Abstract
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 Fe 3+, Ni 3+, Fe 2+ and Ti 4+ ions. It is known that Fe 3+ yields pale yellow and Ni 3+ a golden color while the charge transfer mechanism between Fe 2+ and Ti 4+ gives sapphire its blue color. It is obvious that the green color is due to a high ratio of Fe 2+ to Ti 4+. 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 Fe 2+ ions converted to Fe 3+ was detected by electron spin resonance spectrometry and found to increase with temperature. The sapphire with trace Ti 4+ 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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