Electronic and Chemical State of Aluminum from the Single- (K) and Double-Electron Excitation (KL_II&III, KL_I) X-ray Absorption Near-Edge Spectra of α-Alumina, Sodium Aluminate, Aqueous Al³⁺·(H₂O)₆, and Aqueous Al(OH)₄^–

Abstract

We probe, at high energy resolution, the double electron excitation (KLII&II) X-ray absorption region that lies approximately 115 eV above the main Al K-edge (1566 eV) of α-alumina and sodium aluminate. The two solid standards, α-alumina (octahedral) and sodium aluminate (tetrahedral), are compared to aqueous species that have the same Al coordination symmetries, Al(3+)·6H2O (octahedral) and Al(OH)4(-) (tetrahedral). For the octahedral species, the edge height of the KLII&III-edge is approximately 10% of the main K-edge; however, the edge height is much weaker (3% of K-edge height) for Al species with tetrahedral symmetry. For the α-alumina and aqueous Al(3+)·6H2O the KLII&III spectra contain white line features and extended absorption fine structure (EXAFS) that mimics the K-edge spectra. The KLII&III-edge feature interferes with an important region in the EXAFS spectra of the crystalline and aqueous standards. The K-edge spectra and K-edge energy positions are predicted using time-dependent density functional theory (TDDFT). The TDDFT calculations for the K-edge X-ray absorption near-edge spectra (XANES) reproduce the observed transitions in the experimental spectra of the four Al species. The KLII&II and KLI onsets and their corresponding chemical shifts for the four standards are estimated using the delta self-consistent field (ΔSCF) method.

Supersaturated calcium carbonate solutions are classical

“…The BHLYP ( 50 , 58 ) exchange-correlation functional was used for all calculations. This approach has been successfully used in several studies including the K-edges of oxygen, carbon, and fluorine in a number of molecular systems ( 32 ); ruthenium L 3 -edge in [Ru(NH 3 ) 6 ] 3+ ( 32 ); ruthenium L 3 -edge in a series of model Ru(II) and Ru(III) complexes and mixed-valence metal (Ru/Fe) dimers ( 59 ); K-edge spectra of oxygen, nitrogen, and sulfur in cysteine ( 60 ); dissolved lithium polysulfide species in Li–S batteries ( 61 ); Al K-edge studies of the aluminum distribution in zeolites ( 62 ); alpha-alumina, sodium aluminate, and aqueous Al 3+ species ( 33 ); Cl K-edge spectra in actinide hexachloride complexes ( 63 ); Na K-edge studies of the hydration structure of Na + ( 13 ); and the valence-to-core x-ray emission in transition metal complexes ( 64 ).…”

Revisiting the hydration structure of aqueous Na+

“…For the liquid samples, a membrane-window cell was used that is similar to what has been previously described 20 that incorporated a 200 nm thick Si 3 N 4 . Liquid transfer lines extended outside of the vacuum chamber for ease of sample introduction.…”

“…These features appear in the χ(k) data at k = 3.9 and 5.1 Å ☞1 and they correspond to the KL II,III , and KL I transitions, respectively, that are similar to those observed for aqueous Al 3+ . 20 While the KL II,III transition is relatively strong and easy to identify, the KL I transition is much weaker and the relative edge height is difficult to discern. Although the k range is relatively short, the Na-O distance is well resolved.…”

“…X-ray absorption fine structure spectroscopy (XAFS) is another method that effectively probes these length scales, and in particular, the extended-XAFS (EXAFS) region is important since it contains the photoelectron backscattering processes that are due to spatial distribution of atoms in the sample. While EXAFS has been used to measure ion hydration structure with the same spatial resolution as XRD and ND, it is rarely been applied to light element ions [17][18][19][20] since the low K edge energies create challenges for transmission through solutions and x-ray windows. In this study, we use the latest generation of low energy beamlines to acquire the first EXAFS spectra for aqueous Na + .…”

Inference of principal species in caustic aluminate solutions through solid-state spectroscopic characterization

“…The relatively small energy difference between tetrahedral and octahedral Al (ca. 2 eV), paired with structural diversity of previously studied materials leads to uncertainty in assignment of the coordination number of 3 66. However, the extensive characterization discussed above, and presence of the 1581.7 eV feature in the XANES spectrum of 3 indicate that the 1568.6 eV edge maximum correspond to tetrahedrally coordinated Al.…”