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.     

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.     

Vanadium K‐edge XANES in vanadium‐bearing model compounds: a full multiple scattering study

A systematic study is presented on a set of vanadium‐bearing model compounds, representative of the most common V coordination geometries and oxidation states, analysed by means of vanadium K‐edge X‐ray absorption near‐edge spectroscopy calculations in the full multiple scattering (FMS) framework. Analysis and calibration of the free parameters of the theory under the muffin‐tin approximation (muffin‐tin overlap and interstitial potential) have been carried out by fitting the experimental spectra using the MXAN program. The analysis shows a correlation of the fit parameters with the V coordination geometry and oxidation state. By making use of this correlation it is possible to approach the study of unknown V‐bearing compounds with useful preliminary information.     

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.     

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.     

A Li K‐edge XANES study of salts and minerals

The first comprehensive Li K‐edge XANES study of a varied suite of Li‐bearing minerals is presented. Drastic changes in the bonding environment for lithium are demonstrated and this can be monitored using the position and intensity of the main Li K‐absorption edge. The complex silicates confirm the assignment of the absorption edge to be a convolution of triply degenerate p‐like states as previously proposed for simple lithium compounds. The Li K‐edge position depends on the electronegativity of the element to which it is bound. The intensity of the first peak varies depending on the existence of a 2p electron and can be used to evaluate the degree of ionicity of the bond. The presence of a 2p electron results in a weak first‐peak intensity. The maximum intensity of the absorption edge shifts to lower energy with increasing SiO₂ content for the lithium aluminosilicate minerals. The bond length distortion of the lithium aluminosilicates decreases with increasing SiO₂ content, thus increased distortion leads to an increase in edge energy which measures lithium's electron affinity.     

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.     

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.     

Calibration method at the N K‐edge using interstitial nitrogen gas in solid‐state nitrogen‐containing inorganic compounds

The standard method of soft X‐ray beamline calibration at the N K‐edge uses the ν = 0 peak transition of gas‐phase N₂. Interstitial N₂ gas trapped or formed within widely available solid‐state ammonium‐ and amine‐containing salts can be used for this purpose, bypassing gas‐phase measurements. Evidence from non‐nitrogen‐containing compounds (KH₂PO₄) and from He‐purged ammonium salts suggest that production of N₂ gas is through beam‐induced decomposition. Compounds with nitrate or nitrite as anions produce coincident features and are not suitable for this calibration method.     

Quantitative local structure determination in mica crystals: ab initio simulations of polarization XANES at the potassium K‐edge

An attempt to refine the local structure of a layered structure such as mica is made by combining angle‐resolved XANES (AXANES) and single‐crystal X‐ray diffraction (SC‐XRD) experiments. Ab initio calculations of AXANES spectra of several tri‐octahedral micas have been used to further interpolate experimental data and to deduce physico/chemical effects. Structural distortions have been found highly correlated with the compositional disordering that arises from electronic interactions between anions and cations, and extend the interlayer entering deep into nearby tetrahedral and octahedral sheets. Multiple occupations at the same atomic site have been investigated in detail both in the parallel and perpendicular components of AXANES spectra. Finally, the best fit obtained, computed in the framework of the multiple‐scattering theory, is presented and the limitations of the muffin‐tin potential in layered systems are briefly discussed.     

Effect of atomic vibrations in XANES: polarization‐dependent damping of the fine structure at the Cu K‐edge of (creat)2CuCl4

Polarization‐dependent damping of the fine structure in the Cu K‐edge spectrum of creatinium tetrachlorocuprate [(creat)₂CuCl₄] in the X‐ray absorption near‐edge structure (XANES) region is shown to be due to atomic vibrations. These vibrations can be separated into two groups, depending on whether the respective atoms belong to the same molecular block; individual molecular blocks can be treated as semi‐rigid entities while the mutual positions of these blocks are subject to large mean relative displacements. The effect of vibrations can be efficiently included in XANES calculations by using the same formula as for static systems but with a modified free‐electron propagator which accounts for fluctuations in interatomic distances.     

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.     

Carbon K‐edge spectra of carbonate minerals

Carbon K‐edge X‐ray spectroscopy has been applied to the study of a wide range of organic samples, from polymers and coals to interstellar dust particles. Identification of carbonaceous materials within these samples is accomplished by the pattern of resonances in the 280–320 eV energy region. Carbonate minerals are often encountered in the study of natural samples, and have been identified by a distinctive resonance at 290.3 eV. Here C K‐edge and Ca L‐edge spectra from a range of carbonate minerals are presented. Although all carbonates exhibit a sharp 290 eV resonance, both the precise position of this resonance and the positions of other resonances vary among minerals. The relative strengths of the different carbonate resonances also vary with crystal orientation to the linearly polarized X‐ray beam. Intriguingly, several carbonate minerals also exhibit a strong 288.6 eV resonance, consistent with the position of a carbonyl resonance rather than carbonate. Calcite and aragonite, although indistinguishable spectrally at the C K‐edge, exhibited significantly different spectra at the Ca L‐edge. The distinctive spectral fingerprints of carbonates provide an identification tool, allowing for the examination of such processes as carbon sequestration in minerals, Mn substitution in marine calcium carbonates (dolomitization) and serpentinization of basalts.     

