Electronic Structure Of Oxides Research Articles

Structure and electronic properties of δ-Bi2O3 tuned by vacancy and doping: A first-principles study

Pure Bi2O3 with high ionic conductivities is considered as a candidate material for an electrolyte in solid oxide fuel cells and oxygen separation membranes. However, its lower structural and thermal stability prevent it application in ion conductivity and photocatalysis at suitable temperatures. Metal oxides are usually used to stabilize its structure to lower temperatures and the underlying mechanism is still unclear. To shed light on the issue, vacancy ordered structures of pure and doped δ-Bi2O3 have been studied by first-principles calculations. It have been shown that the structure with combined <110> and <111> vacancy arrangements is energetically favorable compared to either <100>, <110> or <111> vacancy ordered structures. Electronic structure analyses have further verified that δ-Bi2O3 has a semiconductor character with an energy gap of 2.0 eV, consistent with the experiment results. The site occupation of doping ions is further analyzed by formation energy, geometry and electronic structures. It is evident that the substitution sites of doping ions depend on the type of the doping ions. The ions with large ion sizes tend to occupy the Bi(2) sites while the ions with small ion sizes tend to occupy the Bi(1) sites. At the same time, the probability of the Y ions occupying the oxygen vacancy sites and the optical properties of the Y-doped Bi2O3 are explored. Our investigations reveal that the electronic structure of oxides could be tuned by vacancy and interstitial defects for better conductivity, photocatalytic properties.

The influence of A- and B-cation substitution on electronic structure of SrFeO3 and SrCoO3

Oxides with perovskite structure SrFeO3 and SrCoO3 are ancestors of wide range of technologically important substances, used as oxide ionic conductors. Previously we use the substitution of the cations in the B positions by high-charge ions Mo6+ W6+ Ta5+ Nb5+ to modify the properties of this class of oxygen-deficient oxides. Doping with these elements suppressed phase transitions in oxides based on cobalt-ferrite strontium-barium, reduced the size of nanodomains and rise chemical and structure stability of these oxides. Now we performed the theoretical study of the influence of A- and B- substitution on electronic structure of SrFeO3 and SrCoO3 in the order to clarify how the doping by high-charge cations affects the properties and electronic structure of oxides.

Structural and electronic properties of monolayer group III monochalcogenides

We investigate the structural, mechanical, and electronic properties of the two-dimensional hexagonal structure of group III-VI binary monolayers, $MX$ ($M=\text{B}$, Al, Ga, In and $X=\text{O}$, S, Se, Te) using first-principles calculations based on the density functional theory. The structural optimization calculations and phonon spectrum analysis indicate that all of the 16 possible binary compounds are thermally stable. In-plane stiffness values cover a range depending on the element types and can be as high as that of graphene, while the calculated bending rigidity is found to be an order of magnitude higher than that of graphene. The obtained electronic band structures show that $MX$ monolayers are indirect band-gap semiconductors. The calculated band gaps span a wide optical spectrum from deep ultraviolet to near infrared. The electronic structure of oxides ($M\mathrm{O}$) is different from the rest because of the high electronegativity of oxygen atoms. The dispersions of the electronic band edges and the nature of bonding between atoms can also be correlated with electronegativities of constituent elements. The unique characteristics of group III-VI binary monolayers can be suitable for high-performance device applications in nanoelectronics and optics.

A copper oxide's electronic structure

Physics![Figure][1] Superconducting copper-oxide electronic structurePHOTO: NICOLLE R. FULLER Physicists still do not understand why some copper-oxide compounds become superconducting at relatively high temperatures. Even more basic issues have remained controversial as well, such as the electronic structure of the normal (i.e., nonsuperconducting) state from which superconductivity emerges. Sebastian et al. chose YBa2Cu3O6.56 as a good representative of a subclass of these compounds and used high magnetic fields to suppress its superconductivity and reach its normal state. They analyzed the wiggles of the electrical resistivity to map out the shape of the so-called Fermi surface, which separates the quantum states filled with electrons from the empty ones. They found an undulating shape that suggested that the electronic structure was organized differently from what one expects based on the crystal lattice structure alone. Nature 10.1038/nature13326 (2014). [1]: pending:yes

Adjusting the Electronic and Optical Properties of Mesoporous MCM-41 Materials by Ti Doping

