Oxidation Kinetics of Nanocrystalline Hexagonal RMn1-xTixO3 (R = Ho, Dy).

Hexagonal manganites, RMnO3 (R = Sc, Y, Ho-Lu), are potential oxygen storage materials for air separation due to their reversible oxygen storage and release properties. Their outstanding ability to absorb and release oxygen at relatively low temperatures of 250-400 °C holds promise of saving energy compared to current industrial methods. Unfortunately, the low temperature of operation also implies slow kinetics of oxygen exchange in these materials, which would make them inefficient in applications such as chemical looping air separation. Here, we show that the oxidation kinetics of RMnO3 can be improved through Ti4+-doping as well as by increasing the rare earth cation size. The rate of oxygen absorption of nanocrystalline RMn1-xTixO3 (R = Ho, Dy; x = 0, 0.15) was investigated by thermogravimetric analysis, X-ray absorption near-edge structure, and high-temperature X-ray diffraction (HT-XRD) with in situ switching of atmosphere from N2 to O2. The kinetics of oxidation increases for larger R and even more with Ti4+ donor doping, as both induce expansion of the ab-plane, which reduces the electrostatic repulsion between oxygen in the lattice upon oxygen ion migration. Surface exchange rates and activation energies of oxidation were determined from changes in lattice parameters observed through HT-XRD upon in situ switching of atmosphere.

Electronic Structure and Surface Chemistry of Hexagonal Boron Nitride on HOPG and Nickel Substrates.

The effect of point defects and interactions with the substrate are shown by density functional theory calculations to be of significant importance for the structure and functional properties of hexagonal boron nitride (h-BN) films on highly ordered pyrolytic graphite (HOPG) and Ni(111) substrates. The structure, surface chemistry, and electronic properties are calculated for h-BN systems with selected intrinsic, oxygen, and carbon defects and with graphene hybrid structures. The electronic structure of a pristine monolayer of h-BN is dependent on the type of substrate, as h-BN is decoupled electronically from the HOPG surface and acts as bulk-like h-BN, whereas on a Ni(111) substrate, metallic-like behavior is predicted. These different film/substrate systems therefore show different reactivities and defect chemistries. The formation energies for substitutional defects are significantly lower than for intrinsic defects regardless of the substrate, and vacancies formed during film deposition are expected to be filled by either ambient oxygen or carbon from impurities. Significantly lower formation energies for intrinsic and oxygen and carbon substitutional defects were predicted for h-BN on Ni(111). In-plane h-BCN hybrid structures were predicted to be terminated by N-C bonding. Substitutional carbon on the boron site imposes n-type semiconductivity in h-BN, and the n-type character increases significantly for h-BN on HOPG. The h-BN film surface becomes electronically decoupled from the substrate when exceeding monolayer thickness, showing that the surface electronic properties and point defect chemistry for multilayer h-BN films should be comparable to those of a freestanding h-BN layer.

Confinement-Driven Inverse Domain Scaling in Polycrystalline ErMnO3.

The research on topological phenomena in ferroelectric materials has revolutionized the way people understand polar order. Intriguing examples are polar skyrmions, vortex/anti-vortex structures, and ferroelectric incommensurabilties, which promote emergent physical properties ranging from electric-field-controllable chirality to negative capacitance effects. Here, the impact of topologically protected vortices on the domain formation in improper ferroelectric ErMnO3 polycrystals is studied, demonstrating inverted domain scaling behavior compared to classical ferroelectrics. It is observed that as the grain size increases, smaller domains are formed. Phase field simulations reveal that elastic strain fields drive the annihilation of vortex/anti-vortex pairs within the grains and individual vortices at the grain boundaries. The inversion of the domain scaling behavior has far-reaching implications, providing fundamentally new opportunities for topology-based domain engineering and the tuning of the electromechanical and dielectric performance of ferroelectrics in general.

