High-temperature UV-VIS-NIR spectroscopy of chromium-doped glasses

Chromium-doped glasses with the basic composition (in mol%) 16 Na2O (.) 10 CaO (.) 74 SiO2 were melted under different redox conditions. From these gl asses, UV-VIS-NIR absorption spectra were recorded at temperatures up to 12 00 degreesC. While the intensity of the peak attributed to Cr6+ decreases, some of the peaks caused by Cr3+ increase in intensity at higher temperatur e. All peaks are slightly shifted to larger wavelengths and get broader wit h increasing temperature. Glasses melted under oxidizing conditions were sl owly cooled as well as quenched. Using EPR spectroscopy, in the quenched sa mple, Cr5+ was detected in a larger concentration than in the slowly cooled sample. Otherwise, the Cr6+ concentration was larger in the slowly cooled sample. This is explained by a redox reaction, i.e. a disproportionation of Cr5+ to Cr6+ and Cr3+ during cooling. As shown by high-temperature spectro scopy of the quenched sample, this redox reaction is frozen in below 550 de greesC.