Using pressure to probe thermodynamic anomalies in tetrahedrally-bonded materials

Tetrahedrally-bonded materials, such as silicon, diamond, or gallium nitride, are characterized by a low coordination number of 4 in the crystalline phase and, in general, can exhibit a liquid phase with higher density and coordination. This leads to interesting thermodynamic behavior, including the lowering of the melting temperature with increasing pressure and the possible existence of distinct low- and high-density liquid phases. Using molecular dynamics simulations, we explored the role of pressure and the degree of tetrahedrality on the structure and phase equilibria between the crystalline and liquid phases of tetrahedrally-bonded materials. In addition to the thermodynamic melting point, we determined the temperature of mechanical stability (spinodal temperature) as a function of pressure. The latter temperature is relevant to the laser pulse rapid melting of tetrahedrally-bonded materials. The results of our simulations indicate the possibility of the existence of a thermodynamically stable low-density liquid phase of silicon at high pressures. Our simulation also suggests that GaN is unlikely to exhibit anomalous thermodynamic behavior due to a high degree of tetragonality preventing the formation of high-density liquid, even at high pressures.