Acoustic and optical modes in vitreous silica, germania and beryllium fluoride

Normal modes of vibration in vitreous SiO 2 , GeO 2 and BeF 2 are discussed from the point of view of the relative phase of motion of neighbouring atoms in the system. The notion of a phase quotient is introduced, as a criterion for determining the extent to which modes are acoustic or optical in character. Use of the phase quotient, in conjunction with normal mode assignments derived in an earlier paper, enables one to make tentative predictions about the infrared activity of the different spectral bands. The results are in substantial agreement with available experimental data.