Normal mode assignments in vitreous silica, germania and beryllium fluoride

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Abstract
Detailed assignments are given for normal modes of vibration in vitreous silica, germania and beryllium fluoride. They have been obtained by calculating the atomic displacement eigenvectors for selected modes throughout the spectrum, and separating the kinetic energy of these modes into contributions from cation motion, bond-stretching, bond- bending and bond-rocking motions of the anions. Bands in the 1050, 500-800 and 400 cm-1 regions of the silica spectrum can be attributed largely to bond-stretching, bond-bending and bond-rocking vibrations respectively, while a low frequency shoulder below about 350 cm-1 is found to be mainly acoustic in character. In the vibrational spectrum of vitreous germania, bands which occur at about 850, 500, 370 and 200 cm-1 are similar in their respective assignments to those listed above for silica. In the case of beryllium fluoride, the highest frequency band, at around 700 cm-1, is dominated by cation motion.

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