Oxygen Nonstoichiometry and Valence State of Manganese in La1-x Ca x MnO3+δ.

Perovskites of the ABO3 type, such as LaMnO3, can be used as air electrodes in solid oxide fuel cells and electrolyzers. Their properties can be tuned by A- and B-site substitutions. The influence of La substitution by Ca on the oxygen nonstoichiometry has been investigated frequently, but the results depend highly on the synthesis and atmospheric conditions. In this work, a series of La1-x Ca x MnO3+δ (x = 0-0.5) was synthesized using conventional solid-state synthesis under an air atmosphere. The structures of the materials were studied in detail with powder X-ray diffraction. The initial oxygen nonstoichiometries were determined using thermogravimetric reduction. The samples were subsequently analyzed in terms of defect chemistry in dependence of temperature, atmosphere, and Ca content via thermogravimetric analysis. The changes in the manganese charge states were investigated by X-ray absorption near-edge spectroscopy experiments. The influence of intrinsic and extrinsic effects on the Mn-valence state of the differently Ca-substituted samples as calculated from thermogravimetric analysis and as determined directly from X-ray absorption near-edge spectroscopy is presented.

Molecular Level Synthesis of InFeO3 and InFeO3/Fe2O3 Nanocomposites.

New heterometallic In-Fe alkoxides [InFe(OtBu)4(PyTFP)2] (1), [InFe2(OneoPen)9(Py)] (2), and [InFe3(OneoPen)12] (3) were synthesized and structurally characterized. The arrangement of metal centers in mixed-metal framework was governed by the In:Fe ratio and the coordination preferences of Fe(III) and In(III) centers to be in tetrahedral and octahedral environments, respectively. 3 displayed a star-shaped so-called "Mitsubishi" motif with the central In atom coordinated with three tetrahedral {Fe(OneoPen)4}- anionic units. The deterministic structural influence of the larger In atom was evident in 1 and 2 which displayed the coordination of neutral coligands to achieve the desired coordination number. Thermal decomposition studies of compounds 1-3 under inert conditions with subsequent powder diffraction studies revealed the formation of Fe2O3 and In2O3 in the case of 3 and 2, whereas 1 intriguingly produced elemental In and Fe. In contrary, the thermal decomposition of 1-3 under ambient conditions produced a ternary oxide, InFeO3, with additional Fe2O3 present as a secondary phase in a different stoichiometric ratio predetermined through the In:Fe ratio in 2 and 3. The intimate mixing of different phases in InFeO3/Fe2O3 nanocomposites was confirmed by transmission electron microscopy of solid residues obtained after the decomposition of 1 and 2. The pure InFeO3 particles demonstrated ferromagnetic anomalies around 170 K as determined by temperature-dependent field-cooled and zero-field-cooled magnetization experiments. A first-order magnetic transition with an increase in the ZFC measurements was explained by temperature-induced reduction of the Fe-Fe distance and the corresponding increase in superexchange.

Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces.

The ability to tailor oxide heterointerfaces has led to novel properties in low-dimensional oxide systems. A fundamental understanding of these properties is based on the concept of electronic charge transfer. However, the electronic properties of oxide heterointerfaces crucially depend on their ionic constitution and defect structure: ionic charges contribute to charge transfer and screening at oxide interfaces, triggering a thermodynamic balance of ionic and electronic structures. Quantitative understanding of the electronic and ionic roles regarding charge-transfer phenomena poses a central challenge. Here, the electronic and ionic structure is simultaneously investigated at the prototypical charge-transfer heterointerface, LaAlO3 /SrTiO3 . Applying in situ photoemission spectroscopy under oxygen ambient, ionic and electronic charge transfer is deconvoluted in response to the oxygen atmosphere at elevated temperatures. In this way, both the rich and variable chemistry of complex oxides and the associated electronic properties are equally embraced. The interfacial electron gas is depleted through an ionic rearrangement in the strontium cation sublattice when oxygen is applied, resulting in an inverse and reversible balance between cation vacancies and electrons, while the mobility of ionic species is found to be considerably enhanced as compared to the bulk. Triggered by these ionic phenomena, the electronic transport and magnetic signature of the heterointerface are significantly altered.

Magnetic Field-Assisted Chemical Vapor Deposition of Iron Oxide Thin Films: Influence of Field-Matter Interactions on Phase Composition and Morphology.

Magnetic field-assisted CVD offers a direct pathway to manipulate the evolution of microstructure, phase composition, and magnetic properties of the as-prepared film. We report on the role of applied magnetic fields (0.5 T) during a cold-wall CVD deposition of iron oxide from [FeIII(OtBu)3]2 leading to higher crystallinity, larger particulates, and better out-of-plane magnetic anisotropy, if compared with zero-field depositions. Whereas selective formation of homogeneous magnetite films was observed for the field-assisted process, coexistence of hematite and amorphous iron(III) oxide was confirmed under zero-field conditions. Comparison of the coercive field (11 vs 60 mT) indicated lower defect concentration for the field-assisted process with nearly superparamagnetic behavior. X-ray photoemission electron microscopy (X-PEEM) in absorption mode at the O-K and Fe-L3,2 edges confirmed the selective formation of magnetite (field-assisted) and hematite (zero-field) with coexisting amorphous phases, respectively, emphasizing the importance of field-matter interactions in the phase-selective synthesis of magnetic thin films.

Volatile Rhenium(I) Compounds with Re-N Bonds and Their Conversion into Oriented Rhenium Nitride Films by Magnetic Field-Assisted Vapor Phase Deposition.

