Glassformer fragilities and landscape excitation profiles by simple calorimetric and theoretical methods

In this paper we introduce two key notions related to understanding the 'gl assy state' problem. One is the notion of the 'excitation profile' for an a morphous system, and the other is the notion of the 'simple glassformer'. T he attributes of the latter may be used, in quite different ways, to calcul ate and characterize the former. The excitation profile itself directly ref lects the combined phonon/configuron density of states, which in turn deter mines the liquid fragility. In effect, we are examining the equivalent, for liquids, of the low temperature Einstein-Debye regime for solids though, i n the liquid heat capacity case, there is no equivalent of the Dulong/Petti t classical limit for solids. To quantify these notions we apply simple calorimetric methods in a novel m anner. First we use DTA techniques to define some glass-forming systems tha t are molecularly simpler than any described before, including cases which are 80 mol% CS2, or 100% S2Cl2. We then use the same data to obtain the fra gility of these simple systems by a new approach, the 'reduced glass transi tion width' method. This method will be justified using data on a wider var iety of well characterized glassformers, for which the unambiguous F-1/2 fr agility measures are available. We also describe a new DTA method for obtai ning F-1/2 fragilities in a single scan. We draw surprising conclusions abo ut the fragility of the simplest molecular glassformers, the mixed LJ glass es, which have been much studied by molecular dynamics computer simulation. These ideas are then applied to a different kind of simple glass - one whos e thermodynamics is dominated by breaking and making of covalent bonds - fo r which case the excitation profile can be straight-forwardly modeled. Comp arisons with the profile obtained from computer studies of the molecularly simple glasses are made, and the differences in profiles implied for strong vs. fragile systems are discussed. The origin of fragility in the relation between the vibrational and configurational densities of states is discuss ed, and the conditions under which high fragility can convert to a first or der liquid-liquid transition, is outlined.