X-ray diffraction and Raman scattering measurements on silica xerogels

The dynamics and the structure of silica xerogels at different degrees of densification have been studied by small angle X-ray scattering and Raman scattering, with the aim to investigate the effects of the inhomogeneous structure on the excitations that give rise to the excess in the density of states (Boson peak). The results are compared with those of specific heat measurements on the same samples. The present results show that the existence of the pores, whose medium size is in the 30-60 A A range, does not appreciably affect the density of states in the Boson peak frequency range. On the contrary, at lower frequencies (x < 20 cm À1), a strong pore-size dependence is observed in the Raman spectra.