Preparation and characterization of sol-gel glasses containing chromium

The optical and structural characteristics of sol-gel SiO₂ powders and coatings containing Cr are reported. The samples were characterized using X-ray diffraction, micro-Raman, visible photoacoustic absorption spectroscopy (PAS), optical transmission spectroscopy and diffuse reflectance. Glasses with a wide range of colors were obtained either by varying the chromium concentration, in the range of 0.1-20% w/w, or by air heat treatments in the range of 100-500 °C. These optical changes are explained in terms of the different chemical states adopted by the Cr into the SiO₂ matrix. Three absorption bands at 370, 460 and 600 nm were observed in the PAS, diffuse reflectance and optical transmission measurements. According to the ligand field theory, the last two bands are identified with electronic transitions in the d-d levels of Cr³⁺; these two bands are better defined in the PAS measurements. The band at 370 nm, due to a ligand-to-metal charge transfer, in which chromium has the Cr⁶⁺ valence state, is only observed using optical transmission measurements. X-Ray measurements show the formation of Cr₂O₃ particles only in the samples with 20% w/w of Cr after annealing at 500 °C. The Raman spectra are not sensitive to the chromium oxide particles, but reveal the existence of isolated chromate- and dichromate-like structures.