Influence of impurities on Cr³⁺ luminescence properties in Brazilian emerald and alexandrite

Emerald and alexandrite, the chromiferous varieties of beryl, Be₃Al₂[Si₆O₁₈], and chrysoberyl, BeAl₂O₄, respectively, are, as gems, of high economical and technological interest. The Cr³⁺ properties in natural beryl and chrysoberyl samples have been studied by mainly photoluminescence technique and electron paramagnetic resonance (EPR) as a function of Cr content (50-11370 ppm) as well as impurities, such as Fe and V. In emeralds, the Cr³⁺ crystal field value is linked to the Cr content and decreases when Cr increases. In chrysoberyl with low Cr content, the vanishing of Cr³⁺ emission from Cr³⁺ ions located on the inversion-symmetry site suggests a competition between Cr and V regarding chemical substitution. The Fe³⁺ ion substitutes efficiently on the mirrorsymmetry site, with a strong impact on the Cr³⁺ lifetime on this site.