RELAXATIONAL AND VIBRATIONAL DYNAMICS IN THE GLASS-TRANSITION RANGE OF A STRONG GLASS FORMER B2O3

The structural relaxation behavior of a strong glass former B2O3 has b een investigated over broad temperature (300-1275 K) and frequency (0. 5 GHz-10 THz) ranges using depolarized light scattering. The spectra c learly show nonmonotonic temperature behavior with some dynamical cros sover at about T-c approximate to 800-900 K Above T-c the spectra deve lop qualitatively according to the general scenario predicted by the m ode-coupling theory (MCT), including a fast beta process and a much sl ower alpha process in addition to a vibrational contribution. However, there is disagreement between the observed functional form of the fas t relaxational dynamics and that predicted by MCT. The disagreement se ems to be related to the influence of low-lying vibrational contributi ons, the so-called boson peak, which generally seems to be more pronou nced in strong glass formers. Below T-c the spectra do not follow MCT predictions, not even qualitatively; the main signature is a decrease of the level of the fast relaxation spectrum. Analysis in terms of an alternative phenomenological approach, in which the fast relaxation co ntribution is related to the damping of the vibrational modes (giving rise to the boson peak), reveals some crossover of the damping rate at about the same temperature T-c as the crossover of the fast relaxatio n dynamics itself, and with similar temperature dependence as that rec ently reported for the Brillouin linewidth. We suggest that these vari ations are related to the temperature dependence of the relative stren gth of the fast relaxation. We show that apart from differences in the vibrational contribution, strong and fragile glass formers differ con cerning the temperature range of transition (between T-c and T-g), bei ng narrow for fragile systems (T-c/T-g approximate to 1.2) and broad f or stronger ones (T-c/T-g approximate to 1.6 for B2O3).