Synthesis and symmetry of perovskite oxynitride CaW(O,N)3.

Perovskite oxynitrides, in addition to being promising electrocatalysts and photoabsorbers, present an interesting case study in crystal symmetry. Full or partial ordering of the O and N anions affects global symmetry and influences material performance and functionality; however, anion ordering is challenging to detect experimentally. In this work, we synthesize a novel perovskite oxynitride CaW(O,N)3 and characterize its crystal structure using both X-ray and neutron diffraction. Through co-refinement of the diffraction patterns with a range of literature and theory-derived model structures, we demonstrate that CaW(O,N)3 adopts an orthorhombic Pnma average structure and exhibits octahedral distortion with evidence for preferred anion site occupancy. However, through comparison with a large, low-symmetry unit cell, we identify the presence of disorder that is not fully accounted for by the high-symmetry model. We compare CaW(O,N)3 with SrW(O,N)3 to demonstrate the broader presence of such disorder and identify contrasting features in the electronic structures. This work signifies an updated perspective on the inherent crystal symmetry present in perovskite oxynitrides.

Emergent Magnetism with Continuous Control in the Ultrahigh-Conductivity Layered Oxide PdCoO2.

The current challenge to realizing continuously tunable magnetism lies in our inability to systematically change properties, such as valence, spin, and orbital degrees of freedom, as well as crystallographic geometry. Here, we demonstrate that ferromagnetism can be externally turned on with the application of low-energy helium implantation and can be subsequently erased and returned to the pristine state via annealing. This high level of continuous control is made possible by targeting magnetic metastability in the ultrahigh-conductivity, nonmagnetic layered oxide PdCoO2 where local lattice distortions generated by helium implantation induce the emergence of a net moment on the surrounding transition metal octahedral sites. These highly localized moments communicate through the itinerant metal states, which trigger the onset of percolated long-range ferromagnetism. The ability to continuously tune competing interactions enables tailoring precise magnetic and magnetotransport responses in an ultrahigh-conductivity film and will be critical to applications across spintronics.

Real-space imaging of periodic nanotextures in thin films via phasing of diffraction data.

New properties and exotic quantum phenomena can form due to periodic nanotextures, including Moire patterns, ferroic domains, and topologically protected magnetization and polarization textures. Despite the availability of powerful tools to characterize the atomic crystal structure, the visualization of nanoscale strain-modulated structural motifs remains challenging. Here, we develop nondestructive real-space imaging of periodic lattice distortions in thin epitaxial films and report an emergent periodic nanotexture in a Mott insulator. Specifically, we combine iterative phase retrieval with unsupervised machine learning to invert the diffuse scattering pattern from conventional X-ray reciprocal-space maps into real-space images of crystalline displacements. Our imaging in PbTiO3/SrTiO3 superlattices exhibiting checkerboard strain modulation substantiates published phase-field model calculations. Furthermore, the imaging of biaxially strained Mott insulator Ca2RuO4 reveals a strain-induced nanotexture comprised of nanometer-thin metallic-structure wires separated by nanometer-thin Mott-insulating-structure walls, as confirmed by cryogenic scanning transmission electron microscopy (cryo-STEM). The nanotexture in Ca2RuO4 film is induced by the metal-to-insulator transition and has not been reported in bulk crystals. We expect the phasing of diffuse X-ray scattering from thin crystalline films in combination with cryo-STEM to open a powerful avenue for discovering, visualizing, and quantifying the periodic strain-modulated structures in quantum materials.

Chemical design of electronic and magnetic energy scales of tetravalent praseodymium materials.

Lanthanides in the trivalent oxidation state are typically described using an ionic picture that leads to localized magnetic moments. The hierarchical energy scales associated with trivalent lanthanides produce desirable properties for e.g., molecular magnetism, quantum materials, and quantum transduction. Here, we show that this traditional ionic paradigm breaks down for praseodymium in the tetravalent oxidation state. Synthetic, spectroscopic, and theoretical tools deployed on several solid-state Pr4+-oxides uncover the unusual participation of 4f orbitals in bonding and the anomalous hybridization of the 4f1 configuration with ligand valence electrons, analogous to transition metals. The competition between crystal-field and spin-orbit-coupling interactions fundamentally transforms the spin-orbital magnetism of Pr4+, which departs from the Jeff = 1/2 limit and resembles that of high-valent actinides. Our results show that Pr4+ ions are in a class on their own, where the hierarchy of single-ion energy scales can be tailored to explore new correlated phenomena in quantum materials.

