MD simulation of phase transformations in liquid carbon

The supercooled liquid of carbon is investigated by means of molecular-dynamics simulation. The dynamics of a glass and a supercooled liquid is compared and the glass transition temperature is determined by two methods: analyzing (i) the temperature dependence of thermodynamic coefficients and (ii) relaxation time of liquid. The pressure dependences of the glass transition temperature and the diamond melting temperature are found. The percolation properties of structures of sp3 atoms formed in liquid carbon with different numbers of embedded diamond crystallites are investigated. It is shown that the percolation cluster of 4-fold coordinated atoms forms when their total concentration in structure reaches a value close to 0.38 irrespective of the number of embedded crystallites. It turns out that the stability of diamond crystallites embedded into supercooled carbon liquid correlates with the presence of the percolation cluster of 4-fold coordinated atoms. The correspondence of diamond crystallite stability with percolation disappears at a temperature more than 5000 K. The topological criterion for the definition of tetrahedral amorphous carbon is proposed: amorphous carbon is tetrahedral if a percolation cluster exists in it and the embedded diamond crystallites are stable.