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Hexagonal graphite to cubic diamond transition from equilibrium lines and barrier calculations

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  • Shen Qiu

Abstract

Phase equilibrium lines of hexagonal graphite (hg) and cubic diamond (cd) phases of carbon as well as a saddle-point equilibrium line between the two phase equilibrium lines are studied by first-principles total-energy calculations. The Gibbs free energies (G) of the three equilibrium lines determine the transition pressure p t =70 kbar (0.070 Mbar) from hg phase to cd phase and the barrier height at p t of ΔG=178 mRy/atom that stabilizes the two phases against a phase transition. The cd phase becomes unstable at V=13.6 au 3 /atom (p=26 Mbar) where the curvature at the equilibrium point of the energy curve (denoted E V (c/a) curve) goes to zero. The hg and cd phase equilibrium lines cross at V=14.5 au 3 /atom where the regular hg phase (with one minimum in each E V (c/a) curve) ends and the irregular hg phase (with two minima in each E V (c/a) curve) develops. The feature of “two phase equilibrium lines cross” was not observed in our previous work [S.L. Qiu, P.M. Marcus, J. Phys.: Condens. Matter 24, 225501 (2012); S.L. Qiu, P.M. Marcus, Eur. Phys. J. B 86, 425 (2013)] where the two interacting crystal phases have a common unit cell with different c/a ratios. This work demonstrates that the saddle-point equilibrium line along with the two phase equilibrium lines are all needed for a complete description of crystal phases and their transitions under pressure. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2014

Suggested Citation

  • Shen Qiu, 2014. "Hexagonal graphite to cubic diamond transition from equilibrium lines and barrier calculations," The European Physical Journal B: Condensed Matter and Complex Systems, Springer;EDP Sciences, vol. 87(7), pages 1-8, July.
    • Handle: RePEc:spr:eurphb:v:87:y:2014:i:7:p:1-8:10.1140/epjb/e2014-50260-8
    DOI: 10.1140/epjb/e2014-50260-8
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    Solid State and Materials;

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