Relaxations around the substitutional chromium site in emerald: X-ray absorption experiments and density functional calculations

We determine the local crystallographic and electronic structure around the chromium impurity in emerald (a chromium doped beryl) by combining ab initio density functional calculations and x-ray absorption experiments in both x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) regions. The isotropic and natural linear dichroic signals at the Cr K edge have been recorded using single crystals and a reliable method has been applied to remove diffraction peaks. The contributions of both electric dipolar and electric quadrupolar transitions in the XANES preedge are calculated and analyzed within group theory. This study reveals that the introduction of chromium in beryl leads to relaxations that are mainly local. The oxygen atoms in the coordination shell relax to a bond distance similar to that for Cr in ruby (alpha-Al2O3:Cr3+). Angular relaxations are found to affect essentially the chromium first neighbors.