Removing Fluoride-Terminations from Multilayered V2CT x MXene by Gas Hydrolyzation.

Two-dimensional MXenes have shown great promise for many different applications, but in order to fully utilize their potential, control of their termination groups is essential. Here we demonstrate hydrolyzation with a continuous gas flow as a method to remove F-terminations from multilayered V2CT x particles, in order to prepare nearly F-free and partly bare vanadium carbide MXene. Density functional theory calculations demonstrate that the substitution of F-terminations is thermodynamically feasible and presents partly nonterminated V2CO as the dominating hydrolyzation product. Hydrolyzation at elevated temperatures reduced the F content but only subtly changed the O content, as inferred from spectroscopic data. The ideal hydrolyzation temperature was found to be 300 °C, as a degradation of the V2CT x phase and a transition to vanadium oxycarbides and V2O3 were observed at higher temperature. When tested as electrodes in Li-ion batteries, the hydrolyzed MXene demonstrated a reduced polarization compared with the pristine MXene, but no change in intercalation voltage was observed. Annealing in dry Ar did not result in the same F reduction, and the importance of water vapor was concluded, demonstrating hydrolyzation as a new and efficient method to control the surface terminations of multilayered V2CT x post etching. These results also provide new insights on the thermal stability of V2CT x MXene in hydrated atmospheres.

On the Durability of Protective Titania Coatings on High-Voltage Spinel Cathodes.

TiO2 -coating of LiNi0.5-x Mn1.5+x O4 (LNMO) by atomic layer deposition (ALD) has been studied as a strategy to stabilize the cathode/electrolyte interface and mitigate transition metal (TM) ion dissolution. The TiO2 coatings were found to be uniform, with thicknesses estimated to 0.2, 0.3, and 0.6 nm for the LNMO powders exposed to 5, 10, and 20 ALD cycles, respectively. While electrochemical characterization in half-cells revealed little to no improvement in the capacity retention neither at 20 nor at 50 °C, improved capacity retention and coulombic efficiencies were demonstrated for the TiO2 -coated LNMO in LNMO||graphite full-cells at 20 °C. This improvement in cycling stability could partly be attributed to thinner cathode electrolyte interphase on the TiO2 -coated samples. Additionally, energy-dispersive X-ray spectroscopy revealed a thinner solid electrolyte interphase on the graphite electrode cycled against TiO2 -coated LNMO, indicating retardation of TM dissolution by the TiO2 -coating.

Limitations of Ultrathin Al2O3 Coatings on LNMO Cathodes.

This study demonstrates the application of Al2O3 coatings for the high-voltage cathode material LiNi0.5-x Mn1.5+x O4-δ (LNMO) by atomic layer deposition. The ultrathin and uniform coatings (0.6-1.7 nm) were deposited on LNMO particles and characterized by scanning transmission electron microscopy, inductively coupled plasma mass spectrometry, and X-ray photoelectron spectroscopy. Galvanostatic charge discharge cycling in half cells revealed, in contrast to many published studies, that even coatings of a thickness of 1 nm were detrimental to the cycling performance of LNMO. The complete coverage of the LNMO particles by the Al2O3 coating can form a Li-ion diffusion barrier, which leads to high overpotentials and reduced reversible capacity. Several reports on Al2O3-coated LNMO using alternative coating methods, which would lead to a less homogeneous coating, revealed the superior electrochemical properties of the Al2O3-coated LNMO, suggesting that complete coverage of the particles might in fact be a disadvantage. We show that transition metal ion dissolution during prolonged cycling at 50 °C is not hindered by the coating, resulting in Ni and Mn deposits on the Li counter electrode. The Al2O3-coated LNMO particles showed severe signs of pitting dissolution, which may be attributed to HF attack caused by side reactions between the electrolyte and the Al2O3 coating, which can lead to additional HF formation. The pitting dissolution was most severe for the thickest coating (1.7 nm). The uniform coating coverage may lead to non-uniform conduction paths for Li, where the active sites are more susceptible to HF attack. Few benefits of applications of very thin, uniform, and amorphous Al2O3 coatings could thus be verified, and the coating is not offering long-term protection from HF attack.

