Solid energy calibration standards for P K‐edge XANES: electronic structure analysis of PPh4Br

P K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy is a powerful method for analyzing the electronic structure of organic and inorganic phosphorus compounds. Like all XANES experiments, P K‐edge XANES requires well defined and readily accessible calibration standards for energy referencing so that spectra collected at different beamlines or under different conditions can be compared. This is especially true for ligand K‐edge X‐ray absorption spectroscopy, which has well established energy calibration standards for Cl (Cs₂CuCl₄) and S (Na₂S₂O₃·5H₂O), but not neighboring P. This paper presents a review of common P K‐edge XANES energy calibration standards and analysis of PPh₄Br as a potential alternative. The P K‐edge XANES region of commercially available PPh₄Br revealed a single, highly resolved pre‐edge feature with a maximum at 2146.96 eV. PPh₄Br also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh₃ rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh₃ and PPh₄⁺ revealed large differences in the molecular orbital energies that were ascribed to differences in the phosphorus oxidation state (III versus V) and molecular charge (neutral versus +1). Time‐dependent density functional theory calculations corroborated the experimental data and allowed the spectral features to be assigned. The first pre‐edge feature in the P K‐edge XANES spectrum of PPh₄Br was assigned to P 1s → P‐C π* transitions, whereas those at higher energy were P 1s → P‐C σ*. Overall, the analysis suggests that PPh₄Br is an excellent alternative to other solid energy calibration standards commonly used in P K‐edge XANES experiments.     

K‐edge XANES analysis of sulfur compounds: an investigation of the relative intensities using internal calibration

Sulfur K‐edge XANES (X‐ray absorption near‐edge structure) spectroscopy is an excellent tool for determining the speciation of sulfur compounds in complex matrices. This paper presents a method to quantitatively determine the kinds of sulfur species in natural samples using internally calibrated reference spectra of model compounds. Owing to significant self‐absorption of formed fluorescence radiation in the sample itself the fluorescence signal displays a non‐linear correlation with the sulfur content over a wide concentration range. Self‐absorption is also a problem at low total absorption of the sample when the sulfur compounds are present as particles. The post‐edge intensity patterns of the sulfur K‐edge XANES spectra vary with the type of sulfur compound, with reducing sulfur compounds often having a higher post‐edge intensity than the oxidized forms. In dilute solutions (less than 0.3–0.5%) it is possible to use sulfur K‐edge XANES reference data for quantitative analysis of the contribution from different species. The results show that it is essential to use an internal calibration system when performing quantitative XANES analysis. Preparation of unknown samples must take both the total absorption and possible presence of self‐absorbing particles into consideration.     

Reference spectra of important adsorbed organic and inorganic phosphate binding forms for soil P speciation using synchrotron‐based K‐edge XANES spectroscopy

Direct speciation of soil phosphorus (P) by linear combination fitting (LCF) of P K‐edge XANES spectra requires a standard set of spectra representing all major P species supposed to be present in the investigated soil. Here, available spectra of free‐ and cation‐bound inositol hexakisphosphate (IHP), representing organic P, and of Fe, Al and Ca phosphate minerals are supplemented with spectra of adsorbed P binding forms. First, various soil constituents assumed to be potentially relevant for P sorption were compared with respect to their retention efficiency for orthophosphate and IHP at P levels typical for soils. Then, P K‐edge XANES spectra for orthophosphate and IHP retained by the most relevant constituents were acquired. The spectra were compared with each other as well as with spectra of Ca, Al or Fe orthophosphate and IHP precipitates. Orthophosphate and IHP were retained particularly efficiently by ferrihydrite, boehmite, Al‐saturated montmorillonite and Al‐saturated soil organic matter (SOM), but far less efficiently by hematite, Ca‐saturated montmorillonite and Ca‐saturated SOM. P retention by dolomite was negligible. Calcite retained a large portion of the applied IHP, but no orthophosphate. The respective P K‐edge XANES spectra of orthophosphate and IHP adsorbed to ferrihydrite, boehmite, Al‐saturated montmorillonite and Al‐saturated SOM differ from each other. They also are different from the spectra of amorphous FePO₄, amorphous or crystalline AlPO₄, Ca phosphates and free IHP. Inclusion of reference spectra of orthophosphate as well as IHP adsorbed to P‐retaining soil minerals in addition to spectra of free or cation‐bound IHP, AlPO₄, FePO₄ and Ca phosphate minerals in linear combination fitting exercises results in improved fit quality and a more realistic soil P speciation. A standard set of P K‐edge XANES spectra of the most relevant adsorbed P binding forms in soils is presented.     