Probing lithium mobility at a solid electrolyte surface.

Solid-state electrolytes overcome many challenges of present-day lithium ion batteries, such as safety hazards and dendrite formation1,2. However, detailed understanding of the involved lithium dynamics is missing due to a lack of in operando measurements with chemical and interfacial specificity. Here we investigate a prototypical solid-state electrolyte using linear and nonlinear extreme-ultraviolet spectroscopies. Leveraging the surface sensitivity of extreme-ultraviolet-second-harmonic-generation spectroscopy, we obtained a direct spectral signature of surface lithium ions, showing a distinct blueshift relative to bulk absorption spectra. First-principles simulations attributed the shift to transitions from the lithium 1 s state to hybridized Li-s/Ti-d orbitals at the surface. Our calculations further suggest a reduction in lithium interfacial mobility due to suppressed low-frequency rattling modes, which is the fundamental origin of the large interfacial resistance in this material. Our findings pave the way for new optimization strategies to develop these electrochemical devices via interfacial engineering of lithium ions.

Revealing Site Occupancy in a Complex Oxide: Terbium Iron Garnet.

Complex oxide films stabilized by epitaxial growth can exhibit large populations of point defects which have important effects on their properties. The site occupancy of pulsed laser-deposited epitaxial terbium iron garnet (TbIG) films with excess terbium (Tb) is analyzed, in which the terbium:iron (Tb:Fe)ratio is 0.86 compared to the stoichiometric value of 0.6. The magnetic properties of the TbIG are sensitive to site occupancy, exhibiting a higher compensation temperature (by 90 K) and a lower Curie temperature (by 40 K) than the bulk Tb3 Fe5 O12 garnet. Data derived from X-ray core-level spectroscopy, magnetometry, and molecular field coefficient modeling are consistent with occupancy of the dodecahedral sites by Tb3+ , the octahedral sites by Fe3+ , Tb3+ and vacancies, and the tetrahedral sites by Fe3+ and vacancies. Energy dispersive X-ray spectroscopy in a scanning transmission electron microscope provides direct evidence of TbFe antisites. A small fraction of Fe2+ is present, and oxygen vacancies are inferred to be present to maintain charge neutrality. Variation of the site occupancies provides a path to considerable manipulation of the magnetic properties of epitaxial iron garnet films and other complex oxides, which readily accommodate stoichiometries not found in their bulk counterparts.

Structural Chirality of Polar Skyrmions Probed by Resonant Elastic X-Ray Scattering.

An escalating challenge in condensed-matter research is the characterization of emergent order-parameter nanostructures such as ferroelectric and ferromagnetic skyrmions. Their small length scales coupled with complex, three-dimensional polarization or spin structures makes them demanding to trace out fully. Resonant elastic x-ray scattering (REXS) has emerged as a technique to study chirality in spin textures such as skyrmions and domain walls. It has, however, been used to a considerably lesser extent to study analogous features in ferroelectrics. Here, we present a framework for modeling REXS from an arbitrary arrangement of charge quadrupole moments, which can be applied to nanostructures in materials such as ferroelectrics. With this, we demonstrate how extended reciprocal space scans using REXS with circularly polarized x rays can probe the three-dimensional structure and chirality of polar skyrmions. Measurements, bolstered by quantitative scattering calculations, show that polar skyrmions of mixed chirality coexist, and that REXS allows valuation of relative fractions of right- and left-handed skyrmions. Our quantitative analysis of the structure and chirality of polar skyrmions highlights the capability of REXS for establishing complex topological structures toward future application exploits.

Large Exchange Coupling Between Localized Spins and Topological Bands in MnBi2 Te4.

