Properties Of Transition Metal Oxides Research Articles

Oxide Wizard: An EELS Application to Characterize the White Lines of Transition Metal Edges

Physicochemical properties of transition metal oxides are directly determined by the oxidation state of the metallic cations. To address the increasing need to accurately evaluate the oxidation states of transition metal oxide systems at the nanoscale, here we present "Oxide Wizard." This script for Digital Micrograph characterizes the energy-loss near-edge structure and the position of the transition metal edges in the electron energy-loss spectrum. These characteristics of the edges can be linked to the oxidation states of transition metals with high spatial resolution. The power of the script is demonstrated by mapping manganese oxidation states in Fe3O4/Mn3O4 core/shell nanoparticles with sub-nanometer resolution in real space.

Electron‐Beam‐Induced Perovskite–Brownmillerite–Perovskite Structural Phase Transitions in Epitaxial La2/3Sr1/3MnO3 Films

Structural phase transitions driven by oxygen-vacancy ordering can drastically affect the properties of transition metal oxides. The focused electron beam of a transmission electron microscope (TEM) can be used to control structural phase transitions in epitaxial La2/3Sr1/3MnO3. The ability to induce and characterize oxygen-deficient structural phases simultaneously in a continuous and controllable manner opens up new pathways for atomic-scale studies of transition metal oxides and other complex materials.

Pseudocapacitive oxide materials for high-rate electrochemical energy storage

Electrochemical energy storage technology is based on devices capable of exhibiting high energy density (batteries) or high power density (electrochemical capacitors). There is a growing need, for current and near-future applications, where both high energy and high power densities are required in the same material. Pseudocapacitance, a faradaic process involving surface or near surface redox reactions, offers a means of achieving high energy density at high charge–discharge rates. Here, we focus on the pseudocapacitive properties of transition metal oxides. First, we introduce pseudocapacitance and describe its electrochemical features. Then, we review the most relevant pseudocapacitive materials in aqueous and non-aqueous electrolytes. The major challenges for pseudocapacitive materials along with a future outlook are detailed at the end.

Role of screened exact exchange in accurately describing properties of transition metal oxides: Modeling defects in LaAlO3

The properties of many intrinsic defects in the wide-band-gap semiconductor LaAlO${}_{3}$ are studied using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE) [J. Chem. Phys. 118, 8207 (2003)]. As in pristine structures, exact exchange included in the screened hybrid functional alleviates the band-gap underestimation problem, which is common to semilocal functionals; this allows accurate prediction of defect properties. We propose correction-free defect energy levels for bulk LaAlO${}_{3}$ computed using HSE that might serve as a guide in the interpretation of photoluminescence experiments.

Surface symmetry-breaking and strain effects on orbital occupancy in transition metal perovskite epitaxial films

The electron occupancy of 3d-orbitals determines the properties of transition metal oxides. This can be achieved, for example, through thin-film heterostructure engineering of ABO(3) oxides, enabling emerging properties at interfaces. Interestingly, epitaxial strain may break the degeneracy of 3d-e(g) and t(2g) orbitals, thus favoring a particular orbital filling with consequences for functional properties. Here we disclose the effects of symmetry breaking at free surfaces of ABO(3) perovskite epitaxial films and show that it can be combined with substrate-induced epitaxial strain to tailor at will the electron occupancy of in-plane and out-of-plane surface electronic orbitals. We use X-ray linear dichroism to monitor the relative contributions of surface, strain and atomic terminations to the occupancy of 3z(2)-r(2) and x(2)-y(2) orbitals in La(2/3)Sr(1/3)MnO(3) films. These findings open the possibility of an active tuning of surface electronic and magnetic properties as well as chemical properties (catalytic reactivity, wettability and so on).

Electronic properties of oxides: Chemical and theoretical approaches

Abstract An original analysis of the electronic and chemical properties of oxides is proposed based on the electronegativity χ and the chemical hardness η. This model which has been applied to various oxide based metals, degenerate semiconductors and optical properties of transition metal oxides allows explaining their electronic behaviors: Strong electronegativity and weak chemical hardness characterize oxides of transition elements with high oxidation state. Strong electronegativity and strong chemical hardness feature insulators with a large optical gap. Weak electronegativity and moderate chemical hardness describe alkali and alkaline earth oxides and weak electronegativity and strong chemical hardness are for ionic oxides with a relatively large optical gap. For a few illustrative case studies, ab intio electronic band structure calculations within the density functional theory framework are used.

