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

Abstract

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

arXiv comment6 Pages, 4 Figures

References in corpus (11)

• Van der Waals Density Functional for General Geometries
• Van der Waals density functionals applied to solids
• A Higher-Accuracy van der Waals Density Functional
• High-accuracy first-principles determination of the structural, vibrational and thermodynamical properties of diamond, graphite, and derivatives
• Defect scattering in graphene
• Ferromagnetism in Oriented Graphite Samples
• Nucleation mechanism for the direct graphite-to-diamond phase transition
• Carbon Nitride Frameworks and Dense Crystalline Polymorphs
• Temperature evolution of infrared- and Raman-active phonons in graphite
• Defect topology and annihilation by cooperative cascading movement of atoms in highly neutron irradiated graphite
• Detonation-induced transformation of graphite to hexagonal diamond