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distance for Be3Si6Al2O18:Cr 3+ and Al2O3:Cr 3+ is found to be practically equal to 1.97 ˚ A, in agreement with recent experimental values. The present calculations confirm that the variations of optical properties due to Cr 3+ impurities along the series of ionic oxides can be understood merely through the CrO6 9− unit but subject to the electric field due to the rest of the lattice ions. As a salient feature it is proved that changes in electronic density and covalency due to the internal field are not the cause of the color shift. Therefore, the red color of ruby is not due to the polarization of the electronic cloud around chromium as a result of the C3 local symmetry. The present study also demonstrates that the variation of the ligand field splitting parameter, 10Dq, induced by the internal electric field comes mainly from the contributions of first shells of ions around the CrO6 9− unit. As a consequence, 10Dq in emerald is not influenced by the internal field, as the contribution from Be 2+ first neighbors is practically compensated by that of Si 4+ second neighbors. In contrast, in ruby the t2g levels are shifted by the internal field 0.24 eV more than the eg ones, so explaining the color shift in this gemstone in comparison with emerald. This result is shown to arise partially from the asymmetric form of the internal electrostatic potential along the C3 axis in Al2O3.

physical review b

Internal electric fields and color shift in Cr<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow /><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>-based gemstones

Internal electric fields and color shift in Cr3+-based gemstones