Contrasting behaviour of acoustic modes in network and non-network glasses

High-frequency acoustic excitations have been investigated in typical network and non-network glasses, B2O3 and Ca0.4K0.6 (NO3)(1.4) (CKN) using inelastic X-ray scattering. CKN exhibits linear dispersion of the excitations throughout the investigated range (1-8 nm(-1)) and to energies more than five times above the boson peak energy. For B2O3 we observe for the first time a clear crossover in the dispersion relation just above the boson peak. The contrasting behaviour is attributed to differences in connectivity and fluctuations of force constants on a mesoscopic length scale.