Initiating action of continuous heating on the formation of phases in the solid state synthesis of complex oxides of rare earth elements

The kinetics of the solid state synthesis of complex oxides of rare earth elements has been investigated by the x-ray diffraction method for various sizes of the crystallites of initial simple oxides. It has been shown that the continuous heating of the samples undergoing the solid state synthesis stimulates the formation of phases at lower temperatures and in shorter times than long-term step annealing. It has been revealed that the effectiveness of the initiating action of the continuous heating on the formation of phases increases with decreasing size of the crystallites of the initial components.     

Relaxation energies of oxygen auger transitions in some oxides

Relaxation effects are of major importance in the so-called chemical shifts observed in XPS and AES. These chemical shifts mainly arise from changes in the electrostatic environment due to the field of the neighbouring atoms in the initial neutral state and from electron redistribution in the surrounding electron cloud in order to screen the final state holes of the excited atom. Using a three-step model for the Auger process, we succeeded in deriving cross-relaxation energies from the KLL spectrum of oxygen in oxides, provided a careful calibration of experimental binding and Auger kinetic energies is achieved. It has been shown that the extra-atomic relaxation energy increases with ionicity for oxides of non-transition metals. In the case of transition metal oxides, it has been found that the cross-relaxation energies are larger than for the other oxides; it is believed that this is due to a more efficient screening effect of the d-electrons of the neighbouring metal atoms.     

Photoelectron binding energy shifts observed during oxidation of group IIA,IIIA and IVA elemental surfaces

An extensive re-evaluation of XPS binding energies (BE's) and binding energy shifts (ΔBE's) from metals, oxides and the carbonates of the group II, III and IVA elements (exceptions are Be, Mg and Hf) has been carried out using a substrate specific BE referencing approach. From this, O-1s BE's are found to fall into surface oxide, bulk oxide and carbonate groupings, with bulk oxides showing the lowest BE's followed by surface oxides (+∼1.5 eV) and then carbonates (+∼3.0 eV). The O-1s BE's from the bulk oxides also appear to scale with 1/d, where d is inter-atomic distance. The same is noted in the ΔBE's observed from the metallic counterparts during oxidation of the elemental surfaces. This, and the decreasing BE exhibited by Ca, Sr and Ba on oxidation is explained within the charge potential model as resulting from competing inter- and intra-atomic effects, and is shown to be consistent with partial covalency arguments utilizing Madulung potentials. The ΔBE's also fall into groups according to the elements location in the periodic table, i.e. s, p or d block. These trends open up the possibility of approximating ΔBE's arising from initial and final state effects, and bond distances.     

Mechanism of single-walled carbon nanotube growth on natural magnesite

Fe K-edge X-ray absorption fine structure (XAFS) measurements were performed in order to elucidate the formation mechanism of single-walled carbon nanotubes (SWCNTs) grown on natural magnesite by pyrolyzing methane gas. It was clearly shown by XAFS analyses that iron metal fine particles, which were reduced from iron oxides by methane gas, worked as a catalyst for SWCNT growth. Structural characteristics of the initial iron state in the natural magnesite were also discussed.     

Valence state mapping of cobalt and manganese using near-edge fine structures

The properties of transition metal oxides are related to the presence of elements with mixed valences. The spectroscopy analysis of the valence states is feasible experimentally, but a spatial mapping of valence states of transition metal elements is a challenge to existing microscopy techniques. In this paper, with the use of valence state information provided by the white lines and near-edge fine structures observed using the electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM), a novel experimental approach is demonstrated to map the valence state distributions of Mn and Co using the ratio of white lines in the energy-filtered TEM. The valence state map is almost independent of specimen thickness in the thickness range adequate for quantitative EELS microanalysis. An optimum spatial resolution of approximately 2 nm has been achieved for a two-phase Co oxides.     

EELS analysis of cation valence states and oxygen vacancies in magnetic oxides

Transition metal oxides are a class of materials that are vitally important for developing new materials with functionality and smartness. The unique properties of these materials are related to the presence of elements with mixed valences of transition elements. Electron energy-loss spectroscopy (EELS) in the transmission electron microscope is a powerful technique for measuring the valences of some transition metal elements of practical importance. This paper reports our current progress in applying EELS for quantitative determination of Mn and Co valences in magnetic oxides, including valence state transition, quantification of oxygen vacancies, refinement of crystal structures, and identification of the structure of nanoparticles.     

Static and dynamical magnetic characters in doped Cu oxides

The discovery of the spin density wave (SDW) state defined as stripe in the doped Cu oxides becomes a central issue in the elucidation of the high-temperature superconducting mechanism. We present the coexistence of the stripe order and superconductivity, and the change of the stripe structure associated with the insultor–metal transition. Both static and dynamical SDW are robust of the high-temperature superconducting Cu oxides.     