Magnetism in topological materials creates phases exhibiting quantized transport phenomena with potential technological applications. The emergence of such phases relies on strong interaction between localized spins and the topological bands, and the consequent formation of an exchange gap. However, this remains experimentally unquantified in intrinsic magnetic topological materials. Here, this interaction is quantified in MnBi2 Te4 , a topological insulator with intrinsic antiferromagnetism. This is achieved by optically exciting Bi-Te p states comprising the bulk topological bands and interrogating the consequent Mn 3d spin dynamics, using a multimodal ultrafast approach. Ultrafast electron scattering and magneto-optic measurements show that the p states demagnetize via electron-phonon scattering at picosecond timescales. Despite being energetically decoupled from the optical excitation, the Mn 3d spins, probed by resonant X-ray scattering, are observed to disorder concurrently with the p spins. Together with atomistic simulations, this reveals that the exchange coupling between localized spins and the topological bands is at least 100 times larger than the superexchange interaction, implying an optimal exchange gap of at least 25 meV in the surface states. By quantifying this exchange coupling, this study validates the materials-by-design strategy of utilizing localized magnetic order to manipulate topological phases, spanning static to ultrafast timescales.

Emergent ferroelectricity in subnanometer binary oxide films on silicon.

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO2) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO2, conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

Chiral structures of electric polarization vectors quantified by X-ray resonant scattering.

Resonant elastic X-ray scattering (REXS) offers a unique tool to investigate solid-state systems providing spatial knowledge from diffraction combined with electronic information through the enhanced absorption process, allowing the probing of magnetic, charge, spin, and orbital degrees of spatial order together with electronic structure. A new promising application of REXS is to elucidate the chiral structure of electrical polarization emergent in a ferroelectric oxide superlattice in which the polarization vectors in the REXS amplitude are implicitly described through an anisotropic tensor corresponding to the quadrupole moment. Here, we present a detailed theoretical framework and analysis to quantitatively analyze the experimental results of Ti L-edge REXS of a polar vortex array formed in a PbTiO3/SrTiO3 superlattice. Based on this theoretical framework, REXS for polar chiral structures can become a useful tool similar to x-ray resonant magnetic scattering (XRMS), enabling a comprehensive study of both electric and magnetic REXS on the chiral structures.

Ultrathin ferroic HfO2-ZrO2 superlattice gate stack for advanced transistors.

With the scaling of lateral dimensions in advanced transistors, an increased gate capacitance is desirable both to retain the control of the gate electrode over the channel and to reduce the operating voltage1. This led to a fundamental change in the gate stack in 2008, the incorporation of high-dielectric-constant HfO2 (ref. 2), which remains the material of choice to date. Here we report HfO2-ZrO2 superlattice heterostructures as a gate stack, stabilized with mixed ferroelectric-antiferroelectric order, directly integrated onto Si transistors, and scaled down to approximately 20 ångströms, the same gate oxide thickness required for high-performance transistors. The overall equivalent oxide thickness in metal-oxide-semiconductor capacitors is equivalent to an effective SiO2 thickness of approximately 6.5 ångströms. Such a low effective oxide thickness and the resulting large capacitance cannot be achieved in conventional HfO2-based high-dielectric-constant gate stacks without scavenging the interfacial SiO2, which has adverse effects on the electron transport and gate leakage current3. Accordingly, our gate stacks, which do not require such scavenging, provide substantially lower leakage current and no mobility degradation. This work demonstrates that ultrathin ferroic HfO2-ZrO2 multilayers, stabilized with competing ferroelectric-antiferroelectric order in the two-nanometre-thickness regime, provide a path towards advanced gate oxide stacks in electronic devices beyond conventional HfO2-based high-dielectric-constant materials.

Designing Magnetism in High Entropy Oxides.

