Raman scattering and Boson peaks in glasses: temperature and pressure effects

Inelastic light scattering as a function of temperature and pressure was carried out on various oxide and halide glasses. For the low frequency Raman spectra, all of our glasses show a broad feature in the frequency range from 20 to 110 cm−1 (0.60–3.30 THz); the ‘Boson peak’. This Boson peak is associated with the existence of intermediate range order (IRO) in glass. The Boson peak is due to an increase in the vibrational density of states, over the Debye value, caused by localized excitations (phonon localizations). The low frequency Raman scattering containing this dominant spectral line (Boson peak) is interpreted in terms of its relationship to the amplitude and extent of the density fluctuations in glasses and is, thereby, considered a measure of the intermediate range order in these glasses. Phonon localization (the Ioffe–Regel criterion) was used to calculate correlation lengths for the measured glass samples. Characteristic correlation lengths for the glasses in this study were in the range of about 2–5 nanometer in size. We found that the Boson peak energies are highly pressure dependent but show very little change with temperature. The pressure effects in the Raman spectra will be discussed in terms of existing theories. The concept of intermediate range order (IRO) in glass and its applications will also be discussed.