Ab initio calculation of the CrK edge in alpha-Al2O3 : Cr3+

The red color of ruby (alpha-Al2O3:Cr3+) is assigned to transitions between 3d states of the chromium ion. A precise description of the structural and electronic environment of the Cr3+ ion in its slightly distorted octahedral site (C-3 symmetry) is needed to understand the cause of the color. We have analysed the Cr K-edge in alpha-Al2O3:Cr3+ with a reciprocal space approach that allows the computation of self-consistent spin polarized charge density for large unit cells, beyond the traditional ``muffin-tin'' approximation. Structural and electronic information is deduced from the calculation.