Thirteen ways to read a XANES spectrum
The near-edge structure (XANES) of an X-ray absorption spectrum can be used to discuss the local chemical environment of the absorbing atom. The thirteen questions below are grouped as structure and speciation, quantitative analysis, and how a spectrum changes.
The thirteen questions
| Approach | What is observed | Example or question |
|---|---|---|
| Coordination | edge and near-edge shape | separate valence, coordination number, or geometry |
| Crystallinity | spectral shape | crystalline versus amorphous $\mathrm{SiO_2}$ |
| Oxidation state | edge shift and shape | sulfur species; the source reports a shift of about 11 eV |
| Oxidation state and edge position | edge energy | Re metal and the +4, +6, and +7 oxides |
| Mixed phases | a sum of reference spectra | $\mathrm{V_2O_3}$, $\mathrm{V_2O_5}$, and a vanadium sample |
| Redox evolution | shape versus time | $\mathrm{Cr^{3+}/Cr^{6+}}$ during an in-situ reaction |
| Ligand | a shift from the local chemical environment | $\mathrm{As^{3+}}$ and $\mathrm{As^{5+}}$ in water, and with methionine |
| Spectral grouping | pre-edge height and position | group samples by coordination |
| Structural disorder | near-edge detail and the pre-edge | $\mathrm{EuTiO_3}$ versus $\mathrm{PbTiO_3}$ |
| Linear combination fitting | fractions of reference spectra | can known species explain an unknown spectrum? |
| Peak fitting | a step plus peak functions | the C K-edge near-edge shape |
| Principal component analysis | statistically significant components | how many independent changes are in a series |
| Difference spectra | small changes between spectra | XMCD, and before/after a reaction |
1. Structure and speciation
Coordination
Coordination number and geometry change the XANES shape. The source uses the Cr K-edge and compares potassium dichromate, $\mathrm{Cr(VI)}$ as $\mathrm{K_2Cr_2O_7}$, with the $\mathrm{Cr(III)}$ oxide $\mathrm{Cr_2O_3}$, discussing four-coordinate and six-coordinate environments.
The figure legend writes the dichromate as “$\mathrm{K_2CrO_7}$”, which does not match the species in the text. The text uses the ordinary formula $\mathrm{K_2Cr_2O_7}$. The image file is unchanged. Check the original label.
Crystallinity
Crystalline and amorphous $\mathrm{SiO_2}$ can be told apart by XANES shape. The source shows Quartz and Silica reference spectra.
Oxidation state and edge position
From $\mathrm{S^{2-}}$ to $\mathrm{S^{6+}}$, the source reports a shift of about 11 eV. Sulfur speciation is relevant to life science, catalysis, petroleum, photovoltaics, and environmental work. DOI: 10.1021/ja00191a012.
The source also compares Re metal with Re oxides in the +4, +6, and +7 states. As the atomic charge changes, core-electron screening and the binding energy change, and the edge moves with them. DOI: 10.1107/S0909049510006230.
Mixed phases and coordination groups
Placing $\mathrm{V_2O_3}$, $\mathrm{V_2O_5}$, and an unknown vanadium compound on one plot shows differences in valence and coordination. The source treats a weaker pre-edge and an edge that moves to lower energy as spectral signs of reduction.
For Ti K-edge data on zircon, plotting pre-edge height against pre-edge position separates coordination environments into clusters. DOI: 10.1016/0016-7037(96)00144-5.
Ligands and the local chemical environment
Aqueous $\mathrm{As^{3+}}$ and $\mathrm{As^{5+}}$ differ at the edge. The source also compares the same pair after methionine is added, and notes that the direction of the shift is not the same as for the aqueous references.
The source also compares the Fe K-edges of hematite ($\mathrm{Fe_2O_3}$) and pyrite ($\mathrm{FeS_2}$). The shape difference is used to say that the ligand and the neighbouring chemistry also affect the near-edge features.
Structural disorder
Near-edge detail can be used to discuss local disorder. The source compares cubic perovskite $\mathrm{EuTiO_3}$ with tetragonally distorted $\mathrm{PbTiO_3}$ and notes a stronger pre-edge on $\mathrm{PbTiO_3}$. DOI: 10.1080/00150199808009173.
2. Reaction progress and quantitative analysis
Redox versus time
In-situ XANES can follow the edge through a reaction. The source shows $\mathrm{Cr^{3+}}$ converting toward $\mathrm{Cr^{6+}}$ within 4 minutes, with each spectrum collected in 3 s. Temperature ramps and electrochemical cycling are listed as other in-situ settings. DOI: 10.1021/es901759w.
Linear combination fitting
Linear combination fitting (LCF) writes an unknown spectrum as a linear combination of two or more known references. It is used to ask whether those known species can account for the measured shape.
Peak fitting
A spectrum can be fit as a step plus peak functions. The source lists an atan or erfc step, and Gaussian, Lorentzian, or Voigt peaks. For a C K-edge XANES, the peaks discussed are the main $1s \rightarrow \pi^{}$ transition, Rydberg or mixed-valence transitions, and a higher-energy $1s \rightarrow 2\pi^{}$ transition. This kind of analysis is more informative on a related series than on a single spectrum, and the physical meaning of a chosen peak shape is still limited. DOI: 10.1016/j.orggeochem.2004.10.011.
Principal component analysis
Principal component analysis (PCA) decomposes a set of related spectra into statistical components. Those components can be used to build reference spectra and to test whether a species is present in the set. The source links the number of statistically significant components to the number of species in the data. DOI: 10.1051/jp4/1997163.
Difference spectra
A difference spectrum highlights small changes along a series. X-ray magnetic circular dichroism (XMCD) is one common use; the difference can be used to discuss magnetic moment and magnetic order. The source also shows the density of states of Pt nanoparticles before and after a hydrogenation treatment. DOI: 10.1209/epl/i1998-00359-2.