Abstract
In this paper, the results of investigation of luminescent properties of topazes from Ouro Preto, Brazil are presented. It is established that all photoluminescence characteristics of variously colored topazes are due to three structurally non-equivalent Cr3+ centers isomorphically substituting Al3+ in the topaz structure and forming [CrO4F2]7−, [CrO4OH,F]7−, and [CrO4(OH)2]7− complexes. Kinetics of thermal annealing indicates different thermal stability of these centers. Less stable [CrO4OH, F]7−- and [CrO4(OH)2]7− complexes diminish in temperature range 950–1,100°C, accompanied by appearance of corundum phase. The most stable [CrO4F2]7− centers completely decay at 1,250°C and luminescence spectrum of product obtained becomes identical to that of Cr3+ in mullite indicating that topaz completely transforms to mullite.
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Notes
Although the results obtained are measured on different samples, K1–K6, their luminescence characteristics are found to be qualitatively practically identical differing, due to different chromium contents, only by intensity of the emission. As will be shown below, due to somewhat different state of chromium in differently colored samples (valence, clustering), relatively weak differences were observed only in kinetics of thermal transformations of chromium-bearing luminescence centers.
We could not observe such narrowing of the spin-allowed bands of Cr3+ in optical absorption spectra (Taran et al. 2003) because at high-temperature annealing the samples become non-transparent (foggy) and, thus inappropriate for optical absorption measurements.
As far as we are aware, polarized spectrum of Cr3+-doped mullite is published for the first time.
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Acknowledgments
G.M. da Costa (Ouro Preto) kindly provided data on thermal treatment of topaz. The Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, provided research stipendium to M.N.T. We are grateful to these individual and institution.
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Tarashchan, A.N., Taran, M.N., Rager, H. et al. Luminescence spectroscopic study of Cr3+ in Brazilian topazes from Ouro Preto. Phys Chem Minerals 32, 679–690 (2006). https://doi.org/10.1007/s00269-005-0042-1
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DOI: https://doi.org/10.1007/s00269-005-0042-1