Water-like Anomalies and Phase Behavior of a Pair Potential that Stabilizes Diamond

Water, silicon, silica, and other liquids that favor tetrahedral order display thermodynamic, dynamic, and structural anomalies in the pressure range in which they form tetrahedrally coordinated crystals. The tetrahedral order in these liquids is induced by anisotropic hydrogen bonding or covalent interactions, or, in ionic melts, by an appropriate size ratio of the ions. Simple isotropic two-length scale models have been extensively used to understand the origin of anomalies in complex liquids. However, single-component isotropic liquids characterized to date generally do not stabilize tetrahedral crystals, and in the few cases that they do, it was found that the liquids do not display anomalies in the region of the tetrahedral crystal. This poses the question of whether it is possible for isotropic pair potentials to display water-like phase behavior and anomalies. In this work, we use molecular dynamics simulations to investigate the phase behavior and the existence and loci of anomalies of a single-component purely repulsive isotropic pair potential that stabilizes diamond in the ground state over a wide range of pressures. We demonstrate that, akin to water, silica, and silicon, the isotropic potential of Marcotte, Stillinger, and Torquato (MST) presents structural, dynamic, and thermodynamic anomalies in the region of stability of the tetrahedral crystal. The regions of anomalies of MST are nested in the T-p plane following the same hierarchy as in silica: the region of diffusional anomalies encloses the region of structural anomalies, which in turn contains the region of thermodynamic anomalies. To our knowledge, MST is the first example of pair potential for which water-like anomalies are associated with the formation of tetrahedral order.