Abstract
We have investigated the development of the longitudinal acoustic branch in the network glass former Li2O-2B2O3 using inelastic X-ray scattering. We find a pronounced crossover from a propagating to a non-propagating behaviour around Qc≈4nm_1 and at energies close to the boson peak energy (E≈ 1.5E BP). The crossover is interpreted as the onset of strong scattering of the acoustic modes as the Ioffe-Regel limit is reached. From the temperature independence of the width of the excitations in the dispersive regime we conclude that the damping of the modes is essentially due to the disordered structure.