Atomistic structure and anomalous heat capacity of low-density liquid carbon: Molecular dynamics study with machine learning potential

Liquid carbon remains the source of several unsolved questions related to its structure and region of thermodynamic stability. Experiments demonstrate a drastic decrease in the density for liquid carbon along the graphite melting line in the pressure range P = 1-3 GPa and the nature of this phenomenon is unclear. A recent experimental study [A.M. Kondratyev and A.D. Rakhel, PRL (2019)] revealed another peculiar and still unexplained feature of the liquid carbon e its excessive heat capacity. Using classical molecular dynamics with machine learning potential GAP-20, we study the structural properties of liquid carbon and demonstrate that at P < 1-2 GPa it resembles a net of sp-hybridized chains, rather than a typical covalent liquid, with nanoscale porosity of this phase being responsible for the density decrease. We also show that excessive heat capacity could be a direct manifestation of a smooth transition from a high-density sp(2)-hybridized phase into a low-density sp-hybridized. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Liquid carbon presents unresolved questions regarding its structure and thermodynamic stability. Experimental observations show a significant density decrease along the graphite melting line, and the reasons behind this phenomenon remain unclear. Recent studies suggest that liquid carbon exhibits a network of sp-hybridized chains and the nanoscale porosity of this phase contributes to the density decrease. Additionally, the excessive heat capacity could be attributed to a smooth transition between high-density sp(2)-hybridized phase and low-density sp-hybridized phase.