Light scattering from disorder and glass-transition-dynamics in GeSBr2

Abstract

Low frequency Raman spectra (5–50 cm⁻¹) show very obvious effects of disorder in the glass, and of the changes in the system which occur at the liquid–glass transition. Yet this frequency range has been considered in only very few glass formers. Here we report on this frequency range for the glass former system GeSBr₂ which constitutes a statistically interrupted network. The temperature range studied was from far below the glass transition temperature (T_g=−30 °C) to well above it (T=−168 °C to +71 °C). In the glass, the loss of q conservation for phonon observation (due to static disorder) leads to a broad inelastic scattering peak around 10–15 cm⁻¹, the so‐called boson peak. The Raman spectrum is interpreted in terms of a correlation length R_c of phonon propagation and of phonon–photon‐coupling; R_c is a measure of intermediate range order and amounts to ∼10 Å, independent of temperature. As T is raised, the minimum between this peak and the elastic peak is gradually filled up due to fast dynamic processes which increase rapidly above T_g. This scattering is interpreted in terms of fast dynamical processes, and the connection is made to the dynamical aspects of the liquid–glass transition. The strength and the time constants of fast processes are discussed in terms of two phenomenological models; both models describe the observed spectra well using a small number of parameters. The relaxation time found for the fast dynamical process (10⁻¹¹–10⁻¹² s) is in rough agreement with the value found independently in Brillouin scattering.