Polyamorphism in glassy network materials

One dramatic feature of network liquids is the emergence at low temperatures and high pressures of polyamorphism, where multiple distinct liquid phases are accessed in a single material. Polyamorphism can arise from the competition between distinct local structures corresponding to bonded and nonbonded ordering. Thermal bond breaking leads to a phase transition often accompanied by thermodynamic anomalies, such as the familiar density maximum in water at atmospheric pressure and 4 °C. Water exhibits network interactions in the form of hydrogen bonding. The polyamorphic transition in water, however, is difficult to study due to the rapid crystallization of supercooled water and due to glassy effects at low temperatures. In the present study, we propose a simple microscopic model where the glassy and thermodynamic properties are both calculated directly from the microscopic potentials. The model contains a liquid-liquid phase transition, which, after tuning the microscopic parameters, may be located either above, near, or below the glass transition. By applying the Random First Order Transition (RFOT) theory of the glass transition to this simple microscopic model, we shine light on the interplay of polyamorphism and glassy properties in network liquids. We show the connection between the thermodynamic water-like anomalies and corresponding anomalies in the glassy kinetics. The analysis unveils key details on the way glassy dynamics modifies the phase transition kinetics. When the parameters of the model are tuned to produce a phase diagram resembling that of water, the liquid-liquid phase transformation near Tg occurs via "nanonucleation," resulting in nonclassical nucleation properties that are predicted from the RFOT theory.