The Rise and Fall of Anomalies in Tetrahedral Liquids

TL;DR

This study explores how thermodynamic anomalies and structural changes in tetrahedral liquids vary with the degree of tetrahedrality, revealing that certain anomalies appear only at intermediate levels of tetrahedrality.

Contribution

It systematically maps the relationship between tetrahedrality and anomalies in liquids, providing insights into the structural origins of these anomalies across different materials.

Findings

Density anomaly appears at intermediate tetrahedralities

Heat capacity anomaly occurs at high tetrahedralities

Structural changes involve increased four-coordination and tetrahedral order

Abstract

The thermodynamic liquid-state anomalies and associated structural changes of the Stillinger-Weber family of liquids are mapped out as a function of the degree of tetrahedrality of the interaction potential, focusing in particular on tetrahedrality values suitable for modeling C, H2O, Si, Ge and Sn. We show that the density anomaly, associated with a rise in molar volume on isobaric cooling, emerges at intermediate tetrahedralities (e.g. Ge, Si and H2O) but is absent in the low (e.g. Sn) and high (e.g. C) tetrahedrality liquids. The rise in entropy on isothermal compression associated with the density anomaly is related to the structural changes in the liquid using the pair correlation entropy. An anomalous increase in the heat capacity on isobaric cooling exists at high tetrahedralities but is absent at low tetrahedralities (e.g. Sn). Structurally, this heat capacity anomaly originates in a sharp rise in the fraction of four-coordinated particles and local tetrahedral order in the liquid as its structure approaches that of the tetrahedral crystal.