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Structure locale autour des impuretés dans les gemmes, étudiée par spectroscopies et calculs ab initio

Abstract : Colorless corundum (alpha-Al_2O_3) and beryl (Be_3Si_6Al_2O_18) are colored when transition metal ions are present as impurities in the structure. Ruby (alpha-Al_2O_3:Cr) is red, emerald (Be_3Si_6Al_2O_18:Cr) is green. Corundum doped with iron is yellow, corundum doped with titanium is pink. Both iron and
titanium are present in the blue sapphire (alpha-Al_2O_3:Fe-Ti). Color is generally interpreted within the cristal field theory. The different colors of doped corundum and beryl should come from a specific crystal field around each coloring ion. This specific crystal field should be linked to a precise local structure around the coloring ion.

We want to describe precisely the local structure around impurities in corundum and beryl. To do that, we have registered X-ray absorption spectra (EXAFS and XANES) and we have made ab initio calculations. The local structure around impurities deduced from our experiments (EXAFS) are compared to ab initio energy minimisation calculations. We have used an efficient method to extract structural and electronic information from XANES. All of these calculations use the density functional theory.

This study reveals that the local structure around an impurity M in corundum is similar to the local structure around M in alpha-M_2O_3. Relaxations due to the introduction of the impurity are very local in nature.
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Contributor : Emilie Gaudry Connect in order to contact the contributor
Submitted on : Wednesday, February 14, 2007 - 9:30:30 PM
Last modification on : Thursday, March 25, 2021 - 3:01:39 AM
Long-term archiving on: : Tuesday, April 6, 2010 - 10:32:14 PM



  • HAL Id : tel-00131051, version 1


Emilie Gaudry. Structure locale autour des impuretés dans les gemmes, étudiée par spectroscopies et calculs ab initio. Matériaux. Université Pierre et Marie Curie - Paris VI, 2004. Français. ⟨tel-00131051⟩



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