Size-Dependent Temperature−Pressure Phase Diagram of Carbon

The polymorphic behavior of carbon in consideration of surface stress contributions to the internal pressure of nanocrystals is investigated through nanothermodynamics. It was found that nanodiamond is more stable than nanographite when the crystal size approaches the deep nanoscale. With theoretical and experimental data, a size-dependent temperature−pressure phase diagram of carbon is developed and established. It shows that the diamond/graphite/liquid triple point shifts toward lower temperature and pressure regions with decreasing nanocrystal size. This study also reveals a possible origin of metastable structure formation on the nanometer scale.