Hydrothermal synthesis of hexagonal YMnO3 and YbMnO3 below 250 °C.

The hydrothermal synthesis of hexagonal YMnO3 and YbMnO3 are reported using high KOH mineraliser concentrations (>10 M) and low temperatures (<240 °C). The relation between reaction parameters and resulting phase purity were mapped by ex situ and in situ X-ray diffraction. Excess Y2O3 resulted in two-phase product with hexagonal YMnO3 with different lattice parameters. An unusual microstructure was observed in which particles have a hexagonal shape with a highly crystalline edge and either a hollow or polycrystalline interior. An Ostwald ripening mechanism was proposed to explain this phenomenon. Solid-state reactions and density functional theory calculations were performed to determine plausible defect chemistry which can lead to the observed phases with different lattice parameters.

Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide.

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena.

Publisher Correction: Conductivity control via minimally invasive anti-Frenkel defects in a functional oxide.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Conductivity control via minimally invasive anti-Frenkel defects in a functional oxide.

Utilizing quantum effects in complex oxides, such as magnetism, multiferroicity and superconductivity, requires atomic-level control of the material's structure and composition. In contrast, the continuous conductivity changes that enable artificial oxide-based synapses and multiconfigurational devices are driven by redox reactions and domain reconfigurations, which entail long-range ionic migration and changes in stoichiometry or structure. Although both concepts hold great technological potential, combined applications seem difficult due to the mutually exclusive requirements. Here we demonstrate a route to overcome this limitation by controlling the conductivity in the functional oxide hexagonal Er(Mn,Ti)O3 by using conductive atomic force microscopy to generate electric-field induced anti-Frenkel defects, that is, charge-neutral interstitial-vacancy pairs. These defects are generated with nanoscale spatial precision to locally enhance the electronic hopping conductivity by orders of magnitude without disturbing the ferroelectric order. We explain the non-volatile effects using density functional theory and discuss its universality, suggesting an alternative dimension to functional oxides and the development of multifunctional devices for next-generation nanotechnology.

A Fast, Low-Temperature Synthesis Method for Hexagonal YMnO3 : Kinetics, Purity, Size and Shape as Studied by In Situ X-ray Diffraction.

The reaction mechanisms, phase development and kinetics of the hydrothermal synthesis of hexagonal-YMnO3 from Y2 O3 and Mn2 O3 using in situ X-ray diffraction are reported under different reaction conditions with temperatures ranging from 300 to 350 °C, and using 1, 5 and 10 m KOH, and 5 m NaOH mineraliser. Reactions initiated with Y2 O3 hydrating to Y(OH)3 , which then dehydrated to YO(OH). Higher temperatures and KOH concentrations led to faster, more complete dehydrations. However, 1 m KOH led to YO(OH) forming concurrently with Y(OH)3 before Y(OH)3 fully dehydrated but yielded a very low phase purity of hexagonal-YMnO3 . Using NaOH mineraliser, no YO(OH) was observed. Dehydration also initiated at a higher temperature in the absence of Mn2 O3 . The evolution of Rietveld refined scale factors was used to determine kinetic information and approximate activation energies for the reaction. The described hydrothermal synthesis offers a fast, low-temperature method for producing anisometric h-YMnO3 particles.

Crystal Structure Influences Migration along Li and Mg Surfaces.

Dendrite formation on Li metal anodes hinders commercialization of more energy-dense rechargeable batteries. Here, we use the migration energy barrier (MEB) for surface transport as a descriptor for dendrite nucleation and compare Li to Mg. Density functional theory calculations show that the MEB for the hexagonal close-packed structure is 40 and 270 meV lower than that of the body-centered cubic structure for Li and Mg, respectively. This is suggested as a reason why Mg surfaces are less prone to form dendrites than Li. We show that the close-packed facets exhibit lower MEBs because of smaller changes in atomic coordination during migration and thereby less surface distortion.

