr8/7TiS3 were grown using a molten-salt flux method. Single-crystal X-ray diffraction measurements confirm the presence of long-range order with a periodicity corresponding to Sr8/7TiS3, which is further corroborated by atomic-scale observations using scanning transmission electron microscopy. Polarization-resolved Fourier-transform infrared spectroscopy of Sr8/7TiS3 crystals shows Δn ≈ 2.5, in excellent agreement with the theoretical predictions. Overall, these findings demonstrate the compositional tunability of optical properties in SrxTiS3 compounds by control over atomic scale modulations and suggest that similar strategies could be extended to other compounds having modulated structures. 

We have revisited the Kittel model that describes antiferroelectricity (AFE) in terms of two sublattices of spontaneous polarization with antiparallel couplings. By constructing a comprehensive phase diagram including the antiferroelectric, ferroelectric, and paraelectric phases in the parameter space, we have identified an AFE phase with stable antipolar states and metastable polar states (SAMP) when the coupling between the two sublattices is weak. We find that the metastability of the polar state in the SAMP AFE phase can lead to apparent ferroelectric behavior, depending on the measurement timescale—for example, the frequency of the applied electric field. This explains the observed ferroelectric behavior of orthorhombic hafnia, which is predicted to be antipolar from density functional theory calculations. 

The atomic structures at epitaxial film–substrate interfaces determine the scalability of thin films and can result in new phenomena. However, it is challenging to control the structure of the interface. In this work, we report the strong tunability of the epitaxial interface of improper ferroelectric hexagonal ferrites deposited on spinel ferrites, achieving the artificial selection of two types of interfaces that are related by a 90° rotation of in-plane epitaxial relations and feature either disordered or hybrid reconstruction. The hybrid-type interface exhibits characteristic structures of both hexagonal ferrites and spinel ferrites, which remove the critical thickness for improper ferroelectricity. This tunable interfacial structure provides critical insight into controlling interfacial clamping to maintain robust improper ferroelectricity at the two-dimensional limit.