Résumé
The boson peak (BP) of silica at several densities and of lithium borate glasses has been measured using hyper-Raman scattering (HRS). In the former case, the HRS spectral responses greatly differ from Raman ones but match very well the neutron-scattering measurements. In borate glasses, the Raman and hyper-Raman BPs are rather similar. These results are consistent with the picture that librational motions of rigid structural units (i.e. tetrahedra in silica and triangles and rings in borates) contribute to the excess of low frequency modes. In disordered networks, rigid librations are expected to couple with rigid translations (the second family of “external” modes), leading to extended hybrid excitations. The temperature dependence of the BP of vitreous silica was also measured from 20 to 800 K. It can be scaled with the very high frequency limit of the sound velocity. This indicates that this limit velocity provides a measure of temperature-dependent structural changes in silica.