Influence of impurities on Cr (super 3+) luminescence properties in Brazilian emerald and alexandrite

Emerald and alexandrite, the chromiferous varieties of beryl, Be (sub 3) Al (sub 2) [Si (sub 6) O (sub 18) ], and chrysoberyl, BeAl (sub 2) O (sub 4) , respectively, are, as gems, of high economical and technological interest. The Cr (super 3+) 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 Cr (super 3+) crystal field value is linked to the Cr content and decreases when Cr increases. In chrysoberyl with low Cr content, the vanishing of Cr (super 3+) emission from Cr (super 3+) ions located on the inversion-symmetry site suggests a competition between Cr and V regarding chemical substitution. The Fe (super 3+) ion substitutes efficiently on the mirror-symmetry site, with a strong impact on the Cr (super 3+) lifetime on this site.