Carbon-carbon interactions in warm dense titanium carbide

Encouraged by experimental reports of diamond precipitation in carbon-containing materials under warm dense matter conditions and in shock compression experiments, we examine the behavior of carbon in TiC using density functional theory--molecular dynamics simulations. Two polymorphs of TiC are considered, the ambient-pressure $B1$ ($Fm\overline{3}m$) structure in which C-C interactions are prevented by geometric constraints and a high-pressure $Cmcm$ structure in which C atoms condense into zigzag chains. Chemistry-inspired bonding analyses confirm the covalently bound nature of these chains and further illuminate important interatomic interactions in both structures. Upon melting of $B1$ TiC, new short-range C-C interactions develop in the liquid, while the preexisting C-C interactions in $Cmcm$ TiC persist in the liquid. The resulting carbon networks in the melt provide a promising environment for eventual diamond nucleation.