Oxidation states of trace elements in synthetic corundum

Abstract

Natural and synthetic corundums have various colors due to trace elements in their structures. Despite the abundance of research on impurities in natural corundum, little is known about trace elements in synthetic corundum. This project is aimed to quantify trace elements and identify their oxidation states related to coloration in red, blue, and yellow synthetic corundums. A total of 15 corundums, synthesized by melt growth process, were investigated by advanced analytical techniques. X-ray diffraction results indicate that all synthetic corundums contain crystalline Al2O3. Measurements from Electron Probe Micro-Analyzer, UV-Vis-NIR, and X-ray Absorption Near Edge Structure spectrometry suggest that Cr3+ is significant trace element in red samples. A small amount of Cr3+ substitutes in some Al3+ sites in corundum structure. Cr3+ has unpaired electrons, which can be excited and cause light absorption around 552-580 nm, resulting in red color. Interestingly, a high amount of V3+ is detected in a few red samples and contributed to a unique purple shade. In addition, the electron charge transfer between Fe2+ and Ti4+ pair leads to the absorption around 576-590 nm, causing blue light transmission. Yellow samples have Ni and Cr3+ as major trace elements. Ni is likely the cause of yellow color due to a vacancy in oxygen position or trapped electron hole color center, leading to light absorption around 402-420 nm in synthetic samples.