RELAXATIONAL AND LOW-WAVE-NUMBER VIBRATIONAL DYNAMICS OF STRONG GLASSFORMERS B2O3 AND GEO2 PROBED BY RAMAN-SCATTERING

Low-wavenumber Raman scattering results are presented on the structura l dynamics of two network-forming glasses: the moderately strong glass former B2O3 and the extremely strong GeO2. Studies were performed fro m temperatures ca. 10 It to above their respective glass transition te mperatures (T-g approximate to 526 K for B2O3 and 800 K for GeO2). Two distinct spectral contributions were detected, related to vibrational and fast relaxational dynamics, respectively, and conventionally refe rred to as the boson peak (BP) and quasi-elastic scattering (QS). The fast relaxation spectrum is discussed in terms of the mode coupling th eory (MCT), which has been shown to describe fragile glass formers suc cessfully. A quantity proportional to the fast relaxation strength may be deduced from the integral intensity of QS relative to BP of the sp ectra. It turns out that for T < T-g, the so-obtained QS intensity of the two glasses is similar and smoothly temperature dependent. As T-g is passed, there is a pronounced change in behaviour and dramatic diff erences between the two systems are observed. Thus, the fast relaxatio nal dynamics are clearly affected by T-g, in contrast to the MCT predi ctions of a single crossover temperature T-c > T-g. Moreover, the expe rimentally obtained value of the MCT exponent a, describing the shape of the fast dynamics, greatly exceeds the limiting value (a = 0.395) o f the theory, the deviation increasing with increasing strength of the system (a approximate to 0.7 for B2O3 and 0.9 for Ge0(2)). The differ ence between the two glasses is discussed in terms of their difference s in fragility as manifested in the behaviour of the specific heat aro und T-g.