Ferromagnetic ordering in dilute magnetic dielectrics with and without free carriers

The state of art in the theoretical and experimental studies of transition metal doped oxides (dilute magnetic dielectrics) is reviewed. The available data show that the generic non-equilibrium state of oxide films doped with magnetic impurities may either favor ferromagnetism with high Curie temperature or result in highly inhomogeneous state without long-range magnetic order. In both case concomitant defects (vacancies, interstitial ions) play crucial part.     

Fermi-liquid ground state in the n-type Pr(0.91)LaCe(0.09)CuO(4-y) copper-oxide superconductor

We report nuclear magnetic resonance studies on the low-doped n-type copper-oxide Pr(0.91)LaCe(0.09)CuO(4-y) (T(c)=24 K) in the superconducting state and in the normal state uncovered by the application of a strong magnetic field. We find that when the superconductivity is removed the underlying ground state is the Fermi liquid state. This result is at variance with that inferred from previous thermal conductivity measurement and appears to contrast with that in p-type copper oxides with a similar doping level where high-T(c) superconductivity sets in within the pseudogap phase. The data in the superconducting state are consistent with the line-node gap model.     

Anomalous magnetism of the nanocrystalline oxide TiO<Subscript>2</Subscript> surface

The magnetic properties of an oxygen-deficient nanocrystalline undoped titanium dioxide synthesized by the gas-phase, electric-explosion, and chemical method have been studied. The defect state was controlled using reduction treatments in vacuum or in a hydrogen atmosphere. It is shown that the defect state of the surface of nanocrystalline oxides (for example, the existence of vacancies in the anion sublattice and other defects) has a dominant influence on the formation of the magnetic properties of the samples under study. The main contributions to the magnetism of TiO₂ nanoparticles after the reduction treatments are the paramagnetic contribution of the matrix, the paramagnetic Curie–Weiss contribution, and the contribution of the spontaneous magnetic moment provided by the existence of regions with different spin ordering. A heterogeneous magnetic state is found to exist in the TiO₂ nanopowders; for example, at low temperatures, shifted hysteresis loops are observed as a result of a possible set of magnetic states with different spin orders. It is shown that a soft compaction or grinding of nanopowders in an agate mortar lead to substantial increase in the magnetization, sometimes, by a factor of more than two, regardless of the nanopowder synthesis method and the initial phase state of TiO₂ (anatase or rutile structures). This experimental fact proves the key role of the surface defects and the magnetic moment carriers with different spin configurations localized mainly on the nanoparticle surface. The compaction changes the magnetization only in the case when the initial magnetic state has a nonlinear “quasi-superparamagnetic” character of the magnetization curve. As a result of predominant exchange interaction between the nanoparticles with a frustrated character of spin ordering on the nanoparticles surface, the ferromagnetic contribution increases as nanoparticles contact.     

CrLVV Auger emission and photoreduction of hexavalent Cr oxides

The CrLVV Auger emission has been used to investigate the local chemical environment of Cr(VI) in connection with the photoreduction produced by a non-monochromatic X-ray source. We compared Cr(VI) in oxides on anodized Al exposed to a chromate solution, with Cr(VI) in oxides on Al/Cr alloys polarized in a borate solution. The former were found to be much more sensitive to photoreduction than the latter. This is correlated with the intensity of the CrLVV Auger emission, attributed to a charge transfer transition in the final state of the photoemission process.     

加压下ZnO结构相变和电子结构的第一性原理计算

基于密度泛函(DFT)理论的第一性原理,计算半导体ZnO纤锌矿结构和岩盐矿结构状态方程及其在高压下的相变,分析加压下体相ZnO的晶格常数、电子态密度和带隙随压力的变化关系,并将计算结果与文献中的理论和实验数据进行比较.验证在计算金属氧化物时,应用局域密度(LDA)近似计算出的相变压力普遍偏高,采用广义梯度(GGA)近似得到的结果与实验符合较好.     

Spatial resolution of transmission electron spectroscopy for the study of ordered materials

The structure of barium-rich oxides of the Ba-Bi-O system was studied with the transmission electron spectroscopy (TEM) and electron diffraction techniques. The results show that TEM cannot reveal the presence of an ordered state in a material composed of irregular intergrown crystallites, if their sizes are less than several nanometers.     

Modelling and speciation of nitrogen oxides in engines

The present study extends the Nitrogen Oxide Relaxation Approach (NORA [Vervisch et al., Combust. Flame 158 (2011) 1480–1490]) for NO_x modelling in engines by introducing alternative chemical routes to the thermal pathway as well as a speciation of nitrogen oxides into nitrogen monoxide (NO), nitrogen dioxide (NO₂) and nitrous oxide (N₂O). Perturbations of equilibrium state are considered to identify nitrogen oxide reactivity in various mixture and thermodynamic conditions. A common Intrinsic Low-Dimensional Manifold (ILDM) is identified for NO and NO₂, while nitrous oxides appear to stay in near-equilibrium state for any in-cylinder conditions. The relaxations back towards the equilibrium state after the perturbations are analysed to extract the characteristic times of relaxation, an image of the species reactivity. Tabulation of equilibrium mass fractions and characteristic relaxation times as a function of mixture and thermodynamic conditions allows nitrogen oxide modelling to be performed for internal combustion engines at very low computational cost through idealised ILDMs that are independent of the combustion ones. Results show the accuracy of the modelling approach for nitrogen oxide emissions of a Diesel engine at part and full load.     

