Optical Phonons in Amorphous Silicon Oxides. I. Calculation of the Density of States and Interpretation of LoTo Splittings of Amorphous Sio₂

Abstract Until now there exists no calculation of the phonon density of states of amorphous SiO 2 and related materials, which could interpret the experimentally detected splittings between longitudinal and transverse modes in their optical phonon spectra. This problem is solved by taking into account the influence of the vibrationally induced polarization on the motion of atoms. A simple model for the description of this polarization in non‐crystalline SiO 2 is developed and used for the calculation of the density of states of LO and TO phonons in the framework of a coherent potential approximation for structurally disordered systems. This new method leads also to the determination of the IR response function and to simple analytical expressions for the microscopic parameters of the model, i.e. the bond angle at the oxygen atom and transverse dynamic effective charges. The values of these parameters are determined from IR transmission spectra of thin SiO 2 layers prepared with the help of different methods. The calculated density of states and IR response function are in good agreement with corresponding experimental results.