Study on the Relation between the Intensity Distribution of Infrared Absorption Peak at 3 309 cm-1 and Trace Elements in Color Zones of Changle Sapphire
CHEN Chao-yang, SHAO Tian, Andy Hsitien Shen*
Gemmological Institute，China University of Geoscience (Wuhan)，Wuhan 430074，China
Abstract：The infrared absorption peak at 3 309 cm-1 caused by the vibration of OH often appears in natural sapphire. This peak is significant for the identification of heat treatment sapphire. The color of sapphire produced in Changle County, Shandong Province is often dark blue, and the color zones are usually well developed. The absorption peak at 3 309 cm-1 is often found in FTIR spectrum of Changle Sapphire. At present, the intensity distribution of this peak in color zones of sapphire is still not studied, and the assignment of this peak is still controversial. In this paper, the intensity distribution of peak at 3 309 cm-1 in Changle sapphire color zones and the relationship between this peak and trace elements are studied, and the assignment of this peak is further speculated. In spectroscopic technology, FTIR area scanning was innovatively used to analyze the intensity distribution of peak at 3 309 cm-1 in the color zones. In the spectroscopic analysis, the assignment of the peak at 3 309 cm-1 was creatively predicted based on the charge compensation theory in sapphire and the distribution of trace elements in the color zones. We found that the intensity distribution of this peak showed a trend of increasing from the lower left corner to the upper right corner in the scanning area. Along the direction of this peak increasing, we measured the trace elements contents at five points by Laser Ablation Inductively Coupled Plasma Mass Spectrometer. Based on the charge compensation theory, Ti4+ prefers to compensate with Mg2+ in sapphire. If the content of Ti4+ is higher than Mg2+, almost all Mg2+ will compensate with Ti4+ and remaining Ti4+ will compensate with Fe2+ to form Fe2+-Ti4+ pairs which produce the blue color. The content of Ti in the colorless region is low, and all Ti4+ will compensate with Mg2+. So there is almost no Fe2+-Ti4+ pair in colorless region and the peak at 3 309 cm-1 is very weak. The contents of Fe2+-Ti4+ pairs in the blue region determine the depth of blue. The intensity of 3 309 cm-1 peaks in the blue region is obviously higher than that in colorless region, but the intensity of this peak in the dark blue region is not necessarily stronger than that in the blue region. The intensity of this peak has no certain relationship with the contents of Fe2+-Ti4+ pairs in sapphire. The intensity distribution of this peak shows a positive correlation with the contents of Ti. The more the Ti contents are, the stronger the is peak. We found this peak was positively correlated with the content of Ti, and we further speculated that the defect cluster containing Ti and OH produced the peak at 3 309 cm-1. The Fe2+ is not necessary to produce a peak at 3 309 cm-1 but to compensate with the Ti4+ to produce the blue color.
Key words：Changle sapphire; FTIR spectrum area scanning; Absorption peak at 3 309 cm-1; Defect cluster
陈超洋，邵 天，沈锡田. 昌乐蓝宝石色带区域3 309 cm-1红外吸收峰强度分布与微量元素关系[J]. 光谱学与光谱分析, 2020, 40(07): 2138-2142.
CHEN Chao-yang, SHAO Tian, Andy Hsitien Shen. Study on the Relation between the Intensity Distribution of Infrared Absorption Peak at 3 309 cm-1 and Trace Elements in Color Zones of Changle Sapphire. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2020, 40(07): 2138-2142.
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