Symmetry role on the preedge X—ray absorption fine structure at the metal K edge

The preedge features in a system with “even”symmetry,apart from quadrupolar transition contribution,are mainly dipolar in character,associated with the existence of unoccupied states made up of mixed cation-4p with higher-neighboring cation-3d orbitals,and reflect the density of states due to the medium-range order of the system.In “odd“symmetry materials these preedge features ate the result of a transition from the ls to a final density of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation 3d orbitals,similar to atetrahedral configuration.These results are validated for Fe as a photoabsorber by comparing XAS spectra of Fe2SiO4(fayalite) to ab initio full multiple scattering calculations at the fe K edge,but pertain to all systems containing sixfold-coordinated cations.     

Experimental and theoretical XANES study of the effcets of Fe—Mg solid solution in the enstatit—ferrosilite series

Synthetic orthopyroxenes in the join enstatite-ferrosilite (Mg2Si2O6-Fe2Si2O6)have been studied by XANES with the aim to interpret the variations found in the spectra and evaluate the effects of the Mg-Fe substitution in the orthopyroxene(OPX) structure.The experimental Fe-and Mg-K edge XANES spectra show variations of the peaks intensity as a function of the chemical composition of the samples along the EN-FS join.In the region of the Fe edge,the intensity ratio between peak A and B(IB/IA)varies linearly(R=0.99) with the Mg/Fe substitution. Multiple scattering calculation have been performed both at the Fe and Mg K-edge,to study the geometrical modifications of the M1 and M2 octahedral sites induced by the Mg-Fe substitutions.The spectra,obtained locating Fe(or Mg)in both M1 and M2 and weighing the contributions using the occupancy data for the M1 and M2 sites determined by XRD,allow to discriminate the effects to the total spectrum due to Fe(Mg) located in each sites separately.     

INVESTIGATION OF BONDING IN NANO—SiO2 BY Si L2，3 X—RAY ABSORPTION NEAR—EDGE STRUCTURE SPECTROSCOPY

The Si L2,3 X-ray absorption near-edge structure(XANES) can be used to probe the local structure around Si and derive electronic information of the unoccupied s- and d-like partial density of states in nano-size SiO2.We present Si L2,3-edge for three different size silicates acquired by total electron yield(TEY) at the photoemission atation of Beijing Synchrotron Radiation Facility (BSRF).The Si L2,3-edge spectra are interpreted based on ab initio full multiple-scattering(MS) calculation.The Si L2,3-edge of nano-size materials has XANES similar to that of α-quartz.The similarities between the Si L2,3-edge shapes attest to a common molecular-orbital picture of their Si-O bonding and the same coordination state.However,a considerable broadening of Si L2,3-edge XANES spectra as decrease of particle size is also an indicative of polyhedral distortions.     

Symmetry dependence of x—ray absorption near—edge structure at rhe metal K edge of 3d transition metal compounds

The pre-edge features in system with “even” symmetry,apart from quadrupolar tranition contribution,are mainly dipolar in character,associated with the existence of unoccupied states made up of mixed cation np with higher-neighboring cation-(n-l)d orbitals,and reflect the density of states due to the medium-range order of the system,while in “ood” symmetry materials these pre-edge features are the result of a transition from the 1s to final denisty of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation d orbitals.In the latter case,they contain not only the p but also the d base of orbitals,similar to a tetrahedral configuration.These results are validated for Fe as a photoabsorber by comparing x-ray absorption near-edge spectra of Fe2SiO4(fayalite) and Fe2O3(hematite) to ab initio full multiple scattering calculations at the Fe K edge,but pertain to all systems containing sixfold-coordinated cations.     

Oxygen 1s ELNES study of perovskites （Ca,Sr,Ba）TiO3

The O K-edge spectra of perovskites XTiO3 with X=Ca,Sr and Ba have been measured using electro energy-loss spectroscopy (EELS) and are analyzed using the ab initio full mulitiple-scattering(MS) calculations.The near-edge structures arise mainly from covalency by direct and/or indirect interaction between oxygen and metal.The coordination number of the cation and the site symmetry also infuence the spectral shape and structures.Comparison of full MS simulations of different clusters around the excited atom allows to interpret and correlate the features present in the spectra of each compound to its specific atomic arrangement and electronic structure.     

