Constant-pressure first-principles studies on the transition states of the graphite-diamond transformation

We have investigated the activation barriers and the intermediate paths of the transformation to cubic diamond and that to hexagonal diamond from graphite under pressure, allowing both atomic geometry and unit-cell shape lo vary, in order to clarify the difference of the microscopic mechanisms between them. For this investigation, we have developed a method, of finding a saddle point of the potential surface automatically on the basis of constant-pressure first-principles molecular dynamics. At the transition states, the length of the interlayer bonding is universal irrespective of the transformations and pressures, while there is a difference in the lateral displacement of atoms on the paths. It is found that the activation barrier from graphite to cubic diamond is lower than that to hexagonal diamond by similar to 70 meV/atom. These results suggest that, whenever collective slide of graphite planes is allowed, the transformation to cubic diamond is favored, and that hexagonal diamond can be obtained only when such slide is prohibited.