Transition states in Ei reactions

The pyrolysis of amine oxides, sulfoxides, selenoxides, and esters to form alkenes is believed to be a concerted reaction with a cyclic transition state. Phosphine oxides, sulfones, and nitro compounds are unreactive. This study seeks to identify reasons for the lack of reactivity of the latter. Transition states were located for all substrates progressing from RHF/3‐21G* to the MP2/6‐31+G(d,p) level (in certain cases). For sulfones and nitro compounds, two possible reasons for lack of reactivity were considered: (1) Atoms approaching one another in the transition state may be considered to participate in a local n_O → σ*_CH interaction. The second oxygen in the sulfone or nitro compound lowers the energy of the ‘non‐bonded electrons’ at oxygen leading to a greater mismatch in energy with the antibonding C? H orbital (in comparison to the sulfoxide or amine oxide). (2) The sulfone and nitro ‘lone pairs’ are not really ‘alone’, and available to react. In fact, complex bonding arrangements exist in the SO₂ and NO₂ groups. The situation is less clear for phosphine oxides, although ground state stability of bonds is important. Implications concerning the Hammond postulate are covered. Copyright © 2009 John Wiley & Sons, Ltd.     

Active oxidation: Silicon etching and oxide decomposition basic mechanisms using density functional theory

The etching of silicon atom from the Si(1 0 0)-p(2 × 2) surface, i.e. the desorption of SiO molecules from this surface, either clean or pre-oxidized, is investigated at the density functional theory level. The reaction paths for desorption are given as a function of the initial oxidation state of the extracted silicon atom. The associated activation energies and the atomic configurations are discussed. Particularly, it is shown that desorption of SiO molecules takes place during conventional thermal oxide growth (∼2 eV activation) via non-oxidized silicon atoms. Further SiO extraction mechanisms of higher silicon oxidation states required higher temperatures. In particular, doubly oxidized silicon atoms (Si²⁺) are able to decompose with an activation of ∼4 eV which corresponds to the actual temperature where decomposition of oxides is observed. This confirms the statement that decomposition of oxide layer nucleates at the interface with silicon where Si²⁺ has been detected thanks to XPS experiment.     

Oxidation catalysis by oxide-supported Au nanostructures: the role of supports and the effect of external conditions

Oxide-supported Au nanostructures are promising low-temperature oxidation catalysts. It is generally observed that Au supported on reducible oxides is more active than Au supported on irreducible oxides. Recent studies also suggest that cationic Au(delta+) is responsible for the unique Au/oxide catalytic activity, contrary to the conventional perception that oxide supports donate electronic charge to Au. We have utilized density functional calculations and ab initio thermodynamic studies to investigate the oxidation state of Au nanostructures deposited on reducible and irreducible supports. We find that there are fundamental differences in the electronic structure of Au deposited on the different oxides. We propose a simple model, grounded in the first principles calculations, which can explain the oxide-specific catalytic activity of Au nanostructures and which can account for the presence and the role of cationic Au(delta+).     

Intrinsic ferromagnetism in insulating cobalt doped anatase TiO2

Using complementary experiments we show that the room temperature ferromagnetism observed in anatase Co:TiO(2) films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the +2 state substituting for Ti. Measurements at 300 K yield a M(S) of 1.1 mu(B)/Co atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of Co:TiO(2) leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.     

Mott transition and suppression of orbital fluctuations in orthorhombic 3d1 perovskites

Using t(2g) Wannier functions, a low-energy Hamiltonian is derived for orthorhombic 3d(1) transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for noncubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO3-type distortions) is found to suppress orbital fluctuations in LaTiO3 and even more in YTiO3, and to favor the transition to the insulating state.     

铁氧化物中电子隧道现象的扫描隧道显微术研究

报道了利用扫描隧道显微术(STM)对金属表面氧化物层进行电子隧道谱研究的结果。在对两类铁晶体表面氧化层进行的隧道谱和势垒高度测量结果进行分析后表明,常温条件下形成的氧化层(Ⅰ类)应主要是Fe₃O₄;而在高温氧化条件下形成的表面层(Ⅱ类)的主要成分则应是Fe₂O₃。从而表明(STM)可用于研究铁表面氧化过程的不同阶段,并且由Ⅰ类氧化层的低势垒特性说明STM还可以用于观测此类氧化层的内部结构。类似研究方法还可应用到对一系列 关键词：