SITE PREFERENCE OF IRON IONS IN ALPHA-ALUMINA - AN FE-K XAFS STUDY ONSPECIMENS PREPARED BY A COMBINED SELF-PROPAGATING CHEMICAL COMBUSTIONAND SINTERING TECHNIQUE

Iran-doped alpha-Al2O3 polycrystals were prepared by self-propagating chemical combustion (SPCC), followed by a brief crystallisation and si ntering period under defined oxygen activity. In contrast to technique s using milled and compressed polycrystalline powders, this combustion -sintering method allows the preparation of homogeneously doped alumin a polycrystals in less than 50 h. To determine the site preference and valence of iron dissolved in the oxide matrix, the complex Fe K-edge extended X-ray absorption fine structure (EXAFS) was compared with fin e structures obtained by multiple-scattering cluster simulations. The resulting model of the local structure around the iron was then refine d with respect to the nearest oxygen coordination shell by carrying ou t a multiple-shell EXAFS analysis. Additionally, the valence and the c oordination geometry of the dopant was determined by a comparative X-r ay absorption near-edge structure (XANES) study using standards contai ning either di- or trivalent iron ions with different coordinative env ironments. Iron was found to be trivalent, in the high-spin state and to occupy ''out-of-centre'' displaced aluminium lattice sites preferen tially.