'Strong' and 'superstrong' liquids, and an approach to the perfect glass state via phase transition

We show first that when the excitations described by the 'bond lattice' mod el, and its 'defect' model relatives, are allowed to interact, this simple class of model predicts the possibility of first-order transitions between viscous and fluid liquid states. Looking for support of this prediction, we examine the behavior of a series of 'tetrahedral' liquids, all of which ar e famous for one or another aspect of their behavior. We show that when dif fusivity data for these liquids, SiO2. BeF2, water, and liquid Si, are plot ted on a reduced temperature scale with glass transition temperature as the scaling temperature, a systematic pattern is revealed in which a mild devi ation, in the case of BeF2, from the 'strong' extreme of the normal strong/ fragile glass-former pattern becomes a (contentious) strong deviation in th e case of water and develops finally into the predicted first-order transit ion deviation in the case of liquid Si. Unfortunately, in the latter two ca ses, the systematic strengthening of the anomalous character is associated with a decrease in the temperatures of occurrence, such that in each case t hey fall below the melting point. The corresponding competition with crysta llization makes their observation difficult. Our account of the phenomenolo gy, therefore, depends heavily on computer simulation studies, but extensiv e links to experimental results are given. We relate our findings to the re cent observations that the amorphous states of Si and water are unique amon g glassy systems in showing little or no trace of the very low temperature (glassy state as opposed to liquid state) anomalies formerly considered 'ub iquitous' among glassy systems. We interpret this to mean that systems with strong cooperativity in their excitations are able to access the lower min ima on their respective configuration space potential energy hypersurfaces and thereby to reach states, which are close to the ideal of the 'perfect g lass'. In this state the residual entropy is near zero, and the defect-rela ted boson peak and two-level tunneling system excitations are weak or absen t. Such systems require unusual routes to access their glassy states and th eir properties are more closely related to those of crystals than to those of ordinary glasses. A new designation may be required. The range of such s ystems is large, embracing all the 3:5 and many 2:6 semiconductors. (C) 200 0 Elsevier Science B.V. All rights reserved.