Simulations of high-density amorphous ices under pressure

Currently around 70 different anomalies of water are known. Some of these anomalies are related to solid amorphous forms of water. These forms are classified according to their density and are called low density - (LDA) , high-density- (HDA) and very high-density amorphous ice (VHDA). Relevant research questions in this field are directed to the study of the relationship of these three forms with each other, and to the question, where the so-called glass transition temperature lies, that is the temperature where the amorphous forms convert into a liquid. The focus of this project are HDA and VHDA. In my PhD-thesis I have investigated the relation between the named forms experimentally and found that there is no sharp boundary between them. It was also found that the glass transition temperature shows a maximum at high pressures. However, the latter represents an unexpected result and therefore requires further investigations. Therefore, I would like to trace this issue with computer simulations of water. This I plan to perform in Rome at the university La Sapienza with Professor Francesco Sciortino.

Water is a fascinating substance with an array of unexpected features, commonly called anomalies. A hypothesis to explain this behaviour predicts the existence of two different liquid states of water far below the melting temperature. Due to crystallisation this hypothesis is hard to test in experiments, therefore computer simulations on liquid water were performed in this project. In using a recent and accurate model of water this hypothesis could be confirmed in the simulations and a liquid-liquid phase separation between a low-density liquid (LDL) and a high-density liquid (HDL) was predicted at low temperatures. Indications for a third very high-dense liquid (VHDL) were not found and also the estimated ideal glass transition temperature shows no signs in that direction. Within the above discussed hypothesis it is expected that both liquids transform into glasses upon cooling. This combination of a liquid-liquid transition and vitrified water has interesting consequences. One is polyamorphism, meaning the existence of two types of glasses that show sharp interconversions. Computer simulations and experiments relating to polyamorphism in this project show that water indeed exhibits polyamorphism involving a low-density amorphous ice (LDA) and a high-density amorphous ice (HDA). It was also shown that the polyamorphic transitions between these two forms are very similar to what is expected for a true phase-transition. Through a rigorous theoretical analysis of the simulation data key quantities were identified that show anomalous behaviour during the glass-glass transition corroborating the first-order like nature. Finally, also these studies showed no indications for a distinct very-high density amorphous ice (VHDA).