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We have performed ab-initio molecular dynamics simulations to elucidate themechanism of the phase transition at high pressure from hexagonal graphite (HG)to hexagonal diamond (HD) or to cubic diamond (CD). The transition from HG toHD is found to occur swiftly in very small time of 0.2 ps, with largecooperative displacements of all the atoms. We observe that alternate layers ofatoms in HG slide in opposite directions by (1/3, 1/6, 0) and (-1/3, -1/6, 0),respectively, which is about 0.7 {\AA} along the pm[2, 1, 0] direction, whilesimultaneously puckering by about pm0.25 {\AA} perpendicular to the a-b plane.The transition from HG to CD occurred with more complex cooperativedisplacements. In this case, six successive HG layers slide in pairs by 1/3along [0, 1, 0], [-1, -1, 0] and [1, 0, 0], respectively along with thepuckering as above. We have also performed calculations of the phonon spectrumin HG at high pressure, which reveal soft phonon modes that may facilitate thephase transition involving the sliding and puckering of the HG layers. Thezero-point vibrational energy and the vibrational entropy are found to haveimportant role in stabilizing HG up to higher pressures (>10 GPa) andtemperatures than that estimated (<6 GPa) from previous enthalpy calculations.

Phase Transition Mechanism of Hexagonal Graphite to Hexagonal and Cubic Diamond: Ab-initio Simulation