XANES- and EXAFS-investigations on chromium-doped mullite precursors

Chromium-doped non-crystalline mullite precursors for ceramics were investigated with x-ray absorption spectroscopy at the Cr K edge. They were prepared using a sol-gel-route. 3 wt% Cr2O3 were added to partially substitute aluminium by chromium in the aluminosilicate network. The aim of the study was to characterize the development of the electronic and geometric structure of the precursor at different temperatures prior to its crystallization to mullite. The x-ray absorption spectra of the precursors and of several reference compounds were taken using monochromatized synchrotron radiation at the storage ring ELSA in Bonn, Germany, X-ray absorption near edge structure (XANES) measurements showed that the oxidation number of chromium shifts towards lower values with increasing temperature. Analogously, a transition from tetrahedral to octahedral coordination symmetry takes place. It was observed by extended x-ray absorption fine structure (EXAFS) measurements that interatomic distances concerning the first coordination shell increase with increasing temperature. Changes in the EXAFS amplitude and the appearance of a second coordination shell reveal a development of the short range ordered mullite towards a crystalline mullite.