Effects of heat treatment on blue sapphires as monitored by ESR spectroscopy

We report effects of heat treatments on physical properties and finding optimal heating condition to add value to Thai blue sapphires. The color of sapphire arises from the presence of trace 3d-transition ions in its crystal lattice. For blue sapphire, the color is due to a charge transfer mechanism between Fe2+ and Ti4+ ions. However, iron may adopt both Fe3+ and Fe2+ due to oxygen vacancies. Fe3+ and Fe2+ yield sapphire yellow and green colors, respectively. Therefore, we have to convert as many as possible of Fe3+ to Fe2+ by heating the blue sapphire in N-2 atmosphere for 12 h. Experimental results reveal that the ratio of lattice parameter c/a increases with the heating temperature and reaches maximum at 1700 degrees C, which can be caused by displacement of Fe3+ ions or more Fe3+ ions being converted to Fe2+. ESR signals show that the number of Fe3+ ions decreases roughly linearly with the heating temperature. The intense sky blue color was achieved after the 1500 degrees C heat treatment, having the Fe3+/Fe-RT(3+) ratio similar to 0.78. The optimal heat treatment should therefore be at 1500 degrees C in flowing N-2 atmosphere for Thai blue sapphires which yield intense sky blue color and good crystal clarity. The blue sapphires exhibited good clarity but light sky blue due to the increase in lightness after the treatment at 1700 degrees C. A monoclinic distortion of the corundum structure has been found to start at the 1600 degrees C treatment by ESR spectrometer. This is also clearly evident from low angle shifts of XRD peaks after heating at 1700 degrees C. We can therefore conclude that the color change of Thai blue sapphires arises from the conversion of Fe3+ to Fe2+ and thus the change in crystal field. The monoclinic distortion of the crystal structure may also play an important role in coloring the sapphires after the heat treatment at 1600-1700 degrees C.