Origin of the boson peak in a network glass B2O3

We report coherent inelastic neutron-scattering spectra for the network gla ss former B2O3, measured at temperatures from 100 to 1300 K (T-g similar to 525 K). At the first sharp diffraction peak, which stems from the van der Waals packing of layerlike units, one finds a correlated in-phase motion of these units at low frequency, and in addition a quasi-incoherent contribut ion for all temperatures. The contribution of the in-phase motion in the fr equency range of the boson peak and the fast relaxations is significantly h igher than the one expected for the sound waves.