Anomalies of the Fast Relaxation Dynamics at T g In Strong Glass Formers
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ABSTRACT Structural dynamics of two network forming glasses, B2 0 3 and GeO 2 , has been investigated by Raman scattering over a wide temperature range from -10K to above the respective glass transition temperatures (Tg = 526 K for B2 0 3 and 800 K for GeO 2 ). The spectra are analyzed in terms of two distinct contributions, related to vibrational and fast relaxational dynamics, respectively, and conventionally referred to as the boson peak (BP) and quasielastic scattering (QS). A quantity proportional to the fast relaxation strength may be deduced from the integral intensity of QS relative to BP of the spectra. It turns out, that for T Tg. Moreover, the experimentally obtained value of the MCT exponent a, describing the shape of the fast dynamics, is by far exceeding the limiting value (a=0.395) of the theory, the deviation increasing with the strength of the system (a= 0.7 for B2 0 3 and = 0.9 for GeO 2 ). The observed difference between the two glasses is discussed in terms of the fragility of the system manifested in jumps in the specific heat temperature dependences. INTRODUCTION
Glass Transition Supercooling a liquid at a rate that exceeds the rate of structural rearrangement produce a metastable state which, if cooling proceeds to a sufficiently low temperature, is called a glass. A glass is commonly regarded as a liquid which has lost its ability to flow, the disordered system being locked in one of its infinitely many potential minima by quickly reducing the thermal energy. Structurally a glass is hardly distinguishable from its corresponding melt, yet the enormous (by many orders of magnitude) slowing down of the transport properties in the glass transition range, leads to quite abrupt changes in many other physical parameters (for reviews of the glass transition phenomena see e.g. Refs. 1-3). In particular, the specific heat exhibits an almost step-wise behavior, changing typically by 10 to 100% between liquid- and solid-like values at the transition temperature (Fig.1). This is commonly associated with the "ergodicity breaking" during cooling, such that certain liquid degrees of freedom become "frozen" and kinetically inaccessible. The calorimetrically determined glass transition temperature Tg is conventionally defined as the temperature of onset of the heat capacity increase ACp when heating the system from below Tg at some constant rate (usually 10 K/min). This definition is to some extend arbitrary as the so-obtained Tg slightly depends on the heating rate. This observation merely illustrates the fact that the experimentally determined glass transition is of kinetic origin, being obtained when the average structural relaxation time exceeds the time of observation. 35 Mat. Res. Soc. Symp. Proc. Vol. 455 01997 Materials Research Society
Glass forming liquids represent a wide variety of systems with different molecular interactions from strong covalent bondings, weaker hydrogen bondings to the weak vander-Waals and ionic bonds. The different systems may be classified according to the temperature dep
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