Frequency- and polarization-resolved photocurrents provide a sensitive probe of hidden symmetry and band geometry in quantum materials. Here we study a chiral cubic sillenite whose global crystal symmetry forbids a longitudinal odd-in-B magneto-photocurrent in the Voigt geometry. Nevertheless, we observe a pronounced longitudinal response across the visible range that is predominantly linear in magnetic field, persists below the band gap, and exhibits strong helicity selectivity, with the circular channel exceeding the linear one and reversing sign upon switching light helicity. We resolve this apparent contradiction by identifying defect-enabled, field-selected spin ordering as the mechanism that lowers the effective magnetic symmetry without altering the global crystal structure. First-principles calculations show that O vacancies generate in-gap bound states and localized magnetic moments on neighboring Bi-O units, stabilized by strong SOC. Although symmetry-related vacancy configurations remain energetically degenerate and preserve the macroscopic T symmetry at zero field, an applied magnetic field selects a time-reversal-broken sector of the defect ensemble and reduces the effective magnetic symmetry to the subgroup that leaves B invariant, thereby lifting the longitudinal selection rule. Importantly, this field-selected symmetry reduction does more than activate a nominally forbidden photocurrent: it unmasks latent quantum-geometric responses encoded in the electronic structure. Momentum-resolved calculations show that the dominant circular and linear magneto-photocurrent channels spatially correlate with Berry-curvature-rich and quantum-metric-rich regions of the Brillouin zone, respectively. Our results establish field-selected defect symmetry lowering as a route to revealing hidden quantum geometry and activating forbidden nonlinear photocurrents in chiral quantum materials.

Abstract Relaxor ferroelectrics are characterized by dispersion of the temperature-dependent dielectric constant with frequency and enhanced electromechanical coupling. These properties arise from the dynamic polar response of correlated nanodomains that are strongly associated with atomic-scale compositional disorder, which disrupts long-range ferroelectric ordering and enables nanodomain formation. Here, we report relaxor properties originating from spontaneous low temperature phase competition in fully cation-ordered antiferroelectric PbMg 0.5 W 0.5 O 3 epitaxial films, including the identification of a new low-energy polar phase. Unlike prototypical relaxors, the B -site cations in coherently strained PbMg 0.5 W 0.5 O 3 films exhibit long-range rocksalt chemical ordering. Temperature-dependent polarization studies reveal the switching behaviors associated with the phase transitions from paraelectric to antiferroelectric to ferroelectric, and the characteristic dielectric relaxation is ascribed instead to phase competition between the polar and antipolar phases mediated by temperature and substrate clamping. This phase competition breaks long-range dipole correlation and leads to dielectric dispersion and relaxor behavior. These findings demonstrate a new paradigm for designing relaxor material properties through engineered phase competition.

Accurate characterization of thermodynamic phases and domain structures is crucial for the engineering of polar-textured thin-film electronic devices. In an improvement to the phase prediction, a Landau-GinzburgDevonshire thermodynamic model of ferroic materials is extended to generalized elastic and electromagnetic boundary conditions. The proposed mathematical framework allows for the implementation of a modular computational model which can analyze composite systems with solid solutions, multiple material layers, and domain structures. The model is applied to epitaxially constrained solid solutions of barium and strontium titanates, with the results illustrating the similarity in the evolution of the single-domain and polydomain ferroelectric phases across a wide range of material compositions. In the applications where barium titanate is placed near a phase boundary, further refinement of the characterization of the boundary can be achieved with the inclusion of the inequality of domain sizes and the tilting of the domain wall within the computational model.

