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Anomalies of the Fast Relaxation Dynamics at Tg In Strong Glass Formers

Published online by Cambridge University Press:  10 February 2011

A. Brodin  and
L. M. Torell
A. Brodin
Affiliation:
Department of Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden
L. M. Torell
Affiliation:
Department of Physics, Chalmers University of Technology, S-412 96 Göteborg, Sweden
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Abstract

Structural dynamics of two network forming glasses, B2O3 and GeO2, has been investigated by Raman scattering over a wide temperature range from ∼10 K to above the respective glass transition temperatures (Tg = 526 K for B2O3 and 800 K for GeO2). 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 the so-obtained QS intensity of the two glasses is similar and smoothly temperature dependent, and can qualitatively be described by e.g. the defect model or the soft potential model. As Tg is passed, there is a pronounced change in behavior and dramatic differences between the two systems are observed. Thus, the fast relaxational dynamics is obviously affected by Tg, a finding which is in contrast to the predictions of the mode coupling theory (MCT) of a single crossover temperature Tc > 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 B2O3 and ≈ 0.9 for GeO2). 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.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 455: Symposium T – Glasses and Glass Formers–Current Issues , 1996 , 35
Copyright
Copyright © Materials Research Society 1997

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