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We present a topological model for the composition dependence of glass transition temperature and fragility. Whereas previous topological models are derived for zero temperature conditions, our approach incorporates the concept of temperature-dependent constraints that freeze in as the system is cooled from high temperature. Combining this notion of temperature-dependent constraints with the Adam-Gibbs model of viscosity, we derive an analytical expression for the scaling of glass transition temperature and fragility in the binary Ge(x)Se(1-x) system. In the range of 0<or=x<or=1/3, we reproduce the modified Gibbs-DiMarzio equation of Sreeram et al. J. Non-Cryst. Solids 127, 287 (1991) but without any empirical fitting parameters. The modified Gibbs-DiMarzio equation breaks down for 1/3<x<or=2/5, where the glass transition temperature decreases with increasing germanium content.
Gupta et al. (Thu,) studied this question.