ORIGIN OF THE HIGH-FREQUENCY DOUBLET IN THE VIBRATIONAL-SPECTRUM OF VITREOUS SIO2

The vibrational properties of amorphous SiO2 were studied within first -principles density functional theory. The calculated spectrum is in g ood agreement with neutron data, showing, in particular, a double peak in the high-frequency region. This doublet results from different loc al modes of the tetrahedral subunits and cannot be ascribed to a longi tudinal-optic-transverse-optic (LO-TO) effect. This solves a long-stan ding controversy about the origin of the doublet in neutron spectra. A LO-TO splitting is recovered only when the long-wavelength limit is p robed, as in optical experiments. These findings should be a general f eature of tetrahedral AX(2) amorphous networks.