Glass Transitions and Critical Points in Orientationally Disordered Crystals and Structural Glassformers: 'Strong' Liquids are More Interesting Than We Thought

摘要

When liquids are classified using Tg -scaled Arrhenius plots of relaxationtimes (or relative rates of entropy increase above Tg) across a"strong-fragile" spectrum of behaviors, the "strong" liquids have alwaysappeared rather uninteresting [1, 2]. Here we use updated plots of the sametype for crystal phases of the "rotator" variety [3] to confirm that the samepattern of behavior exists for these simpler (center of mass ordered) systems.However, in this case we can show that the "strong" systems owe their behaviorto the existence of lambda-type order-disorder transitions at highertemperatures (directly observable in the cases where observations are notinterrupted by prior melting). Furthermore, the same observation can be madefor other systems in which the glass transition, at which the ordering isarrested, occurs in the thermodynamic ground state of the system. This promptsan enquiry into the behavior of strong liquids at high temperatures. Using thecase of silica itself, we again find strong evidence from extended ion dynamicssimulations, for a lambda transition at high temperatures, but only if pressureis adjusted to a critical value. In this case the lambda point is identifiableas a liquid-liquid critical point of the type suggested for supercooled water.We recognize the possibility of exploring, a postiori, the consequences ofrapid cooling of laboratory liquid SiO2 from >5000K and multi-GPa pressures,using the phenomenology of damage-induced plasmas in optical fibers. Theramifications of these considerations will be explored to establish a "bigpicture"2 of the relation of thermodynamic transitions to supercooled liquidphenomenology [4, 5].