Site preference of iron ions in α-alumina: An Fe-K XAFS study on specimens prepared by a combined self-propagating chemical combustion and sintering technique

Abstract

Iron-doped α-Al₂O₃ polycrystals were prepared by self-propagating chemical combustion (SPCC), followed by a brief crystallisation and sintering period under defined oxygen activity. In contrast to techniques using milled and compressed polycrystalline powders, this combustion–sintering method allows the preparation of homogeneously doped alumina 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 fine structures obtained by multiple-scattering cluster simulations. The resulting model of the local structure around the iron was then refined with respect to the nearest oxygen coordination shell by carrying out a multiple-shell EXAFS analysis. Additionally, the valence and the coordination geometry of the dopant was determined by a comparative X-ray absorption near-edge structure (XANES) study using standards containing either di- or trivalent iron ions with different coordinative environments. Iron was found to be trivalent, in the high-spin state and to occupy "‘out-of-centre’' displaced aluminium lattice sites preferentially.