Bismuth ferrite (BiFeO3) thin films possess large ferroelectric polarization and antiferromagnetic order, yet their magnetoelectric coupling is limited by weak intrinsic magnetization. Here, a multiferroic morphotropic phase boundary (MPB) is demonstrated wherein the crystal structure, polarization, and magnetic order simultaneously evolve across a chemically induced phase boundary in strain-engineered (1-x)BiFeO3-(x)BaTiO3 thin films. Between 0.1 < x < 0.2, the crystal structure evolves from a monoclinic phase to a newly stabilized tetragonal phase through an intermediate mixed-phase region. This structural transition is accompanied by concurrent changes in magnetic order, resulting in dramatically enhanced functional responses as compared with those of BiFeO3. Specifically, films with x = 0.2 exhibit larger electromechanical strains (≈ 0.3%, about three-times larger than BiFeO3) and a significantly enhanced magnetoelectric-coupling coefficient (αME ≈ 416 mV cm-1 Oe-1, nearly 1000- and 19-times larger than bulk and thin-film BiFeO3, respectively). These enhancements diminish beyond the MPB (x > 0.2) and arise from polarization rotation and evolving spin configurations driven by the near degeneracy of competing ferroic states at the multiferroic MPB. These results establish a rare multiferroic MPB where both the polar and magnetic order evolve simultaneously, providing a promising strategy for designing materials with strongly coupled ferroic order parameters.

The coexistence of ferroelectric and antiferromagnetic order in BiFeO3 makes it promising for next-generation magnetoelectric devices. But, single-phase multiferroics with robust room-temperature polarization and magnetization are rare. Here, enhanced, room-temperature ferroelectric polarization (≈ 120 µC cm-2), saturation magnetization (≈ 40 emu cm-3), and strong magnetoelectric coupling (≈ 400 mV cm-1 Oe-1) are observed in epitaxial (1-x)BiFeO3-(x)BaTiO3 thin films. These values of magnetization and magnetoelectric coupling are, respectively, one- and two-orders of magnitude larger than those same properties in the widely studied parent material BiFeO3. This sought after combination of properties is found in a distinct tetragonal phase, which is different from rhombohedral and super-tetragonal variants of BiFeO3, that emerges at x = 0.2 to 0.3 via combined chemical substitution and epitaxial strain. Structural and physical-property characterization, along with first-principles calculations, reveal a transition from monoclinic to tetragonal symmetry and suggest that short-range ordering of the titanium in the tetragonal phase results in ferrimagnetic spin ordering. This work demonstrates a unique single-phase multiferroic combining strong polarization, magnetization, and magnetoelectric coupling achieved through manipulation of the coupled chemical order and spin order; thereby addressing a major challenge in multiferroics research and providing a path toward practical room-temperature, efficient charge-to-spin and spin-to-charge conversion technologies.

The pursuit of smaller, energy‐efficient devices drives the exploration of electromechanically active thin films (<1 µm) to enable micro‐ and nano‐electromechanical systems. While the electromechanical response of such films is limited by substrate‐induced mechanical clamping, large electromechanical responses in antiferroelectric and multilayer thin‐film heterostructures have garnered interest. Here, multilayer thin‐film heterostructures based on antiferroelectric PbHfO 3 and ferroelectric PbHf 1‐x Ti x O 3 overcome substrate clamping to produce electromechanical strains >4.5%. By varying the chemistry of the PbHf 1‐x Ti x O 3 layer (x = 0.3‐0.6) it is possible to alter the threshold field for the antiferroelectric‐to‐ferroelectric phase transition, reducing the field required to induce the onset of large electromechanical response. Furthermore, varying the interface density (from 0.008 to 3.1 nm −1 ) enhances the electrical‐breakdown field by >450%. Attaining the electromechanical strains does not necessitate creating a new material with unprecedented piezoelectric coefficients, but developing heterostructures capable of withstanding large fields, thus addressing traditional limitations of thin‐film piezoelectrics.

