The origin of diamond's extreme properties lies in its unique structure, whereby each carbon atom is covalently bonded to four others by bonds in a tetrahedral configuration. Graphite contains planes of threefold coordinated carbon atoms, bound by strong bonds. These two crystals (graphite and diamond) have very different physical and chemical properties. Diamond is extremely hard, transparent over a wide range of wavelength and is a wide band gap semiconductor. In contrast, graphite is a soft material with semi-metallic electrical properties.
Even though the ground state energies of diamond and graphite are quite similar (with graphite being slightly more stable then diamond), a high potential barrier separates these two allotropes of carbon. Much effort, both experimental and theoretical, has been devoted to the question of how graphite can be transformed into diamond. According to the phase diagram of carbon, high pressures and high temperatures are required to cause this transformation, i.e. to overcome the potential energy barrier between graphite and diamond.
So called 'industrial diamond' has been synthesized commercially for over 50 years using ``high-pressure high-temperature'' (HPHT) techniques, in which diamond is crystallized from metal solvated carbon at P50-100 kbar and T6000 K. The more recent discovery that it is possible to produce polycrystalline diamond films, or coatings, by a wide variety of chemical vapor deposition (CVD) techniques.
sp bonded carbon has been found to exist not only as cubic crystals but also as hexagonal crystals (Londsdaleite). Cubic and hexagonal diamond have a rather similar structure, differing only in the stacking order of the sp bonded carbon layers. In contrast to the high pressure high temperature (HPHT) cubic diamond growth achieved under hydrostatic pressure, hexagonal diamond was observed to grow when uniaxial pressure was applied to liquid carbon during its solidification.
In the present study we simulate the precipitation and growth of diamond clusters inside an amorphous carbon or hydrogenated amorphous carbon network by rapid quenching of the compressed liquid phase, followed by volume expansion. This procedure is similar to that occurring during the bias-enhanced nucleation process. Our computational method is tight-binding molecular dynamics. This method incorporates electronic structure calculations in the molecular dynamics through an empirical tight-binding Hamiltonian, and bridges the gap between ab initio molecular dynamics and simulations using empirical classical potentials.
The simulations are carried out under both hydrostatic (in all three directions) and uniaxial pressure, exposing the samples to different initial pressures (densities) as well as to different cooling rates. At fast cooling rates (500-1000 K/ps) and high densities (3.7-3.9 g/cc), large diamond crystallites (containing up to 120 atoms) are formed. We find that the probability of precipitation of diamond crystallites increases with density and with cooling rate. Uniaxial compression of the samples does not lead to nucleation of the hexagonal form of diamond; all uniaxially compressed ordered sp clusters were identified to be cubic diamond, with random orientation relative to the compression direction. The diamond clusters generated inside hydrogenated amorphous carbon network are smaller and of the worse quality than those formed without hydrogen atoms. Hydrogen atoms are bonded with sp- and sp-bonded atoms, and are expelled from the sp amorphous or diamond clusters. Quantum confinement effects were not found in the diamond clusters embedded in an amorphous carbon network.
At slower cooling rates (200-500 K/ps), some samples transformed to graphite with an interplanar distance smaller than that of perfect graphite. Graphite formed under hydrostatic pressure has planes with random orientation whereas the planes of graphite formed under uniaxial pressure were oriented in parallel with the direction of compression.