Determination of co-ordination numbers and oxidation states in chromium oxides by extended X-ray absorption fine structure spectroscopy

Abstract

Chromium and iron K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements for several phases in the Sr₂Cr_xFe_{2 –x}O_{5 +y} system have established that there are substantial numbers of chromium atoms with either short (ca. 1.65 Å) or long (ca. 1.91 Å) distances to oxygen. These can be attributed to high oxidation states of chromium in tetrahedral co-ordination and Cr³⁺ ions in octahedral co-ordination, respectively. There is also some evidence for chromium in edge-sharing polyhedra, but not in face-sharing polyhedra, while the iron is in corner-sharing polyhedra. Possible model structures for the 15R phase are discussed.