抵抗変化メモリの動作原理—外部電場下の金属/遷移金属酸化物界面に関する解析評価—

The electronic properties of transition metal oxide (TMO) films in resistance random access memory (ReRAM) devices have been gaining research interest in relation to their relevant application in the field of non-volatile memory devices. TMO films sandwiched between two metallic electrodes in ReRAM devices have the ability to switch its electronic properties between insulator and metal upon being subjected to pulse voltages. However, this switching mechanism has not been fully clarified. As part of this big endeavor to fully elucidate this switching mechanism in ReRAM, we introduce, in this review, the proposed effect of electric field on the metal electrode-TMO interface's electronic properties. We use the Ta/CoO/Ta system in our investigation and evaluated its density of states (DOS) to analyze changes in the bandgap of the interface layers in the presence of an electric field. Analysis shows that the presence of the electric field causes a change in the bandgap of the TMO interface layer near the anode side while no change have been seen at the interface layer near the cathode side. This presumes that the anode side has an important role in switching mechanism as compared with the cathode side.

Electro-forming of vacancy-doped metal-SrTiO3-metal structures

ABSTRACTResistance switching in metal – insulator - metal (MIM) structures with transition metal oxides as the insulator material is a promising concept for upcoming non-volatile memories. The electronic properties of transition metal oxides can be tailored in a wide range by doping and external fields. In this study SrTiO3 single crystals are subjected to high temperature vacuum annealing. The vacuum annealing introduces oxygen vacancies, which act as donor centers. MIM stacks are produced by physical vapor deposition of Au and Ti contacts on the front and rear face of the SrTiO3 crystal. The time dependent forming of the MIM stacks under an external voltage is investigated for crystals with varying bulk conductivities. For continued formation, the resistivity increases up to failure of the system where no current can be measured anymore and switching becomes impossible.

Microwave-Assited Synthesis of RuO2⋅xH2O/Graphene Nanocomposites for Electrochemical Capacitor Applications

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Electrochromic Properties of Nano-Columnar Nickel Oxide

Electrochromic properties of transition metal oxides had much attention in recent years. The electrochromic thin films can be assembly as electrochromic devices (ECDs) and then used for applications in devices such as mirrors, panels and smart windows. A kind of complementary ECD is popular in resent years. Therefore, a specific investigation on nickel oxide (NiO) electrochromic properties is completed in this study. The crystalline structure of the NiO films was analyzed using XRD (PANalytical X’Pert PRO) with Cu-Kα radiation. The atmosphere of oxygen concentration increasing has changed the NiO films crystalline from (200) to (111). The thicknesses and surface microstructures of the NiO films were investigated using a scanning electron microscope (SEM, Philips/FEI XL40 FEG). It is observed that films are relatively smooth deposited without oxygen. The characterization of the electrochromic properties was carried out in a two-electrode cell with an electrochemical analyzer (CHI 611B). The NiOx changes the transmittance of NiO films in the wavelength range of 300-1500 nm and the color of the film changes from transparent to brown. The nano-crack exhibits in the NiO film did enhance the electrochromic properties.

Resonant x-ray scattering in 3d-transition-metal oxides: Anisotropy and charge orderings

The structural, magnetic and electronic properties of transition metal oxides reflect in atomic charge, spin and orbital degrees of freedom. Resonant x-ray scattering (RXS) allows us to perform an accurate investigation of all these electronic degrees. RXS combines high-Q resolution x-ray diffraction with the properties of the resonance providing information similar to that obtained by atomic spectroscopy (element selectivity and a large enhancement of scattering amplitude for this particular element and sensitivity to the symmetry of the electronic levels through the multipole electric transitions). Since electronic states are coupled to the local symmetry, RXS reveals the occurrence of symmetry breaking effects such as lattice distortions, onset of electronic orbital ordering or ordering of electronic charge distributions. We shall discuss the strength of RXS at the K absorption edge of 3d transition-metal oxides by describing various applications in the observation of local anisotropy and charge disproportionation. Examples of these resonant effects are (I) charge ordering transitions in manganites, Fe3O4 and ferrites and (II) forbidden reflections and anisotropy in Mn3+ perovskites, spinel ferrites and cobalt oxides. In all the studied cases, the electronic (charge and/or anisotropy) orderings are determined by the structural distortions.

Synthesis of RuO2·xH2O Nanocomposites using Microwave-Polyol Process

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X-ray spectroscopic techniques are powerful tools for electronic structure investigations of transition metal oxides

Transition metal oxides display uniquely rich physics. Phenomena like superconductivity or colossal magneto resistance are related to collective phase transitions as consequence of a fascinating interplay between the charge, orbital and spin degree of freedom with the crystal lattice. Understanding the underlying electronic properties of transition metal oxides is one major topic in nowadays condensed matter physics. In this paper the investigation of a number of transition metal oxides, with emphasis to ferroelectric and magnetic compounds, by means of different X-ray spectroscopic techniques is presented. X-ray spectroscopic techniques offer unique capabilities for the analysis of spatial distribution of the electron density and chemical bonding. A lot of results shown in this paper are compared to different theoretical electronic structure calculations, i.e. ab initio band structure calculations as well as full multiplet calculations.