Relationship between the structural distortion and the Mn electronic state in La1−xCaxMnO3: a Mn K‐edge XANES study

A theoretical study of the X‐ray absorption near‐edge structure (XANES) spectra at the Mn K‐edge in the La_1?xCa_xMnO₃ series is reported. The relationship between the edge shift, the Ca–La substitution and the distortion of the MnO₆ octahedra in these systems has been studied. It is shown that, by correctly considering these effects simultaneously, the experimental XANES data are consistent with the presence of two different Mn local environments in the intermediate La_1?xCa_xMnO₃ compounds. By taking into account the energy shift associated with the modification of the MnO₆ distortion as Ca substitutes for La, it is possible to reproduce the XANES spectra of the intermediate‐doped compounds starting from the experimental spectra of the end‐members LaMnO₃ and CaMnO₃. These results point out the need to re‐examine the conclusions derived in the past from the simple analysis of the Mn K‐edge XANES edge‐shift in these materials. In particular, it is shown that the modification of the Mn K‐edge absorption through the La_1?xCa_xMnO₃ series is well reproduced by considering the simultaneous presence of both distorted and undistorted octahedra and, consequently, that the existence of charge‐ordering phenomena cannot be ruled out from the XANES data.     

An unconventional method for measuring the Tc L3‐edge of technetium compounds

Tc L₃‐edge XANES spectra have been collected on powder samples of SrTcO₃ (octahedral Tc⁴⁺) and NH₄TcO₄ (tetrahedral Tc⁷⁺) immobilized in an epoxy resin. Features in the Tc L₃‐edge XANES spectra are compared with the pre‐edge feature of the Tc K‐edge as well as other 4d transition metal L₃‐edges. Evidence of crystal field splitting is obvious in the Tc L₃‐edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L₃ absorption edge energy difference between SrTcO₃ (Tc⁴⁺) and NH₄TcO₄ (Tc⁷⁺) shows that the energy shift at the Tc L₃‐edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L₃‐edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L₃‐edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K‐edge.     

Magnesium K‐edge XANES spectroscopy of geological standards

Magnesium K‐edge X‐ray absorption near‐edge structure (XANES) spectra have been investigated to develop a systematic understanding of a suite of Mg‐bearing geological materials such as silicate and carbonate minerals, sediments, rocks and chemical reagents. For the model compounds the Mg XANES was found to vary widely between compounds and to provide a fingerprint for the form of Mg involved in geologic materials. The energy positions and resonance features obtained from these spectra can be used to specify the dominant molecular host site of Mg, thus shedding light on Mg partitioning and isotope fractionation in geologic materials and providing a valuable complement to existing knowledge of Mg geochemistry.     

Electronic structure effects on B K‐edge XANES of minerals

In order to assess the usability of X‐ray absorption near‐edge structure (XANES) for studying the structure of BO_n‐containing materials, the dependence of theoretical XANES at the B K‐edge on the way the scattering potential is constructed is investigated. Real‐space multiple‐scattering calculations are performed for self‐consistent and non‐self‐consistent potentials and for different ways of dealing with the core hole. It is found that in order to reproduce the principal XANES features it is sufficient to use a non‐self‐consistent potential with a relaxed and screened core hole. Employing theoretical modelling of XANES for studying the structure of boron‐containing glasses is thus possible. The core hole affects the spectrum significantly, especially in the pre‐edge region. In contrast to minerals, B K‐edge XANES of BPO₄ can be reproduced only if a self‐consistent potential is employed.     

