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.
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