MICROSTRUCTURE OF P-SE GLASSES AND LOW-FREQUENCY RAMAN-SCATTERING

The vibrational structure of P-Se glasses has been studied by Raman scattering. The vibrational spectra in the frequency range 5-500 cm(-1) show features associated with the "boson peak" at low wavenumbers, and with internal molecular modes above 100cm(-1). The vibrations near 250 cm(-1) associated with Se-Se bonds are found only for the Se-rich glasses. The internal modes of P4Se3 molecules dominate the spectra for glasses with P content in the range 63-85 atomic % P; these glasses show no contribution from Se-Se vibrations, indicating that all Se atoms are incorporated into the molecules. These glasses therefore consist of P4Se3 molecules in a phosphorus matrix. In contrast, the high-Se glasses probably consist of PnSem-units cross linked by selenium bridges. This gives two different structural families within each of which a gradation of behaviour is expected as the Se content is changed, which provide a test of the lognormal spectral form of the boson peak proposed by Denisov and Rylev. This is found to give an adequate fit to the measured spectra over only a limited frequency range, but the normalised peak shape is very similar for both types of structure, adding weight to the argument for a "universal" form of the vibrational state density.