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Spac-Ttme-Correlations for Cation Motion in Alkali Silicates

Published online by Cambridge University Press:  10 February 2011

John Kieffer
John Kieffer*
Affiliation:
Department of Materials Science & Engineering, University of Illinois, Urbana IL 61801
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Abstract

The motion of structural components in alkali silicates has been studied using molecular dynamic simulations. The relevant time correlation functions have been recorded over periods of up to 0.5 nanoseconds. The van Hove correlation functions reveal that alkali cation diffusion occurs by hopping between rather well defined sites, even at temperatures far above the melting point. The accessibility of such sites is controlled by the relaxation of the surrounding network, and constitutes the mechanism of coupling between the motion of network and modifier elements. The intermediate scattering functions exhibit a stretched exponential behavior, with a wavevector dependent stretching exponent. This is the result of a distribution of residence times at individual cation sites. When the diffusion process is examined on a coarser scale, at which Brownian motion is applicable, this distribution remains undetected. As an alternative to the Kohlrausch function, several mechanistic approaches for describing the relaxation behavior of these structures are discussed.

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Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 455: Symposium T – Glasses and Glass Formers–Current Issues , 1996, 331
Copyright
Copyright © Materials Research Society 1997

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