Controlled Growth of Srx Ba1-x Nb2 O6 Hopper- and Cube-Shaped Nanostructures by Hydrothermal Synthesis.

Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. In this work, the ferroelectric material Srx Ba1-x Nb2 O6 (x=0.32-0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube-shaped to hollow-ended structures based on a fundamental understanding of the precursor chemistry. Synchrotron X-ray total scattering and PDF analysis was used to reveal the structure of the Nb-acid precursor, showing Lindqvist-like motifs. The changing growth mechanism, from layer-by-layer growth forming cubes to hopper-growth giving hollow-ended structures, is attributed to differences in supersaturation. Transmission electron microscopy revealed an inhomogeneous composition along the length of the hollow-ended particles, which is explained by preferential formation of the high entropy composition, SBN33, at the initial stages of particle nucleation and growth.

The Effect of Cation Disorder on Ferroelectric Properties of SrxBa1-xNb2O6 Tungsten Bronzes.

The versatile crystal structure of tetragonal tungsten bronzes (A12A24C4B10O30) can accommodate complex stoichiometries including cations in different valence states and vacant cation sites. Here, we report on the effect of thermally induced cation-vacancy disorder in the tetragonal tungsten bronze SrxBa1-xNb2O6 (SBNX). SBNX (x = 0.25, 0.33, 0.50, 0.61) ceramics, prepared by conventional solid-state synthesis, were annealed at varying temperatures and subsequently quenched to room temperature. The Curie temperature of all the SBNX materials increased with higher quenching temperatures, accompanied with ferroelectric hardening. The variation in thermal history also caused structural changes, specifically a contraction of the a lattice parameter and a minor elongation of the c parameter. These effects are discussed in relation to recent first principles calculations of the energy landscape of the cation-vacancy configurations and experimental evidence of thermally induced cation-vacancy disordering.

Electrical half-wave rectification at ferroelectric domain walls.

Domain walls in ferroelectric semiconductors show promise as multifunctional two-dimensional elements for next-generation nanotechnology. Electric fields, for example, can control the direct-current resistance and reversibly switch between insulating and conductive domain-wall states, enabling elementary electronic devices such as gates and transistors. To facilitate electrical signal processing and transformation at the domain-wall level, however, an expansion into the realm of alternating-current technology is required. Here, we demonstrate diode-like alternating-to-direct current conversion based on neutral ferroelectric domain walls in ErMnO3. By combining scanning probe and dielectric spectroscopy, we show that the rectification occurs at the tip-wall contact for frequencies at which the walls are effectively pinned. Using density functional theory, we attribute the responsible transport behaviour at the neutral walls to an accumulation of oxygen defects. The practical frequency regime and magnitude of the direct current output are controlled by the bulk conductivity, establishing electrode-wall junctions as versatile atomic-scale diodes.

High Interfacial Charge Storage Capability of Carbonaceous Cathodes for Mg Batteries.

A rechargeable Mg battery where the capacity mainly originates from reversible reactions occurring at the electrode/electrolyte interface efficiently avoids the challenge of sluggish Mg intercalation encountered in conventional Mg batteries. The interfacial reactions in a cell based on microwave-exfoliated graphite oxide (MEGO) as the cathode and all phenyl complex (APC) as electrolyte are identified by quantitative kinetics analysis as a combination of diffusion-controlled reactions involving ether solvents ( esols) and capacitive processes. During magnesiation, esols in APC electrolytes can significantly affect the electrochemical reactions and charge transfer resistances at the electrode/electrolyte interface and thus govern the charge storage properties of the MEGO cathode. In APC-tetrahydrofuran (THF) electrolyte, MEGO exhibits a reversible capacity of ∼220 mAh g-1 at 10 mA g-1, while a reversible capacity of ∼750 mAh g-1 at 10 mA g-1 was obtained in APC-1,2-dimethoxyethane (DME) electrolyte. The high capacity improvement not only points to the important role of the esols in the APC electrolytes but also presents a Mg battery with high interfacial charge storage capability as a very promising and viable competitor to the conventional intercalation-based batteries.

