Résumé:

Both high-frequency acoustic modes and the boson peak related to SiO4 librations are observed in a single inelastic x-ray scattering experiment. The experimental data are consistent with a picture where the acoustic modes experience a crossover at a frequency Omega(co) beyond which plane waves cease to exist. The spectra evolve with the scattering vector to merge into a broad boson peak at Omega(BP)approximate toOmega(co). The latter, although essentially optic in nature, might hybridize with the resonant acousticlike modes, which can be crucial to their strong scattering.