Nanodielectrics based upon nanoscale Ba(Ti, M-V)O-3, where M = Nb or Ta, were prepared and electrically characterized for their potential use as a high permittivity dielectric layer. Nanocrystals of Ba(Ti, Nb)O-3 (BTNO) and Ba(Ti, Ta)O-3 (BTTO) of average size 20 nm (range 10-50 nm) with a non-centrosymmetric (polarizable) crystal structure were synthesized, dispersed in alcohol solvents and blended with three polymers of known but differing dielectric and electromechanical behavior: Polyvinylpyrrolidone (PVP), Polyfurfuryl alcohol (PFA) and Polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). 0-3 nanoparticle-polymer pressed pellets, films and metal-insulator-metal devices were prepared for electrical characterization. Analysis of the Ba(Ti, M-V)O-3-PVP and Ba(Ti, M-V)O-3 -PFA composites showed a high effective permittivity, low loss, low leakage and voltage tolerance, demonstrating the capability for high energy density capacitance. Effective permittivity, of 52 (BTNO-PFA) and 42 (BTTO-PFA) for pellet nanocomposites and 32 (BTNO-PVP) and 20 (BTNO-PVP) film nanocomposites were observed at 1 MHz respectively. Voltage breakdown strengths of 2133 V/mm (BTNO) and 833 V/mm (BTTO) were demonstrated respectively (threshold 0.1 mu A). Linear and non-linear dielectric behavior was studied by polarization-electric field (P-E) hysteresis measurements. Nanocomposites of BTNO-PVDF-TrFE were prepared to assess the viability of making ferroelectric nanocomposites over a range of polymer-nanoparticle volume fractions.

Switchable order parameters in ferroic materials are essential for functional electronic devices, yet disruptions of the ordering can take the form of planar boundaries or defects that exhibit distinct properties from the bulk, such as electrical (polar) or magnetic (spin) response. Characterizing the structure of these boundaries is challenging due to their confined size and three-dimensional (3D) nature. Here, a chemical antiphase boundary in the highly ordered double perovskite Pb2MgWO6 is investigated using multislice electron ptychography. The boundary is revealed to be inclined along the electron beam direction with a finite width of chemical intermixing. Additionally, regions at and near the boundary exhibit antiferroelectric-like displacements, contrasting with the predominantly paraelectric matrix. Spatial statistics and density functional theory (DFT) calculations further indicate that despite their higher energy, chemical antiphase boundaries (APBs) form due to kinetic constraints during growth, with extended antiferroelectric-like distortions induced by the chemically frustrated environment in the proximity of the boundary. The three-dimensional imaging reveals the interplay between local chemistry and the polar environment, elucidating the role of antiphase boundaries and their associated confined structural distortions and offering opportunities for engineering ferroic thin films.

Ferroelectric nitrides attract immense attention due to their excellent electrical, mechanical, and thermal properties as well as for their compatibility with scalable semiconductor technology. The availability of high-quality nitride films possessing tailorable coercive voltage and field, however, remains challenging, and is a key for deeper exploration of switching dynamics and practical applications in low-power devices. 2D growth of epitaxial thin (≲20 nm) c-axis-oriented Sc0.3Al0.7N films is reported on Al2O3 (0001) and on electrically conductive 4H-SiC (0001), obtained by reflection high-energy electron diffraction-monitored layer-by-layer physical vapor deposition growth. Films exhibit high quality, as evidenced by rocking curve full-width at half-maximum (FWHM) as narrow as ≈0.02°, and an atomically abrupt film-substrate interface with low dislocation density. The coercive field of Sc0.3Al0.7N/4H-SiC (0001) heterostructures is as low as 2.75 MV cm-1. Moreover, a high endurance of >109 cycles at saturation polarization is achieved. Density functional theory calculations of a model system reveal that an improved crystal quality, including atomically abrupt ferroelectric nitride-metal interface, facilitates the reduction in the switching barriers, and leads to reduced coercivity. These findings demonstrate the feasibility of obtaining high-quality epitaxial ferroelectric nitride films on highly scalable and radiation-resistant substrates, and their potential for energy-efficient electronic devices.

