Low-frequency Raman scattering on different types of glass formers used to test predictions of mode-coupling theory

We have measured low-frequency Raman scattering (RS) spectra of the glass formers B2O3, glycerol, m-tricresyl phosphate (m-TCP), and K3Ca2(NO3)7 in a broad temperature range above the glass transition temperature. The basic difference between the spectra is the ratio of relaxational to vibrational contributions: the more fragile the glass, the smaller is the vibrational contribution. Features of the relaxational contributions are quite similar. Testing predictions of mode-coupling theory (MCT) for m-TCP, we find that the dynamics is well described, and as the critical temperature we find Tc∼260 K. In the case of glycerol, due to the strong vibrational contribution to the RS spectrum, a consistent interpolation of the rescaled data is only possible if other exponents than those predicted by MCT are applied; however, the MCT exponents are obtained in a narrow frequency range, provided the contribution of the α process is subtracted first. Thus, MCT describes the dynamics for ν<300 GHz even in the nonfragile glass former glycerol. We extract Tc∼310 K. Combining the RS results with those of NMR in the case of m-TCP, we further find (i) a change of stretching parameter close to Tc and (ii) a change of the temperature dependence of the Debye-Waller factor at T∼280 K which is close to Tc for this system.