The influence of coordination geometry and valency on the K-edge absorption near edge spectra of selected chromium compounds

Soil humic acids may favour the persistence of hexavalent chromium in soil

“…2a and b. The shape of the K-edge and the pre-edge resonances are characteristic for the local symmetry of the investigated atom and can be used as fingerprints in identification of its local structure (Arčon et al, 1998;Bianconi et al, 1991;Lytle et al, 1988;Pantelouris et al, 2004;Wong et al, 1984). Tetrahedrally coordinated Cr(VI) cations with 3d 0 ground state electronic configuration in CaCrO 4 , Ca 10 Cr 6 O 25 and in CrO 4 2À cluster in aqueous solution exhibit a characteristic isolated pre-edge peak, which can be assigned to the electron transition from Cr 1s core level to the lowest unoccupied tetrahedral molecular orbital 7t 2 (Arčon et al, 1998;Bianconi et al, 1991).…”

“…Tetrahedrally coordinated Cr(VI) cations with 3d 0 ground state electronic configuration in CaCrO 4 , Ca 10 Cr 6 O 25 and in CrO 4 2À cluster in aqueous solution exhibit a characteristic isolated pre-edge peak, which can be assigned to the electron transition from Cr 1s core level to the lowest unoccupied tetrahedral molecular orbital 7t 2 (Arčon et al, 1998;Bianconi et al, 1991). On the other hand, octahedrally coordinated Cr(III) cations in Cr 2 O 3 that possess an inversion centre, exhibit a different edge shape with two weak resonances in the pre-edge region assigned to transitions of 1s electron into unoccupied t 2g and e g bands with octahedral symmetry (Pantelouris et al, 2004). In addition, the valence state of Cr cations can be deduced from the energy shift of the absorption edge or from the pre-edge absorption features (Arčon et al, 1998;Pantelouris et al, 2004).…”

“…On the other hand, octahedrally coordinated Cr(III) cations in Cr 2 O 3 that possess an inversion centre, exhibit a different edge shape with two weak resonances in the pre-edge region assigned to transitions of 1s electron into unoccupied t 2g and e g bands with octahedral symmetry (Pantelouris et al, 2004). In addition, the valence state of Cr cations can be deduced from the energy shift of the absorption edge or from the pre-edge absorption features (Arčon et al, 1998;Pantelouris et al, 2004). From the spectra of the Cr 2 O 3 , Ca 10 Cr 6 O 25 and CaCrO 4 reference samples which display III, V and VI chromium oxidation states respectively, we found Cr K-edge shifts of about 3 eV per valence state, while the shift of the tetrahedral pre-edge resonance (in Ca 10 Cr 6 O 25 and CaCrO 4 ) was 1.2 eV per valence state, in agreement with previous observations (Arčon et al, 1998;Wong et al, 1984).…”

The valence and site occupancy of substituting metals in magnetite spinel structure Fe3−xMxO4 (M = Cr, Mn, Co and Ni) and their influence on thermal stability: An XANES and TG-DSC investigation

“…Generally, the energy positions of the XANES spectra depend on the binding energy of the absorbing atom, and hence on the oxidation state, but also on other parameters, such as the nature and number of nearest neighbors [30]. A linear relation between the edge shift and the valence state has been established for several cations in samples with the nearest neighbors of the same chemical species.…”

“…2). For Cr 2 O 3 , Cr atom occupies a distorted octahedral site, with three oxygen atoms as the nearest coordination (R CreO ¼ 0.196 nm) and other three oxygen atoms as the next nearest coordination (R CreO ¼ 0.211 nm) [30], and its XANES spectrum shows a weak pre-edge peak. For CrO 3 , Cr atom occupies a distorted tetrahedral site, with two oxygen atoms as the nearest coordination (R CreO ¼ 0.158 nm) and other two oxygen atoms as the next nearest coordination (R CreO ¼ 0.175 nm), and a strong pre-edge peak can be observed.…”