In magnetic systems, spin and exchange disorder can provide access to quantum criticality, frustration, and spin dynamics, but broad tunability of these responses and a deeper understanding of strong limit disorder are lacking. Here, it is demonstrated that high entropy oxides present a previously unexplored route to designing materials in which the presence of strong local compositional disorder may be exploited to generate tunable magnetic behaviors-from macroscopically ordered states to frustration-driven dynamic spin interactions. Single-crystal La(Cr0.2 Mn0.2 Fe0.2 Co0.2 Ni0.2 )O3 films are used as a model system hosting a magnetic sublattice with a high degree of microstate disorder in the form of site-to-site spin and exchange type inhomogeneity. A classical Heisenberg model simplified to represent the highest probability microstates well describes how compositionally disordered systems can paradoxically host magnetic uniformity and demonstrates a path toward continuous control over ordering types and critical temperatures. Model-predicted materials are synthesized and found to possess an incipient quantum critical point when magnetic ordering types are designed to be in direct competition, this leads to highly controllable exchange bias behaviors previously accessible only in intentionally designed bilayer heterojunctions.

Tunable Nanoscale Evolution and Topological Phase Transitions of a Polar Vortex Supercrystal.

Understanding the phase transitions and domain evolutions of mesoscale topological structures in ferroic materials is critical to realizing their potential applications in next-generation high-performance storage devices. Here, the behaviors of a mesoscale supercrystal are studied with 3D nanoscale periodicity and rotational topology phases in a PbTiO3 /SrTiO3 (PTO/STO) superlattice under thermal and electrical stimuli using a combination of phase-field simulations and X-ray diffraction experiments. A phase diagram of temperature versus polar state is constructed, showing the formation of the supercrystal from a mixed vortex and a-twin state and a temperature-dependent erasing process of a supercrystal returning to a classical a-twin structure. Under an in-plane electric field bias at room temperature, the vortex topology of the supercrystal irreversibly transforms to a new type of stripe-like supercrystal. Under an out-of-plane electric field, the vortices inside the supercrystal undergo a topological phase transition to polar skyrmions. These results demonstrate the potential for the on-demand manipulation of polar topology and transformations in supercrystals using electric fields. The findings provide a theoretical understanding that may be utilized to guide the design and control of mesoscale polar structures and to explore novel polar structures in other systems and their topological nature.

Lattice site-dependent metal leaching in perovskites toward a honeycomb-like water oxidation catalyst.

Metal leaching during water oxidation has been typically observed in conjunction with surface reconstruction on perovskite oxide catalysts, but the role of metal leaching at each geometric site has not been distinguished. Here, we manipulate the occurrence and process of surface reconstruction in two model ABO3 perovskites, i.e., SrSc0.5Ir0.5O3 and SrCo0.5Ir0.5O3, which allow us to evaluate the structure and activity evolution step by step. The occurrence and order of leaching of Sr (A-site) and Sc/Co (B-site) were controlled by tailoring the thermodynamic stability of B-site. Sr leaching from A-site mainly generates more electrochemical surface area for the reaction, and additional leaching of Sc/Co from B-site triggers the formation of a honeycomb-like IrOxHy phase with a notable increase in intrinsic activity. A thorough surface reconstruction with dual-site metal leaching induces an activity improvement by approximately two orders of magnitude, which makes the reconstructed SrCo0.5Ir0.5O3 among the best for water oxidation in acid.

Polarization-Resolved Extreme-Ultraviolet Second-Harmonic Generation from LiNbO_{3}.

Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry, as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic order through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hampers the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In noncentrosymmetric LiNbO_{3}, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO_{3}, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO_{6} octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position while the Nb─O bonds are elongated and contracted by displacements of the O atoms. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. Our result constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.

Extreme Ultraviolet Second Harmonic Generation Spectroscopy in a Polar Metal.

The coexistence of ferroelectricity and metallicity seems paradoxical, since the itinerant electrons in metals should screen the long-range dipole interactions necessary for dipole ordering. The recent discovery of the polar metal LiOsO3 was therefore surprising [as discussed earlier in Y. Shi et al., Nat. Mater. 2013, 12, 1024]. It is thought that the coordination preferences of the Li play a key role in stabilizing the LiOsO3 polar metal phase, but an investigation from the combined viewpoints of core-state specificity and symmetry has yet to be done. Here, we apply the novel technique of extreme ultraviolet second harmonic generation (XUV-SHG) and find a sensitivity to the broken inversion symmetry in the polar metal phase of LiOsO3 with an enhanced feature above the Li K-edge that reflects the degree of Li atom displacement as corroborated by density functional theory calculations. These results pave the way for time-resolved probing of symmetry-breaking structural phase transitions on femtosecond time scales with element specificity.

