Analyzing the coloration of sodium borate glasses caused by sulfur species

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Herein, coloration in sodium borate glasses containing sulfur were investigated and sulfur species in the glasses were determined using ultraviolet–visible (UV–vis), Raman, and fluorescence spectroscopy. Sample coloration varied from blue to brown and depended on alkali content and glass matrix composition. Using UV–vis spectroscopy, several absorption bands were detected at 280, 380, and 580 nm. It was anticipated that the sulfur species, such as S₂, S₂⁻, and S₃⁻, will be formed in the glass samples and will influence sample coloration. Raman spectroscopy suggested that the absorption band at approximately 580 nm was due to the S₃⁻ anion species, whereas fluorescence spectroscopy suggested that the absorption band at approximately 380 nm was due to S₂⁻ anion species. The ratio of these sulfur anion species varied with matrix glass compositions and had a strong effect on the sample coloration. Herein, the origin of these color centers is discussed from the viewpoint of the redox of the melt and borate glass network.