Phonons in AX2 glasses: From molecular to band-like modes

We study the vibrational density of states of glassy AX2 systems such as SiO2 and GeS2 which consist of tetrahedra linked together to form a random network. We show that the higher-frequency modes can be well represented by a model with a single-nearest-neighbor central force. This model leads to an isomorphism of the vibrational modes to the electronic properties of the tetrahedral network studied by Weaire and Thorpe. This allows us to study the metamorphosis of the vibrational modes from molecular-like to band-like as a function of the AXA bond angle and the masses. The spectral limits delineating the allowed frequency regions and the character of the modes are found. These are independent of the detailed nature of the random network. The effect of a noncentral force is considered for the two extreme limits of AXA bond angles (i.e., 90°and 180°). We conclude that it is justified to consider an amorphous network as an assembly of weakly interacting molecular units when the AXA bond angle is close to 90° and the angular forces are weak. We classify BeF2, GeS2, and GeSe2 as being dominated by molecular effects, whereas the solid-state effects are most important in SiO2 and GeO2.