Abstract

The blue color of sapphire is associated with Fe and Ti impurities that replace Al3+ in the corundum structure. Typically, sapphire color depends on its geological origin, such as basaltic or metamorphic localities, and the blue hue can change after heating in an oxidizing environment. Nonetheless, previous studies on the blue color mechanism have left some questions unanswered. Therefore, this research examines how the oxidation states of Fe and Ti influence sapphire color and explores the mechanism of blue coloration before and after heating in oxidizing and reducing atmospheres. This study involved collecting sapphire samples from various gem localities, including basalt-related sapphires from Kanchanaburi, Thailand, and metamorphic-related sapphires from Sri Lanka. The samples were heated in an oxidizing environment at 1100 °C, then at either 1300 °C or 1500 °C in both oxidizing and reducing environments. As a result, after heating under an oxidizing environment at either 1300 °C or 1500 °C, the basalt-related sapphires turned from light blue to pale blue, and the metamorphic-related ones turned colorless. The Fe3+-Ti4+ mixed acceptor states decreased because an electron from the valence band recombined with a hole in the color center during heating. On the other hand, the blue color observed in sapphire samples after heating in a reducing environment at 1300 °C could be explained by electrons being depleted from the hole color center associated with Fe3+-Ti4+ mixed acceptor states within the energy band gap, thereby making them ready to receive electrons from the valence band upon optical excitation. Therefore, it can be concluded that the blue color mechanism in sapphires before and after heating under different atmospheric environments can be explained by an energy-band model involving the presence or absence of Fe3+-Ti4+ mixed acceptor states, as well as a hole color center within the energy band gap. Furthermore, after heating at 1300 °C and 1500 °C in a reducing environment, the oxidation state of Fe gradually decreases from 3+ to 2+. The samples turn black upon heating to 1500 °C, indicating that Fe2+-Ti4+ is responsible for the black color rather than the blue observed in sapphires.

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