Neutron inelastic scattering measurements have been performed on samples of fumed silica (‘‘Cab-O-Sil’’) as a function of energy, momentum-transfer vector Q, and temperature T, and compared with analogous results for amorphous quartz. No evidence is seen for a sharp crossover in the vibrational density of states of Cab-O-Sil between phonon and fracton regimes, but the inelastic scattering shows anomalous behavior in several respects. Neither the T dependence nor the Q dependence nor the absolute intensity of the scattering from Cab-O-Sil is in accord with simple phonon models.

• Received 30 August 1988

DOI:https://doi.org/10.1103/PhysRevB.39.6180