Temperature dependence of pre‐edge features in Ti K‐edge XANES spectra for ATiO3 (A = Ca and Sr), A2TiO4 (A = Mg and Fe), TiO2 rutile and TiO2 anatase

XANES (X‐ray absorption near‐edge structure) spectra of the Ti K‐edges of ATiO₃ (A = Ca and Sr), A₂TiO₄ (A = Mg and Fe), TiO₂ rutile and TiO₂ anatase were measured in the temperature range 20–900 K. Ti atoms for all samples were located in TiO₆ octahedral sites. The absorption intensity invariant point (AIIP) was found to be between the pre‐edge and post‐edge. After the AIIP, amplitudes damped due to Debye–Waller factor effects with temperature. Amplitudes in the pre‐edge region increased with temperature normally by thermal vibration. Use of the AIIP peak intensity as a standard point enables a quantitative comparison of the intensity of the pre‐edge peaks in various titanium compounds over a wide temperature range.     

Trace element analysis of hairs in patients with dementia

The trace elements of scalp hair samples from ≥60‐year‐old dementia patients and normal persons have been studied by X‐ray absorption near‐edge spectroscopy (XANES) in fluorescent mode and wavelength‐dispersive X‐ray fluorescence spectrometry. Comparisons of hair trace element levels of age‐matched dementia patients and normal persons revealed significantly elevated amounts of calcium, chlorine and phosphorus in dementia patients relative to normal persons. The results of XANES measurements identify the chemical forms of deposited calcium and phosphorus in the hair samples of both dementia patients and normal persons to be calcium chloride (CaCl₂) and phosphate (PO₄^3?), respectively. The amount of sulfur in hairs of dementia patients was found to be not significantly different from that in normal persons. The sulfur K‐edge XANES spectra, however, show significantly higher accumulations of sulfur in the sulfate (SO₄^2?) form in hairs of Alzheimer's disease and Parkinson's disease dementia patients. This study presents the possible roles of calcium, chlorine, phosphorus and sulfur in the etiology of dementia in elderly patients.     

Classification of Fe‐bearing species from K‐edge XANES data using two‐parameter correlation plots

Solid iron compounds are extremely common in the environment as well as in meteorites and comets. Fe K‐edge XANES (X‐ray absorption near‐edge structure) measurements can be carried out quickly, theoretically allowing one to categorize many areas within a sample or set of samples in a short time. However, interpretation of such data is not straightforward unless one has the appropriate reference spectra, hence a way of classifying an unknown spectrum to a family group (trivalent, divalent, oxide, silicate etc.) is required. Methods of abstracting Fe XANES spectra to produce pairs of variables which, when plotted, cluster in distinct regions depending on the family are presented. For instance, divalent minerals fall in a different region than trivalent minerals, and sulfides in a different region than oxides.     

Operando QEXAFS studies of Ni2P during thiophene hydrodesulfurization: direct observation of Ni—S bond formation under reaction conditions

Structural changes in Ni₂P/MCM‐41 were followed by quick extended X‐ray absorption fine structure (QEXAFS) and were directly related to changes in X‐ray absorption near‐edge structure (XANES) which had been used earlier for the study of the active catalyst phase. An equation is proposed to correct the transient QEXAFS spectra up to second‐order in time to remove spectral distortions induced by structural changes occurring during measurements. A good correlation between the corrected QEXAFS and the XANES spectral changes was found, giving support to the conclusions derived from the XANES in the previous work, namely that the formation of a Ni—S bond in a surface NiPS phase is involved in the active site for the hydrodesulfurization reaction.     

Manganese speciation in Diplodon chilensis patagonicus shells: a XANES study

X‐ray absorption near‐edge spectroscopy (XANES) at the Mn K‐edge was used to investigate the environment of Mn in situ within the growth increments of the long‐lived freshwater bivalve species Diplodon chilensis patagonicus. Single XANES spectra and Mn Kα fluorescence distributions were acquired at submillimetre resolution (up to 100 µm × 50 µm), at Mn concentrations below the weight percent range (100–1000 µg g^?1) in a high Ca matrix. The position and intensity of the pre‐edge feature in the shell spectrum resembles best that of the Mn(II)‐bearing reference compounds, suggesting that this is the oxidation state of Mn in the bivalve shells. By comparison with the XANES spectra of selected standard compounds, hypotheses about Mn speciation in the shell are also reported. In particular, different factors, such as provenance, ontogenetic age, variable Mn‐concentrations or seasonal shell deposition seem not to influence the speciation of the metal in this bivalve species.     

