The color center of beryllium-treated yellow sapphires

Abstract

Yellow color in gem corundum is commonly caused by Fe3+ impurity replacing Al3+ in the Al2O3 structure. For two decades beryllium-assisted heat treatment has been introduced to produce yellow sapphires from colorless, green, or light blue sapphires. The roles of beryllium atoms in corundum structure have been proposed in different ways either triggering structural defects or being as catalysts. Thus, the research experiments were conducted to evaluate these contradictions by applying the UV–vis excitation spectra and Fe K-edge x-ray absorption near edge structure spectra (XANES) of the samples, in combinations with Tauc plots of the UV–vis absorption spectra. The trace element content of the samples was analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). As a result, iron impurity in the samples has been confirmed as Fe3+ by XANES spectra, and hence, revealed as the cause of yellow color in natural yellow sapphire samples. Besides, the absorptions at 423 nm, 457 nm, 487 nm and 553 nm and emissions at 609 nm and 841 nm of Fe3+-Be2+ mixed donor states were detected by UV–vis excitation, which is the novel finding on the origin of yellow coloration in beryllium-treated sapphires. Therefore, an energy band model for mixed Fe3+-Be2+ donor states is proposed to be responsible for the yellow color center in beryllium-treated yellow sapphires, comparable to Fe3+ and Fe3+/Fe3+ states for the natural yellow sapphires commonly contain high iron.

An application of polychromatic UV-Vis-NIR spectroscopy on detection of beryllium treatment in ruby and yellow sapphire

“…Unlike the monochromatic UV-Vis-NIR spectrophotometer, the polychromatic UV-Vis-NIR spectrophotometer was able to detect the Be signal. In the case of the Be-treated ruby and yellow sapphire samples, excitation signals corresponding to Be were observed at 423 nm, 457 nm, and 487 nm, which are associated with the Fe 3+ -Be 2+ mixed donor states, in agreement with previous research [7]. These excitation peaks were not present in the spectra of the natural ruby and yellow sapphire samples.…”

“…Additionally, the Be-treated ruby sample with a high Fe concentration displays absorption peaks corresponding to the Fe 3+ /Fe 3+ pair at 377 nm and 450 nm. Similarly, the absorption spectra of the natural and Be-treated yellow sapphire samples exhibited patterns consistent with other research findings [7]. Both spectra display absorption peaks for single Fe 3+ at 388 nm and Fe 3+ /Fe 3+ pair at 377 nm and 450 nm, with varying peak intensities depending on the Fe content.…”

“…Without iron, brown coloration was obtained. Hence, the cause of the colour change in yellow sapphires by beryllium heat treatment was proposed to be due to Fe 3+ -Be 2+ mixed-donor states [7]. Moreover, beryllium heat treatment can reduce the blue colour by acceptor-donor pair recombination between Fe 3+ -Ti 4+ mixed-acceptor states and Fe 3+ -Be 2+ mixed-donor states [12].…”

“…However, this technique does not explain of how Be influences the color mechanism. Some research has suggested that the Be 2+ donor state is responsible for producing an orange color in Be-treated synthetic ruby [6], as well as the yellow color in Be-treated yellow sapphire through Fe 3+ -Be 2+ mixed donor states [7], using the polychromatic UV-Vis-NIR spectroscopy.…”

“…In most gem laboratories, a monochromatic UV-Vis-NIR absorption spectrophotometer is used to measure the peak position of trace elements related to the coloration of [8][9]. However, there is no evidence that this technique can be used to detect the Be signal until the detection of the Be 2+ donor state and Fe 3+ -Be 2+ mixed donor states using a polychromatic UV-Vis-NIR spectrophotometer has been reported [6][7]. In some cases, there is no need to differentiate between monochromatic and polychromatic UV-Vis-NIR spectrophotometers as the spectra obtained may be identical [10].…”