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

X-ray absorption spectra at the chromium K-edge are reported for a number of selected chromium compounds of known chemical structure. The spectra were obtained with use of synchrotron radiation available at the ectron tretcher ccelerator ELSA in Bonn. The compounds studied include the tetrahedrally coordinated compounds Ca₂Ge_0.8Cr_0.2O₄, Ba₂Ge_0.1Cr_0.9O₄, Sr₂CrO₄, Ca₂(PO₄)_x(CrO₄)_1?xCl (x=0.25,0.5), Ca₅(CrO₄)₃Cl, CrO₃, the octahedrally coordinated compounds Cr(II)-acetate, CrCl₃, CrF₃, Cr₂O₃, KCr(SO₄)₂ · 12H₂O, CrO₂ and cubic coordinated metallic chromium. In these compounds chromium exhibits a wide range of formal oxidation states (0 to VI). The absorption features in the near edge region are shown to be characteristic of the spatial environment of the absorbing atom. The occurrence of a single pre-edge line easily allows one to distinguish between tetrahedral and octahedral coordination geometry, whereas the energy position of the absorption edge is found to be very sensitive to the valency of the excited chromium atom. Calculations of the ionisation potential of Cr in different oxidation states using the non-relativistic Hartree–Fock method (Froese–Fischer) confirm that the ionisation limit shifts to higher energy with increasing Cr valency. More detailed information on the electronic structure of the different compounds is gained by real-space full multiple scattering calculations using the FEFF8 code.