Optimization of Heat Treatments of African Green Sapphires

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Under suitable heating conditions designed for a particular sapphire it is possible to change the color in order to increase its value. It was found that heating African green sapphires in an oxygen atmosphere could improve their quality and thus increase their prices. The color of green sapphires is due to the presence of trace amounts of Fe3+, Ni3+, Fe2+ and Ti4+ ions. It is known that Fe3+ yields pale yellow and Ni3+ a golden color while the charge transfer mechanism between Fe2+ and Ti4+ gives sapphire its blue color. It is obvious that the green color is due to a high ratio of Fe2+ to Ti4+. By using an X-ray diffractometer, the maximum decrease of the c/a ratio of the hexagonal structure is found to occur after heat treatment at 1200°C. These results are also confirmed by the shift of emission spectra toward shorter wavelengths. Furthermore, the number of Fe2+ ions converted to Fe3+ was detected by electron spin resonance spectrometry and found to increase with temperature. The sapphire with trace Ti4+ ions, appeared slightly bluish green after heating at 1500°C. The optimal temperature is therefore at 1300°C, where we can achieve yellowish green sapphires instead.

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