Enhanced susceptibilities in ferroelectrics often arise near phase boundaries between competing ground states. While chemically-induced phase boundaries have enabled ultrahigh electrical and electromechanical responses in lead-based ferroelectrics, precise chemical tuning in lead-free alternatives, such as (K,Na)NbO3 thin films, remains challenging due to the high volatility of alkali metals. Here, we demonstrate strain-induced morphotropic phase boundary-like polymorphic nanodomain structures in chemically simple, lead-free, epitaxial NaNbO3 thin films. Combining ab initio simulations, thin-film epitaxy, scanning probe microscopy, synchrotron X-ray diffraction, and electron ptychography, we reveal a labyrinthine structure comprising coexisting monoclinic and bridging triclinic phases near a strain-induced phase boundary. The coexistence of energetically competing phases facilitates field-driven polarization rotation and phase transitions, giving rise to a multi-state polarization switching pathway and large enhancements in dielectric susceptibility and tunability across a broad frequency range. Our results open new possibilities for engineering lead-free thin films with enhanced functionalities for next-generation applications.

MXenes represent one‐of‐a‐kind materials to devise radically novel technologies and achieve breakthroughs in optoelectronics. To exploit their full potential, precise control over the influence of stoichiometry on optical and thermal properties, as well as device performance, must be achieved. Here, the characteristics of optoelectronic devices based on Ti 3 C 2 T x and Ti 2 CT x thin films are uncovered, highlighting the striking difference in their photothermal responses to laser irradiation under different experimental conditions. Even though their absorption coefficients at 450 nm are comparable, the thermal excitation and relaxation phenomena display markedly different kinetics: Ti 2 CT x devices show a strong asymmetry during the heating‐cooling cycle, with the heat dissipation kinetics being three orders of magnitude slower than Ti 3 C 2 T x and strongly influenced by environmental conditions. The findings are expected to stimulate fundamental investigations into the photothermal response of MXenes and open exciting prospects for their use in printed and wearable optoelectronics, including memory devices and neuromorphic computing.

Realization of tunable materials that are multifunctional and maintain high performance in dynamically changing environments is a fundamental goal of science and engineering. Tunable dielectrics form the basis of a wide variety of communication and sensing devices and require breakthrough performance improvement to enable next-generation technologies. Using phenomenological modeling, film growth, and characterization, we show that devices consisting of domain-wall-rich Ba0.8Sr0.2TiO3 films close to a polar-domain-variant phase boundary exhibit colossal dielectric tunability of 100:1 (99%) at a voltage (electric field) of ~15 V (750 kV/cm), resulting in a tunability-quality factor product figure of merit that rises to nearly 105, two orders of magnitude higher than the best previous reported values. Remarkably, varying the amplitude of alternating-current bias enables modulation of this tunability by 50%, owing to domain-wall motion. These results suggest that domain engineering is a powerful approach for achieving excellent modulation of functional properties in ferroelectric films.

Highly responsive, voltage‐tunable dielectrics are essential for microwave‐telecommunication electronics. Ferroelectric/relaxor materials have been leading candidates for such functionality and have exhibited agile dielectric responses. Here, it is demonstrated that relaxor materials developed from antiferroelectrics can achieve both ultrahigh dielectric response and tunability. The system, based on alloying the archetypal antiferroelectric PbZrO 3 with the dielectric BaZrO 3 , exhibits a more complex phase evolution than that in traditional relaxors and is characterized by an unconventional multi‐phase competition between antiferroelectric, ferroelectric, and paraelectric order. This interplay of phases can greatly enhance the local heterogeneities and results in relaxor characteristics while preserving considerable polarizability. Upon studying Pb 1‐ x Ba x ZrO 3 for x = 0‐0.45, Pb 0.65 Ba 0.35 ZrO 3 is found to provide for exceptional dielectric tunability under low bias fields (≈81% at 200 kV cm −1 and ≈91% at 500 kV cm −1 ) at 10 kHz, outcompeting most traditional relaxor ferroelectric films. This high tunability is sustained in the radio‐frequency range, resulting in a high commutation quality factor (>2000 at 1 GHz). This work highlights the phase evolution from antiferroelectrics (with lower, “positive” dielectric tunability) to relaxors (with higher, “negative” tunability), underscoring a promising approach to develop relaxors with enhanced functional capabilities and new possibilities.