The objectives of this thesis are to understand the electronic structures of oxides and oxynitrides for photocatalytic water splitting, examine the Casimir interaction between oxides, and explore possible approach to bridge the Casimir force and material properties for advanced material research. The studies were performed in the framework of the density functional theory, many-body perturbation theory, i.e, the GW approximation and Bethe-Salpeter equation, as well as the Casimir-Lifshitz approach.The thesis consists of two sets of results. In the first part (papers I-VI), the electronic structures of oxynitrides, i.e., ZnO-GaN and ZnO-InN, with different compositions and local structures have been studied. The oxynitrides reduce the band-gap energies significantly compared to the binary counterparts, enabling the oxynitrides to act as visible light active photocatalysts. Formation of cluster--like structures further reduces the band-gap and delocalizes the valence bands, benefiting higher optical absorption. Furthermore, the energy levels between oxynitride and water were aligned using a surface model adapted from semiconductor heterostructure.In the second part (papers V-IX), the electronic structures of oxides as well as the Casimir interactions have been examined. In particular, we investigated the differences of optical and electronic properties between SnO2 and TiO2 polymorphs in terms of band-edge characters and electron-phonon coupling. In addition, we synthesized a mesoporous material possessing two types of pore structures (one is hexagonal ordered with pore diameter of 2.60 nm and the other is disordered with pore diameter of 3.85 nm). The pore framework contains four-coordinated titanium and oxygen vacancies, verified by both experimental measurements and density-functional theory calculations. Utilizing the predicted properties of the materials, we studied the Casimir interactions. A stable equilibrium of Casimir force is achieved in planar geometry containing a thin film and porous substrates. Both the force and equilibrium distance are tuned through modification of the material properties, for instance, optical properties and porosity. Furthermore, we adapted this concept to study the interactions between gas bubbles and porous SiO2 in water. A transition from repulsion to attraction is predicted, which highlights that the bubbles may interact differently at different surface regions.

Features of atomic and electronic structure of oxides on porous silicon surface according to XANES data

Based on synchrotron research of the fine structure main parameters of SiL2, 3 X-ray absorption edges (X-ray absorption near edge structure (XANES)) in porous silicon on boron-doped Si(100) wafers, the thickness of the surface oxide layer and the degree of distortions of the silicon-oxygen tetrahedron in this layer were estimated. The thickness of the oxide layer formed on the amorphous layer coating nanocrystals of porous silicon exceeds the thickness of the native oxide on the surface of Si(100) : P and Si(100) : B single-crystal (100) silicon wafers by several times. Distortion of the silicon-oxygen tetrahedron, i.e., the basic unit of silicon oxide, is accompanied by Si-O bond stretching and an increase in the angle between Si-O-Si bonds.

Electronic structure of oxide, peroxide, and superoxide clusters of the 3d elements: A comparative density functional study

The 3d-element transition metal dioxide MO(2), peroxide M(O(2)), and superoxide MOO clusters (M=Sc-Zn), are studied by density functional theory with the B1LYP functional. The reliability of the methods and basis sets employed was tested by a reinvestigation of the monoxides, for which a database of experimental data is available. The global minima on the M+O(2) potential energy surfaces correspond to dioxide structure, the only exception being CuOO, with a superoxide structure. All Zn dioxygen clusters are thermodynamically unstable-their ground states lie higher than the dissociation limit to Zn+O(2). Our calculations are in favor of the high-spin configurations for the FeO(2), CoO(2), and NiO(2) ground states, which are still a subject of extensive theoretical and experimental studies. These assignments are confirmed by the coupled-cluster method, CCSD(T), except for NiO(2). Based on the existence of a stable NiO(2) monoanion in a (4)B(1) state, however, it can be concluded that NiO(2) in its (5)A(1) state should also be stable. The vibrational frequencies are calculated for clusters entrapped in the cubic cell of solid Ar matrix and compared with those obtained for gas-phase clusters. The matrix has no influence on the vibrations of the monoxides and most of the dioxides; however, Co and Ni-dioxoclusters interact strongly with the atoms from the noble gas matrix. The most intense frequencies in the IR spectra are shifted to lower energies and the ordering of the low-lying electronic states by stability is also reversed. According to the electrostatic potential maps, the oxygen atoms in the peroxides are more nucleophilic than those in the dioxides and superoxides. The terminal oxygen atom in superoxides is more nucleophilic than its M-bonded oxygen atom, though charge distribution analysis predicts a smaller negative charge on the terminal oxygen. TiO(2) is the only dioxide in which nucleophilic character in the vicinity of the metal cation is induced.

Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family

Computational and experimental studies using linear muffin tin orbital methods and UV-visible diffuse reflectance spectroscopy, respectively, were performed to quantitatively probe the relationships between composition, crystal structure and the electronic structure of oxides containing octahedrally coordinated d 0 transition metal ions. The ions investigated in this study (Ti 4+, Nb 5+, Ta 5+, Mo 6+, and W 6+) were examined primarily in perovskite and perovskite-related structures. In these compounds the top of the valence band is primarily oxygen 2 p non-bonding in character, while the conduction band arises from the π ∗ interaction between the transition metal t 2 g orbitals and oxygen. For isostructural compounds the band gap increases as the effective electronegativity of the transition metal ion decreases. The effective electronegativity decreases in the following order: Mo 6+>W 6+>Nb 5+∼Ti 4+>Ta 5+. The band gap is also sensitive to changes in the conduction band width, which is maximized for structures possessing linear M–O– M bonds, such as the cubic perovskite structure. As this bond angle decreases (e.g., via octahedral tilting distortions) the conduction band narrows and the band gap increases. Decreasing the dimensionality from 3-D (e.g., the cubic perovskite structure) to 2-D (e.g., the K 2NiF 4 structure) does not significantly alter the band gap, whereas completely isolating the MO 6 octahedra (e.g., the ordered double perovskite structure) narrows the conduction band width dramatically and leads to a significant increase in the band gap. Inductive effects due to the presence of electropositive “spectator” cations (alkali, alkaline earth, and rare-earth cations) tend to be small and can generally be neglected.

Electronic Factors Controlling Bandwidths in Oxides with the Perovskite and Cadmium Halide Structures

The electronic structure of oxides with the perovskite and cadmium halide structures are derived using tightbinding theory and the angular overlap model. The control of the form of the energy bands by the presence of vertex(perovskite) and edge-sharing (cadmium halide) of the coordination octahedra is first explored using onedimensional examples. For one-, two-, and three-dimensional structures of the perovskite type, a very interesting result is uncovered. It is shown that on the band model a band gap may only arise if the oxygen 2s orbital is included in the orbital problem. Prediction of the magnitude of the band gaps in d6 perovskites is thus not a simple matter. This band structure, derived from tight-binding theory via the construction of Bloch orbitals, is compared with an energy band scheme based on local crystal field orbitals and covalent metal-oxygen interactions, which has traditionally been used in this area.

Local geometric and electronic structure of oxides using scanning tunneling microscopy/spectroscopy

The potential of scanning tunneling microscopy (STM) and related scanning probe microscopies is amply demonstrated by the geometric increase in the number of papers utilizing the techniques that are published each year. The power of STM derives, not only from imaging capabilities, but also from associated spectroscopies which can provide local information about the density of states and work function. Although the technique has been applied to a wide range of metal and semiconductor surfaces, application to large bandgap materials such as oxides is difficult. We demonstrate the successful application of STM/STS to oxides with an analysis of the effect of reduction mechanism on the geometric and electronic structure of TiO2 (110). The structure of defects in oxides is critical in that it affects many properties of practical interest including catalysis, photoelectrolysis, and electron transport. The presence of defects can affect the local cation valence, such that the variation of d-orbital occupation results in a range of electronic properties, from insulating to semiconducting in nature. STM/STS is the ideal tool to probe the structure of these defects.

Band theoretical studies of the electronic structure of oxides

The O2− ion is a useful concept in ionic solids. Its electron density and large polarizability is well described by the Watson sphere model. The large variation in the electronic structure of oxides is illustrated by discussing the ionic MgO, the partly ‘covalent’ Cu2O, and TiO and NbO which have defects with respect to the ideal NaCl structure. The variation in physical properties is shown for the oxides in the rutile structure ranging from the insulating TiO2, the metallic RuO2 to the ferromagnetic CrO2. Electron densities, total energies and densities of states are used to study these materials.

Chromium Substitution/Conservation in Alloys for Use in Energy Systems

This paper summarizes the conclusions of a panel of 11 scientists assembled by the U.S. Department of Energy to address the questions regarding our understanding of the scientific basis of chromium alloying additions and to identify the basic material science required to support future substitution/conservation programs. The state of knowledge of how chromium imparts desired properties is explored in the first five sections of this paper which covers the electronic structure of ferrous alloys and electronic structure of oxides, oxidation resistance, corrosion resistance, and mechanical properties. Some of the most promising candidate materials or novel processing techniques for potential new classes of materials are then discussed in sections on alloy design, high-temperature alloys, surface alloying and modification, and structural ceramics.

Surface electronic structure of oxides as established by thermally stimulated electron current measurements

Adsorption and catalysis at the surface of metal oxides is directly related to the electronic constitution in the vicinity of the surface not merely in terms of the predominant electronic character but more importantly the complex assembly of trapping electronic states. Experimental techniques have been developed using both thermally stimulated electron current measurements and thermal glow curves to establish in detail the complex distribution of electronic states. They can lead to quantitative information on both the position and occupancy of trapping states. Preliminary correlation has also been established between trapping states and simple adsorption phenomena. Detailed characteristics have been established for various samples of very high purity zinc oxide and TiO2 including a study of the effect of controlled impurities at a level of a few p.p.m. and variations due to particle size. Direct correlation is made with the adsorption of gases, simple catalytic reactions, the interaction with binders used in electron photography and general characteristics of electron photographic coatings.