Spatially Confined Spin Polarization and magnetic sublattice control in (La,Sr)MnO3-δ Thin Films by Oxygen Vacancy Ordering.

Perovskite oxides are known for their strong structure property coupling and functional properties such as ferromagntism, ferroelectricity and high temperature superconductivity. While the effect of ordered cation vacancies on functional properties have been much studied, the possibility of tuning the functionality through anion vacancy ordering has received much less attention. Oxygen vacancies in ferromagnetic La0.7Sr0.3MnO3-δ thin films have recently been shown to accumulate close to interfaces and form a brownmillerite structure (ABO2.5). This structure has alternating oxygen octahedral and tetrahedral layers along the stacking direction, making it a basis for a family of ordered anion defect controlled materials. We use density functional theory to study how structure and properties depend on oxygen stoichiometry, relying on a block-by-block approach by including additional octahedral layers in-between each tetrahedral layer. It is found that the magnetic and electronic structures follow the layers enforced by the ordered oxygen vacancies. This results in spatially confined electronic conduction in the octahedral layers, and decoupling of the magnetic sub-lattices in the octahedral and tetrahedral layers. These results demonstrate that anion defect engineering is a promising tool to tune the properties of functional oxides, adding a new avenue for developing functional oxide device technology.

Interstitial oxygen as a source of p-type conductivity in hexagonal manganites.

Hexagonal manganites, h-RMnO3 (R=Sc, Y, Ho-Lu), have been intensively studied for their multiferroic properties, magnetoelectric coupling, topological defects and electrically conducting domain walls. Although point defects strongly affect the conductivity of transition metal oxides, the defect chemistry of h-RMnO3 has received little attention. We use a combination of experiments and first principles electronic structure calculations to elucidate the effect of interstitial oxygen anions, Oi, on the electrical and structural properties of h-YMnO3. Enthalpy stabilized interstitial oxygen anions are shown to be the main source of p-type electronic conductivity, without reducing the spontaneous ferroelectric polarization. A low energy barrier interstitialcy mechanism is inferred from Density Functional Theory calculations to be the microscopic migration path of Oi. Since the Oi content governs the concentration of charge carrier holes, controlling the thermal and atmospheric history provides a simple and fully reversible way of tuning the electrical properties of h-RMnO3.

Oxygen Incorporation and Release in Metastable Bixbyite V2O3 Nanocrystals.

A new, metastable polymorph of V2O3 with a bixbyite structure was recently stabilized in colloidal nanocrystal form. Here, we report the reversible incorporation of oxygen in this material, which can be controlled by varying temperature and oxygen partial pressure. Based on X-ray diffraction (XRD) and thermogravimetric analysis, we find that oxygen occupies interstitial sites in the bixbyite lattice. Two oxygen atoms per unit cell can be incorporated rapidly and with minimal changes to the structure while the addition of three or more oxygen atoms destabilizes the structure, resulting in a phase change that can be reversed upon oxygen removal. Density functional theory (DFT) supports the reversible occupation of interstitial sites in bixbyite by oxygen, and the 1.1 eV barrier to oxygen diffusion predicted by DFT matches the activation energy of the oxidation process derived from observations by in situ XRD. The observed rapid oxidation kinetics are thus facilitated by short diffusion paths through the bixbyite nanocrystals. Due to the exceptionally low temperatures of oxidation and reduction, this earth-abundant material is proposed for use in oxygen storage applications.