Calculation of Raman scattering from molecular dynamics (MD) simulations requires accurate modeling of the evolution of the electronic polarizability of the system along its MD trajectory. For large systems, this necessitates the use of atomistic models to represent the dependence of electronic polarizability on atomic coordinates. The bond polarizability model (BPM) is the simplest such model and has been used for modeling the Raman spectra of molecular systems but has not been applied to solid-state systems. Here, we systematically investigate the accuracy and limitations of the BPM parameterized from the density functional theory results for a series of simple molecules, such as CO2, SO2, H2S, H2O, NH3, and CH4; the more complex CH2O, CH3OH, CH3CH2OH, and thiophene molecules; and the BaTiO3 and CsPbBr3 perovskite solids. We find that BPM can reliably reproduce the overall features of the Raman spectra, such as shifts of peak positions. However, with the exception of highly symmetric systems, the assumption of non-interacting bonds limits the quantitative accuracy of the BPM; this assumption also leads to qualitatively inaccurate polarizability evolution and Raman spectra for systems where large deviations from the ground state structure are present.

The prospect of creating ferroelectric or high permittivity nanomaterials provides motivation for investigating complex transition metal oxides of the form Ba(Ti, MV)O3, where M = Nb or Ta. Solid state processing typically produces mixtures of crystalline phases, rarely beyond minimally doped Nb/Ta. Using a modified sol-gel method, we prepared single phase nanocrystals of Ba(Ti, M)O3. Compositional and elemental analysis puts the empirical formulas close to BaTi0.5Nb0.5O3−δ and BaTi0.5Ta0.5O3−δ. For both materials, a reversible temperature dependent phase transition (non-centrosymmetric to symmetric) is observed in the Raman spectrum in the region 533–583 K (260–310 °C); for Ba(Ti, Nb)O3, the onset is at 543 K (270 °C); and for Ba(Ti, Ta)O3, the onset is at 533 K (260 °C), which are comparable with 390–393 K (117–120 °C) for bulk BaTiO3. The crystal structure was resolved by examination of the powder x-ray diffraction and atomic pair distribution function (PDF) analysis of synchrotron total scattering data. It was postulated whether the structure adopted at the nanoscale was single or double perovskite. Double perovskites (A2B′B″O6) are characterized by the type and extent of cation ordering, which gives rise to higher symmetry crystal structures. PDF analysis was used to examine all likely candidate structures and to look for evidence of higher symmetry. The feasible phase space that evolves includes the ordered double perovskite structure Ba2(Ti, MV)O6 (M = Nb, Ta) Fm-3m, a disordered cubic structure, as a suitable high temperature analog, Ba(Ti, MV)O3Pm-3m, and an orthorhombic Ba(Ti, MV)O3Amm2, a room temperature structure that presents an unusually high level of lattice displacement, possibly due to octahedral tilting, and indication of a highly polarized crystal.

Switchable order parameters in ferroic materials are essential for functional electronic devices, yet disruptions of the ordering can take the form of planar boundaries or defects that exhibit distinct properties. Characterizing the structure of these boundaries is challenging due to their confined size and three-dimensional nature. Here, a chemical anti-phase boundary in the highly ordered double perovskite Pb2MgWO6 is investigated using multislice electron ptychography. The boundary is revealed to be inclined along the electron beam direction with a finite width of chemical intermixing. Additionally, regions at and near the boundary exhibit antiferroelectric-like displacements, contrasting with the predominantly paraelectric matrix. Spatial statistics and density functional theory calculations further indicate that despite their higher energy, chemical anti-phase boundaries form due to kinetic constraints during growth, with extended antiferroelectric-like distortions induced by the chemically frustrated environment in the proximity of the boundary. The three-dimensional imaging provides critical insights into the interplay between local chemistry and the polar environment, elucidating the role of anti-phase boundaries and their associated confined structural distortions and offering new opportunities for engineering ferroic thin films.