Optical properties of Mn1.56Co0.96Ni0.48O4 films studied by spectroscopic ellipsometry

Spectroscopic ellipsometry is used to determine the optical properties of the noncrystalline and crystalline Mn1.56Co0.96Ni0.48O4 (MCN) films prepared by chemical solution deposition method in the range of 250–1100 nm. A pronounced difference in the optical functions is found between 500 and 600 °C. Two distinctive peaks, attributed to the critical points of the MCN films, appear in the optical constant spectra for the films annealed at 600 °C while they are not obvious for the films annealed at 500 °C. The presented results are instructive, further understanding the physical properties of transition metal oxides in theory.

Manifestations of quasi-one-dimensional correlations in the electronic properties of transition-metal oxides

As follows from the string model, the pseudogap anomalies in the electronic properties of lightly doped cuprates, observed by different methods, can be explained by the segregation of charge carriers into bosonic stripes. The comparative investigation of the similar anomalies revealed in several “copperless” transition-metal oxides reliably suggests that these anomalies are likely to be due to the analogous processes of quasione-dimensional structurization of paired charge carriers into a metastable network of conducting nanochannels.

Reversible electron-transfer reactions within a nanoscale metal oxide cage mediated by metallic substrates

Transition metal oxides exhibit a rich collection of electronic properties and have many practical applications in areas such as catalysis and ultra-high-density magnetic data storage. Therefore the development of switchable molecular transition metal oxides has potential for the engineering of single-molecule devices and nanoscale electronics. At present, the electronic properties of transition metal oxides can only be tailored through the irreversible introduction of dopant ions, modifying the electronic structure by either injecting electrons or core holes. Here we show that a molybdenum(VI) oxide 'polyoxometalate' molecular nanocluster containing two embedded redox agents is activated by a metallic surface and can reversibly interconvert between two electronic states. Upon thermal activation two electrons are ejected from the active sulphite anions and delocalized over the metal oxide cluster cage, switching it from a fully oxidized state to a two-electron reduced state along with the concomitant formation of an S-S bonding interaction between the two sulphur centres inside the cluster shell.

Effect of rhodium on the properties of bifunctional MxOy/ZrO2 catalysts in the reduction of nitrogen oxides by hydrocarbons

The catalytic properties of transition metal oxides (Cr, Ce, and Co) supported on ZrO2 synthesized by various methods, as well as the effect of rhodium on the performance of the MxOy/ZrO2 oxide systems in NO reduction with hydrocarbons (methane, propane–butane mixture, and propene) were studied. Scanning electron microscopy, ammonia thermoprogrammed desorption (NH3-TPD), XPS, and IR spectroscopy were used to study the physicochemical indices of rhodium-promoted MxOy/ZrO2 oxide catalysts. The enhancement of the redox properties of the oxide catalysts upon the introduction of rhodium does not alter their bifunctional nature in SCR activity: these catalysts have both redox and strong acid Brønsted-sites.

Molecular and Electronic Structures, Brönsted Basicities, and Lewis Acidities of Group VIB Transition Metal Oxide Clusters

The structures and properties of transition metal oxide (TMO) clusters of the group VIB metals, (MO(3))(n) (M = Cr, Mo, W; n = 1-6), have been studied with density functional theory (DFT) methods. Geometry optimizations and frequency calculations were carried out at the local and nonlocal DFT levels with polarized valence double-zeta quality basis sets, and final energies were calculated at nonlocal DFT levels with polarized valence triple-zeta quality basis sets at the local and nonlocal DFT geometries. Effective core potentials were used to treat the transition metal atoms. Two types of clusters were investigated, the ring and the chain, with the ring being lower in energy. Large ring structures (n > 3) were shown to be fluxional in their out of plane deformations. Long chain structures (n > 3) of (CrO(3))(n) were predicted to be weakly bound complexes of the smaller clusters at the nonlocal DFT levels. For M(6)O(18), two additional isomers were also studied, the cage and the inverted cage. The relative stability of the different conformations of M(6)O(18) depends on the transition metal as well as the level of theory. Normalized and differential clustering energies of the ring structures were calculated and were shown to vary with respect to the cluster size. Brönsted basicities and Lewis acidities based on a fluoride affinity scale were also calculated. The Brönsted basicities as well as the Lewis acidities depend on the size of the cluster and the site to which the proton or the fluoride anion binds. These clusters are fairly weak Brönsted bases with gas phase basicities comparable to those of H(2)O and NH(3). The clusters are, however, very strong Lewis acids and many of them are stronger than strong Lewis acids such as SbF(5). Brönsted acidities of M(6)O(19)H(2) and M(6)O(18)FH were calculated for M = Mo and W and these compounds were shown to be very strong acids in the gas phase. The acid/base properties of these TMO clusters are expected to play important roles in their catalytic activities.

Synchrotron Moessbauer Spectroscopy and Resistivity Studies of Iron Oxide Under High Pressure

AbstractThe strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe57 isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mössbauer spectroscopy (nuclear forward scattering -NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.

Kinetic Methods for Studying the Transport Properties of Transition Metal Oxides and Sulphides

Kinetic Methods for Studying the Transport Properties of Transition Metal Oxides and Sulphides