Novel stable crystalline phase for the Stillinger-Weber potential

Tetrahedral liquids such as silicon, germanium, carbon, water, and silica are an important class of materials not only for industrial applications but also for our understanding of nature. The Stillinger-Weber potential is one of the most popular models for computer simulations of these systems with tetrahedral coordination, with the directionality of the interactions introduced via a three-body repulsive term which promotes locally tetrahedral arrangements. This approach has been extended to various tetrahedral liquids, providing valuable insight into the physics of group XIV elements and more recently water. Perhaps surprisingly, a consistent thermodynamic picture of this class of models is still lacking despite their widespread usage. Here we fill this gap by computing equilibrium phase diagrams for the silicon and water parametrizations and report a novel crystal structure which dominates the models' phase diagram at intermediate and high pressure, and thus warrants further theoretical and numerical investigation. Our results redefine the phase behavior of an important class of tetrahedrally coordinated systems, and also suggest that a more stringent test for simulation models is the ability to select the experimentally relevant crystalline phases, as opposed to just reproducing their mechanical stability.