Defect-Driven Oxide Transformations and the Electrochemical Interphase.

ConspectusThe redox reaction pathway is crucial to the sustainable production of the fuels and chemicals required for a carbon-neutral society. Our society is becoming increasingly dependent on devices using batteries and electrolyzers, all of which rely on a series of redox reactions. The overall properties of oxide materials make them very well suited for such electrochemical and catalytic applications due to their associated cationic redox properties and the static site-adsorbate interactions. As these technologies have matured, it has become apparent that defect-driven redox reactions, defect-coupled diffusion, and structural transformations that are both time- and rate-dependent are also critical materials processes. This change in focus, considering not only redox properties but also more complex, dynamic behaviors, represents a new research frontier in the molecular sciences as they are strongly linked to device operation and degradation and lie at the heart of various phenomena that take place at electrochemical interfaces. Fundamental studies of the structural, electronic, and chemical transformation mechanisms are key to the advancement of materials and technological innovations that could be implemented in various electrochemical systems.In this Account, we focus on recent studies and advances in characterizing and understanding the dynamic redox evolution and structural transformations that take place in model perovskites and layered oxides under reactive conditions and correlate them with degradation mechanisms and operations in electrolyzers and batteries. We show that the dynamic evolution of oxygen vacancies and cationic migration in the surface or bulk occurs at the solid-liquid interface, using a combination of different synchrotron-based X-ray spectroscopies and scattering probes. Detailed redox-structure-reactivity correlation studies show how defects and diffusion processes can be tailored to drive various physical and chemical transformations in electrolyzers and batteries. We also highlight a strong correlation between oxygen redox reactivity and structural reorganization in both model thin films and particles, helping to bridge the gap between fundamental studies of the reaction mechanism and device applications. On the basis of these findings, we discuss strategies to probe and tune the redox reactivity and structural stability of the redox-active oxide interphase toward devising efficient pathways for energy and chemical harvesting.

Subterahertz collective dynamics of polar vortices.

The collective dynamics of topological structures1-6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices3. A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.

Reverse-Engineering Strain in Nanocrystallites by Tracking Trimerons.

Although strain underpins the behavior of many transition-oxide-based magnetic nanomaterials, it is elusive to quantify. Since the formation of orbital molecules is sensitive to strain, a metal-insulator transition should be a window into nanocrystallite strain. Using three sizes of differently strained Fe3 O4 polycrystalline nanorods, the impact of strain on the Verwey transition and the associated formation and dissolution processes of quasiparticle trimerons is tracked. In 40 and 50 nm long nanorods, increasing isotropic strain results in Verwey transitions going from TV ≈ 60 K to 20 K. By contrast, 700 nm long nanorods with uniaxial strain along the (110) direction have TV ≈ 150 K-the highest value reported thus far. A metal-insulator transition, like TV in Fe3 O4 , can be used to determine the effective strain within nanocrystallites, thus providing new insights into nanoparticle properties and nanomagnetism.

Amorphization mechanism of SrIrO3 electrocatalyst: How oxygen redox initiates ionic diffusion and structural reorganization.

The use of renewable electricity to prepare materials and fuels from abundant molecules offers a tantalizing opportunity to address concerns over energy and materials sustainability. The oxygen evolution reaction (OER) is integral to nearly all material and fuel electrosyntheses. However, very little is known about the structural evolution of the OER electrocatalyst, especially the amorphous layer that forms from the crystalline structure. Here, we investigate the interfacial transformation of the SrIrO3 OER electrocatalyst. The SrIrO3 amorphization is initiated by the lattice oxygen redox, a step that allows Sr2+ to diffuse and O2- to reorganize the SrIrO3 structure. This activation turns SrIrO3 into a highly disordered Ir octahedral network with Ir square-planar motif. The final Sr y IrO x exhibits a greater degree of disorder than IrO x made from other processing methods. Our results demonstrate that the structural reorganization facilitated by coupled ionic diffusions is essential to the disordered structure of the SrIrO3 electrocatalyst.