Structural and electronic relaxations around substitutional Cr3+ and Fe3+ ions in corundum

International audienceThe local arrangement of atoms surrounding a substitutional Cr3+ and Fe3+ ion in α-Al2O3: Cr3+ (ruby) and in α-Al2O3:(Fe−Ti) (blue sapphire) has been studied experimentally using natural x-ray linear dichroism absorption at the Cr and Fe K edge. The isotropic and dichroic signals have been recorded using single crystals and a reliable method has been applied to remove diffraction peaks. Results given by the analysis of both signals are compared with ab initio density functional calculations. This study reveals that the introduction of an impurity ion in the structure leads to relaxations that are very local in nature. The oxygen atoms in the coordination shell relax to an arrangement similar to that for Cr in α-Cr2O3 or for Fe in α-Fe2O3. Aluminum or oxygen atoms at a farther distance are weakly affected by the presence of the impurity