Spectroscopic Investigation of Properties of Blue Sapphire Samples Depending on Heating Conditions

The 3309, 3232, and 3185 cm^–1 are a series of peaks defined as the 3309 cm^–1-series peaks of the functional group absorptions in the mid-infrared region of gem corundum samples, in particular, the blue sapphires. In this study, the 3309 cm^–1-series peaks were attributed to –Ti–OH stretching. However, the application of revealing these series peaks is still limited because the mechanism of those peaks during the heating process has yet to be clarified. This study showed that the characteristics of the peaks depend strongly on the TiO₂ content in the sapphire samples. Energy dispersive X-ray fluorescence (EDXRF) spectrometry indicated that the samples with Ti content >0.02 wt.% usually show the 3309 cm^–1-series peaks with strong intensity. In addition, the X-ray absorption spectra (XAS) revealed that the oxidation state of Fe is Fe³⁺ while Ti is Ti⁴⁺ for every heating temperature. The UV-Vis-NIR optical absorption showed that the alteration of the bands at 580 and 710 nm, defined as the Fe³⁺/Ti⁴⁺ pair, was related directly to the atmospheric heating conditions. In contrast, the intensity of the 3309 cm^–1-series peaks gradually decreased with increase in heating temperatures in any given atmosphere. This is a result of the mechanism of the bonding between Ti and/or Fe atoms and –OH in blue sapphire structures caused by the heating.