XANES calibrations for the oxidation state of iron in a silicate glass

Abstract

Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57 Fe 2 O 3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 ∞C. The Fe 3+ /SFe ratios were determined previously by 57 Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe 3+ /SFe ratios from XANES spectra were investigated. The energy of the 1s AE 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s AE 4s and crest (1s AE 4p) transitions. The Fe 3+ /SFe ratios determined from linear combinations of end-member spectra (Fe 3+ /SFe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe 2+ or Fe 3+ coordination with the Fe 3+ /SFe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe 3+ /SFe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

A XANES study of chromophores in archaeological glass

“…In particular, the intensity of the pre-edge peak varies considerably as a function of the coordination environment and symmetry, while the pre-edge energy position is strongly influenced, besides the bond distances, by the oxidation state, being the centroid of the pre-edge peak of the Fe 3+ rich minerals and glass varieties shifted towards higher energy with respect to those containing iron in the reduced form [19,21,33,34,35,36]. Hence, the centroid energy position is largely used in literature for estimating iron oxidation state.…”

Limitations of Near Edge X-ray Absorption Fine Structure as a tool for observing conduction bands in chalcopyrite solar cell heterojunctions

“…These include fitting the the crest of the curve immediately after the absorption edge with the initial-state core level [9], extrapolation of the absorption edge to the background [10,11], the maximum of its first derivative [17] as well as consideration of a fitted density of states calculation. In addition, the second derivative can also be used, although this is often in an attempt to differentiate two peaks separated by an energy close to that of the experimental resolution [18].…”

Fe K-edge X-ray absorption spectroscopy study of Pb(Fe2/3W1/3)O3-PbTiO3 multiferroic ceramics

“…As a result, quadrupole transitions are important for centrosymmetric sites, resulting in pre-edge peaks less prominent, while in other cases (no center of inversion) dipolar electronic transitions are responsible for a greater intensity in this region of the spectrum. 18 Thus, transitions to the tetrahedrally coordinated Fe are more intense than transitions in Fe in octahedral coordination. 18 As can be seen in Figures 1(a) and 1(b), no changes are observed on the XANES spectra of PFW and PFW-PT samples with the increasing of temperature or the PT content.…”

“…2 The XANES spectrum of the Fe K-edge comprises peaks which have been identified as transitions between bound states: 1s to 3d (peak in the pre-edge around 7113 eV), 1s to 4s (shoulder in absorption edge), and 1s to 4p (peak after the edge). 18 The peak located in the pre-edge region, related to 1s to 3d transitions, can be attributed to electronic dipole or quadrupole transitions, which are usually up to 100 times weaker than the dipole transitions. 18 This dipole transitions to 3d states should be forbidden but gains intensity due to mixing with 4p orbitals whose dipole transitions are allowed.…”

“…18 The peak located in the pre-edge region, related to 1s to 3d transitions, can be attributed to electronic dipole or quadrupole transitions, which are usually up to 100 times weaker than the dipole transitions. 18 This dipole transitions to 3d states should be forbidden but gains intensity due to mixing with 4p orbitals whose dipole transitions are allowed. This mixture of 3d-4p orbital is increased by the coordination environment of low symmetry.…”