New heteroleptic rhenium(I) compounds, [fac-Re(I)(CO)3(L)] (e.g., L= tfb-dmpda, (N,N-(4,4,4-trifluorobut-1-en-3-on)-dimethyl propylene diamine)), containing anionic and neutral ligands act as efficient precursors to grow polycrystalline rhenium nitride (ReN) films by their vapor phase deposition at 600 °C. Deposition of ReN films under an external magnetic field showed an orientation effect with preferred growth of crystallites along ⟨100⟩ direction. Rhenium complexes reported here unify high stability and reactivity in a single molecule through a Janus-type coordination around a Re center, constituted by a chelating tridentate ligand and three carbonyl groups imparting a facial geometry. Single-crystal diffraction analysis confirmed the structural integrity of the new rhenium compounds. The rigidity of molecular framework was validated in solution via 1D and 2D NMR spectroscopy, in the gas phase via mass spectrometry, and in the solid-state by thermogravimetric analysis and differential scanning calorimetry studies. The analytical data showed that pre-existent Re-N bonds in [fac-Re(I)(CO)3(L)] facilitated low-temperature formation of crystalline ReN deposits confirmed by grazing angle X-ray diffraction analysis. The surface chemical composition and the uniformity of microstructure were provided by X-ray photoelectron spectroscopy (XPS) and scanning and transmission electron microscopy (SEM/TEM), respectively.

Oxygen Exchange Processes between Oxide Memristive Devices and Water Molecules.

Resistive switching based on transition metal oxide memristive devices is suspected to be caused by the electric-field-driven motion and internal redistribution of oxygen vacancies. Deriving the detailed mechanistic picture of the switching process is complicated, however, by the frequently observed influence of the surrounding atmosphere. Specifically, the presence or absence of water vapor in the atmosphere has a strong impact on the switching properties, but the redox reactions between water and the active layer have yet to be clarified. To investigate the role of oxygen and water species during resistive switching in greater detail, isotope labeling experiments in a N2 /H218 O tracer gas atmosphere combined with time-of-flight secondary-ion mass spectrometry are used. It is explicitly demonstrated that during the RESET operation in resistive switching SrTiO3 -based memristive devices, oxygen is incorporated directly from water molecules or oxygen molecules into the active layer. In humid atmospheres, the reaction pathway via water molecules predominates. These findings clearly resolve the role of humidity as both oxidizing agent and source of protonic defects during the RESET operation.

Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy.

Studies of the electrified solid-liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM.

Quantifying redox-induced Schottky barrier variations in memristive devices via in operando spectromicroscopy with graphene electrodes.

The continuing revolutionary success of mobile computing and smart devices calls for the development of novel, cost- and energy-efficient memories. Resistive switching is attractive because of, inter alia, increased switching speed and device density. On electrical stimulus, complex nanoscale redox processes are suspected to induce a resistance change in memristive devices. Quantitative information about these processes, which has been experimentally inaccessible so far, is essential for further advances. Here we use in operando spectromicroscopy to verify that redox reactions drive the resistance change. A remarkable agreement between experimental quantification of the redox state and device simulation reveals that changes in donor concentration by a factor of 2-3 at electrode-oxide interfaces cause a modulation of the effective Schottky barrier and lead to >2 orders of magnitude change in device resistance. These findings allow realistic device simulations, opening a route to less empirical and more predictive design of future memory cells.

Persistent State-of-Charge Heterogeneity in Relaxed, Partially Charged Li1- x Ni1/3 Co1/3 Mn1/3 O2 Secondary Particles.

Ex situ transmission X-ray microscopy reveals micrometer-scale state-of-charge heterogeneity in solid-solution Li1- x Ni1/3 Co1/3 Mn1/3 O2 secondary particles even after extensive relaxation. The heterogeneity generates overcharged domains at the cutoff voltage, which may accelerate capacity fading and increase impedance with extended cycling. It is proposed that optimized secondary structures can minimize the state-of-charge heterogeneity by mitigating the buildup of nonuniform internal stresses associated with volume changes during charge.

Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions.

Surface redox-active centres in transition-metal oxides play a key role in determining the efficacy of electrocatalysts. The extreme sensitivity of surface redox states to temperatures, to gas pressures and to electrochemical reaction conditions renders them difficult to investigate by conventional surface-science techniques. Here we report the direct observation of surface redox processes by surface-sensitive, operando X-ray absorption spectroscopy using thin-film iron and cobalt perovskite oxides as model electrodes for elevated-temperature oxygen incorporation and evolution reactions. In contrast to the conventional view that the transition metal cations are the dominant redox-active centres, we find that the oxygen anions near the surface are a significant redox partner to molecular oxygen due to the strong hybridization between oxygen 2p and transition metal 3d electronic states. We propose that a narrow electronic state of significant oxygen 2p character near the Fermi level exchanges electrons with the oxygen adsorbates. This result highlights the importance of surface anion-redox chemistry in oxygen-deficient transition-metal oxides.

A kinetic study of the decomposition of the cubic perovskite-type oxide Ba(x)Sr(1-x)Co(0.8)Fe(0.2)O(3-delta) (BSCF) (x = 0.1 and 0.5).

The decomposition of the cubic perovskite-type oxide Ba(x)Sr(1-x)Co(0.8)Fe(0.2)O(3-delta) (BSCF) into hexagonal and cubic perovskite-type phases has been examined by means of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED) and X-Ray Diffraction (XRD). SEM and TEM measurements reveal that the new hexagonal phase grows predominantly at the grain boundaries of BSCF ceramics and that the cation composition of the newly formed hexagonal phase differs from that of the starting material. An orientational relationship between the hexagonal and the parent cubic phase was also observed. By means of ex situ XRD the phase fraction of the hexagonal phase was determined as a function of annealing time. A kinetic analysis of the data, based on Avrami-type kinetics, indicates that the decomposition is independent of the initial A-site composition, and the obtained reaction order supports the conclusion that the hexagonal phase grows at the grain boundaries in dense ceramic samples.