Low-frequency Vibrational Dynamics of Amorphous and Crystalline Silica

Crystalline silica shows strong, sharp signals at about 77 and 130 cm−1 in the inelastic neutron scattering spectrum that are missing or strongly different, broadened and shifted to lower frequency for the case of precipitated and fumed silica. The presence or absence of these signals is a sensitive signature of crystallinity or amorphicity in silica. The low-frequency phonon density of states of precipitated and fumed silica is typical for completely amorphous materials. This observation is in perfect agreement with data from X-ray diffraction and high-resolution transmission electron microscopy. The amorphicity is retained during granulation post-treatments.