The Stabilization of Metal Oxide Nanocrystals by the Addition of Alumina

A problem in the study of nanoparticles is that they will tend to grow at moderate temperatures. For example, most oxides (e.g. SnO₂, ZrO₂, MgO) will show significant grain growth at 400°C. This severely limits experimental studies that require measurements over an extensive temperature range. In this contribution we demonstrate that the incorporation of A1₂O₃, can significantly restrict grain growth in MgO and ZrO₂ even at high temperatures.     

Towards a black‐box for biological EXAFS data analysis. II. Automatic BioXAS Refinement and Analysis (ABRA)

In biological systems, X‐ray absorption spectroscopy (XAS) can determine structural details of metal binding sites with high resolution. Here a method enabling an automated analysis of the corresponding EXAFS data is presented, utilizing in addition to least‐squares refinement the prior knowledge about structural details and important fit parameters. A metal binding motif is characterized by the type of donor atoms and their bond lengths. These fit results are compared by bond valance sum analysis and target distances with established structures of metal binding sites. Other parameters such as the Debye–Waller factor and shift of the Fermi energy provide further insights into the quality of a fit. The introduction of mathematical criteria, their combination and calibration allows an automated analysis of XAS data as demonstrated for a number of examples. This presents a starting point for future applications to all kinds of systems studied by XAS and allows the algorithm to be transferred to data analysis in other fields.     

Local structure and magnetism of Fe, Co and Ni doped Cr3Si

The lattice site occupation of 3d-type impurities in Cr₃Si doped with Fe, Co and Ni were studied using the Extended X-ray Absorption Fine Structure (EXAFS) technique, X-rays and magnetic susceptibility measurements. The EXAFS measurements were performed particularly carefully on the K-edge of chromium and cobalt. EXAFS data strengthened by simulations of the spectra show unambiguously that the impurities are occupying mostly Cr-sites. The magnetic state of the alloys has been studied theoretically using the Wien2k code. It is observed that if calculations are not carried out on an appropriately fine mesh of points in k-space, one can get the result indicating ferromagnetism although the overall magnetic moment per formula unit is weak.     

La0.8Sr0.2Co1-xFexO3材料局域结构的研究

采用溶胶-凝胶法制备了La0.8Sr0.2Co1-xFexO3样品,并测量了各样品的X射线衍射谱（XRD）和X射线吸收精细结构（XAFS）谱。XRD分析研究发现Fe的掺杂引起了La0.8Sr0.2CoO3材料相分离;在前期对X射线吸收精细结构谱近边分析的基础上,进一步分析扩展边X射线吸收精细结构谱,表明在La0.8Sr...     

Sr(NO_3)_2·DCH18C6配合物晶体结构与辐射稳定性

二环己基-18-冠-6(DCH18C6)可以有效地从高放废液中分离90Sr,对于减小放射性废物的危害和实现高放废物的减容有重要意义.由于在实际应用中DCH18C6处于射线照射下,其结构可能会被破坏并引起络合能力的变化,因此有必要对该配合物的辐射稳定性进行研究.本文合成了Sr(NO3)2?DCH18C6配合物晶体,并通过单晶X射线衍射(XRD)与扩展X射线吸收精细结构谱(EXAFS)等方法进行了表征,确定Sr2+与周围氧原子的配位数为10,Sr―O平均键长约为0.268 nm/0.266 nm(XRD/EXAFS).配位原子来自DCH18C6的六个氧原子以及两个作为双齿配体的硝酸根的四个氧原子.对该配合物晶体在空气中进行γ辐照,EXAFS结果表明吸收剂量为400 kGy时,Sr―O键长及配位数没有发生变化,配位结构没有被破坏,具有很好的耐辐照稳定性.显微红外光谱(Micro-FTIR)结果进一步证明辐照后冠醚环的部分C―H键氧化为羟基或羰基,但并不影响DCH18C6与Sr2+的配位结构.     

Macroscopic and spectroscopic characterization of selenate, selenite, and chromate adsorption at the solid–water interface of γ-Al2O3

The interaction of selenate, selenite, and chromate with the hydrated surface of γ-Al₂O₃ was studied using a combination of macroscopic pH edge data, electrophoretic mobility measurements, and X-ray absorption spectroscopic analyses. The pH edge data show generally increased oxyanion adsorption with decreasing pH, and indicate ionic strength-(in)dependent adsorption of chromate and selenate across the pH range 4–9, and ionic strength-(in)dependent adsorption of selenite in this pH range. The adsorption of chromate peaks at pH 5.0, whereas for selenate and selenite no pH adsorption maxima are observed. Electrophoretic mobility measurements show that all three oxyanions decrease the zeta potential of γ-Al₂O₃ upon adsorption; however, only selenite decreased the pH_PZC of the γ-Al₂O₃ sorbent. EXAFS data indicate that selenite ions are coordinated in a bridging bidentate fashion to surface AlO₆ octahedra, whereas no second-neighbor Al scattering was observed for adsorbed selenate ions. Combined, the results presented here show that pH is a major factor in determining the extent of adsorption of selenate, selenite, and chromate on hydrated γ-Al₂O₃. The results point to substantial differences between these anions as to the mode of adsorption at the hydrated γ-Al₂O₃ surface, with selenate adsorbing as nonprotonated outer-sphere complexes, chromate forming a mixture of monoprotonated and nonprotonated outer-sphere adsorption complexes, and selenite coordinating as inner-sphere surface complexes in bridging configuration.     

High temperature NMR approach of mixtures of rare earth and alkali fluorides: An insight into the local structure

In situ high temperature nuclear magnetic resonance in molten fluoride mixtures gives some structural picture of the complexes existing in the melt, i.e. of their nature and relative proportion. Thanks to the development of a laser heating system associated with a close crucible in boron nitride, we can describe experimentally the evolution of these complexes from the anions and the cations point of view. By ¹⁹F NMR, we have shown the existence of three kinds of fluorine atoms depending on the composition: free fluorine like in pure LiF (non-bonded), bridging fluorine in melts rich in LnF₃ in addition with terminal fluorine singly bonded to one rare earth. Data obtained by NMR spectroscopy are also combined with EXAFS measurements, again thanks to a specific development of the sample holder adapted with molten fluorides and high temperature. This study is a part of our systematic investigation of the different Alk-LnF₃ systems by NMR and EXAFS spectroscopy.     

Anodic silver (II) oxides investigated by combined electrochemistry,ex situ XPS and in situ X‐ray absorption spectroscopy

Silver (II) oxide layers (AgO) were prepared by anodic oxidation of pre‐oxidized, Ag₂O‐covered silver electrodes in 1 M NaOH (pH 13.8). The oxidized electrodes were investigated using a combination of electrochemical techniques, ex situ X‐ray photoelectron spectroscopy (XPS) and in situ surface‐sensitive grazing incidence X‐ray absorption spectroscopy (EXAFS) under full potential control. The application of these different techniques leads to a detailed, consistent picture of the anodic silver (II) oxide layer formation. The experiments have shown that the chemical composition of the AgO layer varies significantly with oxidation potential, revealing a decreasing oxygen deficiency with increasing anodization potential and oxidation time. XPS as well as EXAFS experiments support the interpretation of the oxide as a mixed valence Ag ⁺ Ag^{3 +} O₂ with different contributions of Ag ⁺ and Ag^{3 +} species, depending on potential and anodization time. Copyright © 2009 John Wiley & Sons, Ltd.