MOLECULAR DYNAMIC SIMULATIONS OF THE INFRARED DIELECTRIC RESPONSE OF SILICA STRUCTURES

The molecular dynamic simulation technique was used to model the vibrational behavior of crystalline (alpha and beta cristobalite) and amorphous silica structures. To this end a refined potential function was developed, which allows one to reproduce the correct structural geometries, the corresponding infrared spectra, and to observe a reversible phase transformation between alpha and beta cristobalite. The complex dielectric constants in the infrared frequency range were calculated from the dipole moment time correlation functions. While idealized cristobalite exhibits the simplest spectrum with only two narrow bands, the increase of structural complexity and reduction of symmetry characteristic for the real cristobalites and amorphous silica, creates additional features in the infrared spectra. These structural changes predominantly affect the coordination of oxygen, and generate a broader spread in the normal modes characterizing the vibrations of this species. A unique method for the identification of atomic trajectories corresponding to the mechanisms responsible for individual absorption bands, which is based on the concept of Fourier transform filtering, is used for the assignment of absorption bands to atomic motions.