Low frequency vibrational modes related to texture in silica aerogels

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Abstract
Low frequency vibrational modes of silica aerogels have been investigated by Raman spectrometry. The position and lineshape of the peaks observed in the range from 5 to 30 cm-1 appear correlated to the initial preparation conditions and to the heat treatment responsible for the densification. The maximum frequency of the peak is proportional to the inverse of the constitutive particle gyration radius deduced from small angle neutron scattering. This gives a determination of the sound velocity inside these particles. Moreover, the temperature dependence of the spectrum for one sample demonstrates the harmonic character of the modes responsible for the peak