From the green color of eskolaite to the red color of ruby: an X-ray absorption spectroscopy study

The best known cause for colors in insulating minerals is due to transition metal ions as impurities. As an example, Cr3+ is responsible for the red color of ruby (alpha-Al2O3: Cr3+) and the green color of eskolaite (alpha-Cr2O3). Using X-ray absorption measurements, we connect the colors of the CrxAl2-xO3 series with the structural and electronic local environment around Cr. UV - VIS electronic parameters, such as the crystal field and the Racah parameter B, are related to those deduced from the analysis of the isotropic and XMCD spectra at the Cr L-2,L-3-edges in Cr0.07Al1.93O3 and eskolaite. The Cr - O bond lengths are extracted by EXAFS at the Cr K-edge in the whole CrxAl2-xO3 (0.07 <= x< 2) solid solution series. The variation of the mean Cr - O distance between Cr0.07Al1.93O3 and alpha-Cr2O3 is evaluated to be 0.01(5) angstrom (approximate to 1%). The variation of the crystal field in the CrxAl2-xO3 series is discussed in relation with the variation of the averaged Cr - O distances.