The status of strontium in biological apatites: an XANES/EXAFS investigation

Osteoporosis represents a major public health problem through its association with fragility fractures. The public health burden of osteoporotic fractures will rise in future generations, due in part to an increase in life expectancy. Strontium‐based drugs have been shown to increase bone mass in postmenopausal osteoporosis patients and to reduce fracture risk but the molecular mechanisms of the action of these Sr‐based drugs are not totally elucidated. The local environment of Sr²⁺ cations in biological apatites present in pathological and physiological calcifications in patients without such Sr‐based drugs has been assessed. In this investigation, X‐ray absorption spectra have been collected for 17 pathological and physiological calcifications. These experimental data have been combined with a set of numerical simulations using the ab initioFEFF9 X‐ray spectroscopy program which takes into account possible distortion and Ca/Sr substitution in the environment of the Sr²⁺ cations. For selected samples, Fourier transforms of the EXAFS modulations have been performed. The complete set of experimental data collected on 17 samples indicates that there is no relationship between the nature of the calcification (physiological and pathological) and the adsorption mode of Sr²⁺ cations (simple adsorption or insertion). Such structural considerations have medical implications. Pathological and physiological calcifications correspond to two very different preparation procedures but are associated with the same localization of Sr²⁺versus apatite crystals. Based on this study, it seems that for supplementation of Sr at low concentration, Sr²⁺ cations will be localized into the apatite network.     

Investigation of damaged interior walls using synchrotron‐based XPS and XANES

Evidence of internal sulfate attack in field exposure was demonstrated by the damaged interior wall of a three‐year‐old house situated in Nakhon Ratchasima Province, Thailand. Partial distension of the mortar was clearly observed together with an expansion of a black substance. Removal of the black substance revealed a dense black layer. This layer was only found in the vicinity of the damaged area, suggesting that this black material is possibly involved in the wall cracking. By employing synchrotron‐based X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption near‐edge structure (XANES) techniques, the unknown sample was chemically identified. The S 2p and O 1s XPS results mutually indicated the existence of sulfate species in the materials collected from the damaged area. The XANES results indicated the presence of ferrous (II) sulfate, confirming sulfate‐induced expansion and cracking. The sulfate attack in the present case appeared to physically affect the structure whereas the chemical integrity at the molecular level of the calcium silicate hydrate phase was retained since there was a lack of spectroscopic evidence for calcium sulfate. It was speculated that internal sulfate probably originated from the contaminated aggregates used during the construction. The current findings would be beneficial for understanding the sulfate‐attack mechanism as well as for future prevention against sulfate attack during construction.     

Phosphorus L2,3‐edge XANES: overview of reference compounds

Synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy is becoming an increasingly used tool for the element speciation in complex samples. For phosphorus (P) almost all XANES measurements have been carried out at the K‐edge. The small number of distinctive features at the P K‐edge makes in some cases the identification of different P forms difficult or impossible. As indicated by a few previous studies, the P L_2,3‐edge spectra were richer in spectral features than those of the P K‐edge. However, experimentally consistent spectra of a wide range of reference compounds have not been published so far. In this study a library of spectral features is presented for a number of mineral P, organic P and P‐bearing minerals for fingerprinting identification. Furthermore, the effect of radiation damage is shown for three compounds and measures are proposed to reduce it. The spectra library provided lays a basis for the identification of individual P forms in samples of unknown composition for a variety of scientific areas.     

Solid‐phase cadmium speciation in soil using L3‐edge XANES spectroscopy with partial least‐squares regression

Cadmium (Cd) has a high toxicity and resolving its speciation in soil is challenging but essential for estimating the environmental risk. In this study partial least‐square (PLS) regression was tested for its capability to deconvolute Cd L₃‐edge X‐ray absorption near‐edge structure (XANES) spectra of multi‐compound mixtures. For this, a library of Cd reference compound spectra and a spectrum of a soil sample were acquired. A good coefficient of determination (R²) of Cd compounds in mixtures was obtained for the PLS model using binary and ternary mixtures of various Cd reference compounds proving the validity of this approach. In order to describe complex systems like soil, multi‐compound mixtures of a variety of Cd compounds must be included in the PLS model. The obtained PLS regression model was then applied to a highly Cd‐contaminated soil revealing Cd₃(PO₄)₂ (36.1%), Cd(NO₃)₂·4H₂O (24.5%), Cd(OH)₂ (21.7%), CdCO₃ (17.1%) and CdCl₂ (0.4%). These preliminary results proved that PLS regression is a promising approach for a direct determination of Cd speciation in the solid phase of a soil sample.     

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.     

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.