Thermodynamic Anomalies in Network-Forming Materials

The density maximum observed in water at 4oC at ambient pressure is perhaps the most famous of the thermodynamic anomalies. However, anomalous behaviour in liquids is more widespread, spanning not only water, but systems such as silicon, germanium, silica and beryllium fluoride. A link between these systems is the dominance of local tetrahedral geometries. It is widely believed that the existence of a liquid-liquid critical point is the source of the anomalous behaviour in water. However, alternative scenarios of the anomalous behaviour exist but are either of little more than theoretical significance or are limiting cases of the so-called liquid-liquid critical point scenario. Recent studies have focused on establishing connections between liquid structure and the observed anomalous behaviour. This work systematically explores the influence of the structure on the thermodynamic anomalies of systems modelled with both a Stillinger-Weber (SW) potential and a polarisable ion model (PIM). These are chosen as models conventionally associated with "covalent" and "ionic" interactions respectively. The liquid structure is changed continuously by varying specific structure-related parameters in either model; the magnitude of the three-body interactions in the SW potential and the anion polarisation in the PIM. The progression of the anomalous behaviour is observed as a function of these parameters and an explanation is given in terms of the evolution of the local environments. The bounds for the emergence of anomalous behaviour are also observed and have been correlated with the polymorphism of the crystalline phase diagram. The relationships between different anomalies are derived from thermodynamic relations and all the reported scenarios were successfully reproduced. Enhanced sampling methods were used to further study SW liquids and a new scenario describing the anomalous behaviour is detected. The progression of this scenario as a function of the model parameter and liquid structure is studied.