Preparation and characterization of sol-gel glasses containing chromium

The optical and structural characteristics of sol-gel SiO2 powders and coat ings containing Cr are reported. The samples were characterized using X-ray diffraction, micro-Raman, visible photoacoustic absorption spectroscopy (P AS), optical transmission spectroscopy and diffuse reflectance. Glasses wit h a wide range of colors were obtained either by varying the chromium conce ntration, in the range of 0.1-20% w/w, or by air heat treatments in the ran ge of 100-500 degreesC, These optical changes are explained in terms of the different chemical states adopted by the Cr into the SiO2 matrix. Three ab sorption bands at 370, 460 and 600 nm were observed in the PAS, diffuse ref lectance and optical transmission measurements. According to the ligand fie ld 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 meas urements. The band at 370 nm? due to a ligand-to-metal charge transfer, in which chromium has the Cr valence state, is only observed using optical tra nsmission measurements. X-Ray measurements show the formation of Cr2O3 part icles only in the samples with 20% w/w of Cr after annealing at 500 degrees C. The Raman spectra are not sensitive to the chromium oxide particles, but reveal the existence of isolated chromate- and dichromate-like structures. (C) 2000 Elsevier Science S.A. All rights reserved.