Extended X‐ray absorption fine structure and multiple‐scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n (n = −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1‐hexene)

A series of Ni dithiolene complexes Ni[S₂C₂(CF₃)]₂ⁿ (n = ?2, ?1, 0) ( 1 , 2 , 3 ) and a 1‐hexene adduct Ni[S₂C₂(CF₃)₂]₂(C₆H₁₂) ( 4 ) have been examined by Ni K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) spectroscopies. Ni XANES for 1 – 3 reveals clear pre‐edge features and approximately +0.7 eV shift in the Ni K‐edge position for `one‐electron' oxidation. EXAFS simulation shows that the Ni—S bond distances for 1 , 2 and 3 (2.11–2.16 Å) are within the typical values for square planar complexes and decrease by ～0.022 Å for each `one‐electron' oxidation. The changes in Ni K‐edge energy positions and Ni—S distances are consistent with the `non‐innocent' character of the dithiolene ligand. The Ni—C interactions at ～3.0 Å are analyzed and the multiple‐scattering parameters are also determined, leading to a better simulation for the overall EXAFS spectra. The 1‐hexene adduct 4 presents no pre‐edge feature, and its Ni K‐edge position shifts by ?0.8 eV in comparison with its starting dithiolene complex 3 . Consistently, EXAFS also showed that the Ni—S distances in 4 elongate by ～0.046 Å in comparison with 3 . The evidence confirms that the neutral complex is `reduced' upon addition of olefin, presumably by olefin donating the π‐electron density to the LUMO of 3 as suggested by UV/visible spectroscopy in the literature.     

(Fe1-xNix)2P电子结构与磁学特性的X射线近边吸收谱研究

利用X射线近边吸收谱对Fe₂P,Ni₂P及其掺杂物(Fe_1-xNi_x)₂P(x=01,025,05)中Fe,Ni,P的K边进行了研究.结合多重散射理论近边计算,讨论了金属原子不同位置格点3f,3g对近边谱特征的贡献,得出当Ni原子取代Fe原子时将优先占据Fe(3f)格点位置;根据第一性原理对能态的计算发现,不考虑磁性时不同格点P的pDOS未占据态电子结构与P-K近边吸收谱实验相符合;与考虑铁磁性Fe₂P 的DOS相比较后结果显示Fe₂P的磁性主要来源于Fe(3g)格点,铁磁性Ni₂P计算的Ni不同格点原子磁矩均接近于0,与它一般显顺磁性结论相一致. 关键词： X射线近边吸收谱 电子结构 多重散射理论 态密度     

An attempt at kidney stone analysis with the application of synchrotron radiation

X‐ray absorption near‐edge spectroscopy (XANES) is a spectroscopic technique using synchrotron light to determine the valence state of excited atoms as well as the electronegativity of their neighbouring atoms. XANES spectra can provide information about the chemical bond in the second coordination shell of the excited atom. In this study, XANES spectra of unknown compounds from human kidney stones were recorded around the K‐edges of sulfur, phosphorus and calcium. The XANES results agree well with the diffractogram data of the same stones obtained through an X‐ray powder diffraction (XRPD) technique. By comparing the measurement techniques presented here, it is shown that XANES requires a smaller amount of each sample than XRPD for analysis.     

Assignment of pre‐edge peaks in K‐edge x‐ray absorption spectra of 3d transition metal compounds: electric dipole or quadrupole?

