The relationship between fragility, configurational entropy and the potential energy landscape of glass forming liquids

摘要

Glass is a microscopically disordered, solid form of matter that results whena fluid is cooled or compressed in such a fashion that it does not crystallise.Almost all types of materials are capable of glass formation -- polymers, metalalloys, and molten salts, to name a few. Given such diversity, organisingprinciples which systematise data concerning glass formation are invaluable.One such principle is the classification of glass formers according to theirfragility\cite{fragility}. Fragility measures the rapidity with which aliquid's properties such as viscosity change as the glassy state is approached.Although the relationship between features of the energy landscape of a glassformer, its configurational entropy and fragility have been analysed previously(e. g.,\cite{speedyfr}), an understanding of the origins of fragility in thesefeatures is far from being well established. Results for a model liquid, whosefragility depends on its bulk density, are presented in this letter. Analysisof the relationship between fragility and quantitative measures of the energylandscape (the complicated dependence of energy on configuration) reveal thatthe fragility depends on changes in the vibrational properties of individualenergy basins, in addition to the total number of such basins present, andtheir spread in energy. A thermodynamic expression for fragility is derived,which is in quantitative agreement with {\it kinetic} fragilities obtained fromthe liquid's diffusivity.