电化学增氧La2CuO4＋δ（0〈δ〈0.12）的研究：Ⅱ.化学相分离

在Ｌａ２ＣｕＯ４＋δ的电化学增氧过程中，额外氧含量δ与电量Ｑ之间存在直线关系：δ＝Ｑ／２，在室温附近至少存在两个相共存区。它们的δ分布范围分别为０．００６－０．０４７和０．０７５－０．１７。其中在较高掺杂浓度范围内的第二个两相区是本研究首次发现的，其存在不能用Ｅｍｅｒｙ理论来解释。     

电化学增氧La2CuO4＋δ（0〈δ〈0.12）的研究：Ⅰ.稳定性与热处理效应

电化学行为和磁化率测量表明：室温下Ｌａ２ＣｕＯ４＋δ的电化学增氧过程偏离了热力学平衡，样品的超导性质与增氧的途径有关。这也是不同实验所得结果差异很大的原因。将电化学增氧的Ｌａ２ＣｕＯ４＋δ在氧气氛中于３８３Ｋ下进行长时间的处理后，样品具有良好的稳定性和重复性。     

On the change of electronic states at the Fermi level by Ce doping in the intermetallic LaRu2

Variation of electronic structure at the Fermi level in superconducting LaRu2 intermetallic system has been studied by X-ray absorption near edge structure(XANES) spectroscopy.The high resolution Ru K-edge XANES have been measured by fluorescence yield method using multi-element solid state detector assembly allowing to reach a signal to noise ratio of the order of △α/α-10^-4.The results show decrease of the superconducting transition temperature.2001 Elsevier Science B.V.All rights reserved.     

EXAFS study of molybdenum oxide on the structure Al2O3

In this paper,MoO3 dispersed on an alumina support was studied.The dispersion threshold was determined by x-ray diffraction and the Mo-O coordination structure was detected by extended x-ray absorption fine structure(EXAFS),which shows that the preferred dispersion phase is madu up of [MoO4] polyhedrons.Copyright 2001 John Wiley and sons,Lad.     

Size distribution of sulfur species in fine and ultrafine aerosol particles using sulfur K-edge XANES

The sulfur species existing in different size aerosol particles were determined based on the ＇white line＇ energy shift and the features of post edge structure of sulfur K-edge XANES （X-ray Absorption Near Edge Structure）. The results indicated that sulfates were dominant sulfur species in aerosol particles. However, small amount of reductive sulfur species were also found in the ultrafine aerosol particles（〈0.1 μm）. Sulfates in aerosols mostly exhibited as （NH4）2SO4 and gypsum. than 0.952 μm, while gypsum was the dominant sulfate Most （NH4）2SO4 distributed in aerosol particles finer in coarse aerosols.     

在(100)SrTiO3上用PLD制备YBa2Cu3O7-δ/Eu2CuO4/LaNiO3三层结构

钙钛矿结构的金属氧化物LaNiO3与高温超导体YBa2Cu3O7-δ有着相近的晶格常数，并且有良好的导电性．我们在(100)SrTiO3基片上，用脉冲激光沉积法(PLD)成功的制备了单取向的LaNiO3，其表面起伏的均方根为1nm，电阻温度特性呈金属性．在此基础上，我们又原位制备了YBa2Cu3O7-δ／Eu2CuO4／LaNiO3三层结构．实验分析表明，高温超导薄膜YBa2Cu3O7-δ单一取向，临界温度在85K以上，超导转变宽度小于1．5K．这一结构对于研究高温超导体的准粒子注入和采用SIS或SINIS结构构成高温超导约瑟夫森结等方面有着重要意义．     

Crystal growth of β’—Gd2（Mo4）3 and in situ observation of its domain structure by the microscope and the synchrotron X—ray topography

We report that β’-Gd2(MoO4)3 crystals have been grown by the Czochralski(CZ) method.We studied the growth conditions fo β’-Gd2(MoO4)3 crystal.A compatison between the resistance-heated method and radio frequency induction-heated method is described.Also,the in situ observation of the domain structure by the microscope and by synchrotron radiation X-ray topography under the conditions of DC polarization voltage and tempertature change were carried out.Experiments showed that multidomain structure including ferroelectric and ferroelastic domains occurred in β’-Gd2(MoO4)3 cystal.Usually ferroeletiric domains appeared in the as-grown crystal and ferroelastic domains appeared in the polished thinner piece.Both ferroelectric and ferroelastic domains disappeared when the temperature of the crystal piece was higher than the Curie tempperature Tc and they could be reproduced when the temperature of the crystal piece was lowered below TC.When a DC polarization voltage ranging from 150 to 500V was applied on a c-axis β’-Gd2(MoO4)3 piece having a thickness of 0.5mm ,the multidomain would gradually transform to a single domain.This resual means that it is possible to make a periodically poled β’-Gd2(MoO4)3 crystal.2001 Elsevier Science B.V.All rights reserved.