The characteristics of pre‐edge peaks in K‐edge x‐ray absorption near edge structure (XANES) spectra of 3d transition metals were reviewed from viewpoints of the selection rule, coordination number, number of d‐electrons, and symmetry of the coordination sphere. The contribution of the electric dipole and quadrupole transition to the peaks was discussed on the basis of the group theory, polarized spectra, and theoretical calculations. The pre‐edge peak intensity for T_d symmetry is larger than those for O_h symmetry for all 3d elements. The intense pre‐edge peak for tetrahedral species of 3d transition metals is not due to 1s–3d transition, but transition to the p component in d–p hybridized orbital. The mixing of metal 4p orbitals with the 3d orbitals depends strongly on the coordination symmetry, and the possibility is predictable by group theory. The transition of 1s electron to d orbitals is electric quadrupole component in any of the symmetries. The d–p hybridization does not occur with regular octahedral symmetry, and the weak pre‐edge peak consists of 1s–3d electric quadrupole transition. The pre‐edge peak intensity for a compound with a tetrahedral center changes as a function of the number of 3d electrons regardless of the kind of element; it is maximized at d⁰ and gradually decreases to zero at d¹⁰. The features of pre‐edge peaks in K‐edge XANES spectra for 4d elements and the L₁‐edge for 5d elements are analogous with those for 3d elements, but the pre‐edge peak is broadened due to the wide natural width of the core level. Copyright © 2008 John Wiley & Sons, Ltd.     

Sample thickness and quantitative concentration measurements in Br K‐edge XANES spectroscopy of organic materials

While XANES spectroscopy is an established tool for quantitative information on chemical structure and speciation, elemental concentrations are generally quantified by other methods. The edge step in XANES spectra represents the absolute amount of the measured element in the sample, but matrix effects and sample thickness complicate the extraction of accurate concentrations from XANES measurements, particularly at hard X‐ray energies where the X‐ray beam penetrates deeply into the sample. The present study demonstrates a method of quantifying concentration with a detection limit approaching 1 mg kg^?1 using information routinely collected in the course of a hard X‐ray XANES experiment. The XANES normalization procedure unambiguously separates the signal of the absorber from any source of background. The effects of sample thickness on edge steps at the bromine K‐edge were assessed and an empirical correction factor for use with samples of variable mass developed.     

X‐ray absorption study of the local structure at the NiO/oxide interfaces

This work reports an X‐ray absorption near‐edge structure (XANES) spectroscopy study at the Ni K‐edge in the early stages of growth of NiO on non‐ordered SiO₂, Al₂O₃ and MgO thin films substrates. Two different coverages of NiO on the substrates have been studied. The analysis of the XANES region shows that for high coverages (80 Eq‐ML) the spectra are similar to that of bulk NiO, being identical for all substrates. In contrast, for low coverages (1 Eq‐ML) the spectra differ from that of large coverages indicating that the local order around Ni is limited to the first two coordination shells. In addition, the results also suggest the formation of cross‐linking bonds Ni—O—M (M = Si, Al, Mg) at the interface.     

Investigation of annealing‐induced oxygen vacancies in the Co‐doped ZnO system by Co K‐edge XANES spectroscopy

To clarify the mechanism of the observed room‐temperature ferromagnetism (RTF), many studies have been focused on dilute magnetic semiconductor systems. Several investigations have demonstrated that oxygen vacancies play a significant role in mediating the RTF behavior so that much effort has been devoted to confirm their presence. In this investigation, X‐ray absorption spectroscopy was combined with ab initio calculations of the electronic structure of Co and Zn in the Zn_0.9Co_0.1O system before and after annealing, which has been recognized as an effective method of originating oxygen vacancies. A feature at about 20 eV after the rising edge of the Co K‐edge XANES that disappears after annealing has been associated with the presence of an oxygen vacancy located in the second shell surrounding the Co atom. Moreover, Zn K‐edge XANES spectra point out that this oxygen vacancy affects the electronic structure near the Fermi level, in agreement with density functional theory calculations.     

Finite lifetime broadening of calculated X‐ray absorption spectra: possible artefacts close to the edge

X‐ray absorption spectra calculated within an effective one‐electron approach have to be broadened to account for the finite lifetime of the core hole. For methods based on Green's function this can be achieved either by adding a small imaginary part to the energy or by convoluting the spectra on the real axis with a Lorentzian. By analyzing the Fe K‐ and L_2,3‐edge spectra it is demonstrated that these procedures lead to identical results only for energies higher than a few core‐level widths above the absorption edge. For energies close to the edge, spurious spectral features may appear if too much weight is put on broadening via the imaginary energy component. Special care should be taken for dichroic spectra at edges which comprise several exchange‐split core levels, such as the L₃‐edge of 3d transition metals.