Structural and dynamical change of liquid carbon with pressure: ab initio molecular dynamics simulations

The structural, electronic and dynamical properties of normal and supercooled liquid carbon were studied by ab initio molecular dynamics simulations with a pressure range of 51.4-1060 GPa. A first-order liquid-liquid phase transition is not found even in supercooled liquid carbon along the 7500 K isotherm, but the structural change in the low-pressure range is more rapid than that at higher pressures because the threefold coordinated (sp(2)) local structure is more easily compressed than the fourfold-(sp(3)), fivefold-and sixfold-coordinated local structures. The calculated electronic density of state shows a shallow dip at 1060 GPa, indicating a nearly metallic bonding at higher pressures. The anomalous increase of diffusivity with increasing pressure is observed, which is similar to that observed in water and silicon dioxide and is due to the gradual breaking of an open network structure. Our results indicate that it is easiest to form diamond crystal nuclei in liquid carbon at about 500 GPa, which corresponds to the position of the maximum in the melting curve.