Optical phonons in SiO2 sol-gel derived colored glasses doped with Cu and Fe

Optical phonons in SiO2 samples prepared by the sol-gel method and colored by doping with Fe and Cu were studied using X-ray diffraction, electron spi n resonance and infrared spectroscopy. The vibration modes in pure and Cu- or Fe-doped glasses are investigated as a function of doping concentration and annealing temperature. It is found that the heat treatments in air atmo sphere have a remarkable influence on the width of the stretching absorptio n bands related to the asymmetric vibrations in the SiO2 matrix. A simple d escription of the different types of vibration modes in the quasilinear Si- O-Si chain is given and correlated with the characteristics of the phonon b ands observed. A relation between the bandwidth of these modes and the chan ge in the phonon scattering rate caused by the Cu or Fe doping is establish ed. Based on this model, the percentage of Cu incorporated on the Si sites into SiO2 matrix was calculated. The behavior of the vibration spectra in F e-doped glasses is different and it indicates less incorporation of this el ement into the SiO2 matrix.