EXAFS studies of Cr-doped mullite

Mullites doped with 7.3 (Cr 6) and 11.5 wt% Cr2O3 (Cr 10) were synthesized by reaction sintering of Al2O3, SiO2 and Cr2O3 powder compacts at 1650 degrees C in air. Prior to the spectroscopic analyses, the powder samples were HF/HCl-washed in order to remove coexisting glassy phases. According to X-ray diffractometry all samples consisted of mullite only. Measurements of the extended X-ray absorption fine structure (EXAFS) of the Cr K edge of mullite were performed at the Photon Factory, National Laboratory of High Energy Physics (KEK), in Tsukuba, Japan. The measured spectra were normalized by first subtracting a pre-edge background and then fitting a smoothly varying cubic-spline background in the region of the EXAFS. For all measured spectra the magnitude of the Fourier transform, which is related to the pair distribution function (PDF) and similar to it, is characterized by two pronounced peaks. The first peak is ascribed to oxygen making up the octahedra surrounding the Cr atoms and the second peak is assumed to be due to Al. It turned out that the second peak could not be fitted satisfactorily with one Al coordination shell alone. The discrepancy can be reduced by assuming that there is an additional contribution from Cr atoms that do not occupy regular lattice sites but are displaced by some amount. The existence of such displaced Cr atoms is known from EPR and crystal field spectroscopy experiments. Since these Cr atoms are not in the center of the Al coordination shell, the Al PDF seen by these Cr atoms is broadened and also slightly asymmetric. From our data analysis we find the displacement of the Cr atoms from the center of their surrounding aluminum coordination shell to be 0.50 Angstrom.