Study of calcium–chromium–iron oxides by extended X-ray absorption fine structure spectroscopy

Abstract

Chromium K-edge EXAFS and XANES measurements for several high-temperature phases in the Ca₂Cr_xFe_{2 –x}O_{5 +y} system have established that there are substantial numbers of chromium atoms with either short (ca. 1.68 Å) or long (ca. 1.88 Å) distances to oxygen. These can be attributed to Cr⁶⁺ ions in tetrahedral co-ordination and Cr³⁺ ions in octahedral co-ordination, respectively. The results are correlated with earlier X-ray and Mössbauer data, and support the description of these materials as intergrowths of brownmillerite-related ordered vacancy structures. They differ significantly from the corresponding strontium materials.