Thin-film materials with large electromechanical responses are fundamental enablers of next-generation micro-/nano-electromechanical applications. Conventional electromechanical materials (for example, ferroelectrics and relaxors), however, exhibit severely degraded responses when scaled down to submicrometre-thick films due to substrate constraints (clamping). This limitation is overcome, and substantial electromechanical responses in antiferroelectric thin films are achieved through an unconventional coupling of the field-induced antiferroelectric-to-ferroelectric phase transition and the substrate constraints. A detilting of the oxygen octahedra and lattice-volume expansion in all dimensions are observed commensurate with the phase transition using operando electron microscopy, such that the in-plane clamping further enhances the out-of-plane expansion, as rationalized using first-principles calculations. In turn, a non-traditional thickness scaling is realized wherein an electromechanical strain (1.7%) is produced from a model antiferroelectric PbZrO3 film that is just 100 nm thick. The high performance and understanding of the mechanism provide a promising pathway to develop high-performance micro-/nano-electromechanical systems. Here, the authors observe that in thin films of antiferroelectric PbZrO3, substrate clamping enhances the electromechanical response, with expansion purely in the out-of-plane direction, achieving 1.7% strain for 100-nm-thick films.

Three-dimensional reciprocal space mapping (3D-RSM) offers crucial insights into the intricate microstructural properties of materials, including spatial domain distribution, directional long-range ordering, multilayer-substrate mismatch, layer tilting, and defect structure. Traditionally, 3D-RSMs are conducted at synchrotron facilities where instrumental resolution is constrained in all three directions. Lab-based sources have often been considered suboptimal for 3D-RSM measurements due to poor instrumental resolution along the axial direction. However, we demonstrate that, by employing three-dimensional reciprocal space x-ray computed tomography (RS-XCT), the same perceived limitation in resolution can be effectively leveraged to acquire high quality 3D-RSMs. Through a combination of ultrafast reciprocal space mapping and computed tomography reconstruction routines, lab-based 3D-RSMs achieve resolutions comparable to those obtained with synchrotron-based techniques. RS-XCT introduces a practical modality for lab-based x-ray diffractometers, enabling high-resolution 3D-RSM measurements on a variety of materials exhibiting complex three-dimensional scattering landscapes in reciprocal space.

Antiferroelectrics, which undergo a field-induced phase transition to ferroelectric order that manifests as double-hysteresis polarization switching, exhibit great potential for dielectric, electromechanical, and electrothermal applications. Compared to their ferroelectric cousins, however, considerably fewer efforts have been made to understand and control antiferroelectrics. Here, it is demonstrated that the polarization switching behavior of an antiferroelectric can be strongly influenced and effectively regulated by point defects. In films of the canonical antiferroelectric PbZrO3 , decreasing oxygen pressure during deposition (and thus increasing adatom kinetic energy) causes unexpected "ferroelectric-like" polarization switching although the films remain in the expected antiferroelectric orthorhombic phase. This "ferroelectric-like" switching is correlated with the creation of bombardment-induced point-defect complexes which pin the antiferroelectric-ferroelectric phase boundaries, and thus effectively delay the phase transition under changing field. The effective pinning energy is extracted via temperature-dependent switching-kinetics studies. In turn, by controlling the concentration of defect complexes, the dielectric tunability of the PbZrO3 can be adjusted, including being able to convert between "positive" and "negative" tunability near zero field. This work reveals the important role and strong capability of defects to engineer antiferroelectrics for new performance and functionalities.