Pressure-induced structural transitions of diamond (100) surfaces

Despite extensive research conducted on the structural transitions of crystalline solids under pressure, the transitions occurring on the solid surfaces that transmit the pressure have been relatively neglected. Here, we investigate the pressure-induced structural transitions of the diamond (100) surface using molecular dynamics simulations combined with the volume-Constant Pressure Molecular Dynamics method for finite systems. Eight possible dimerized configurations were identified through an exhaustive method, considering both translational and rotational symmetries, which in turn define eight diamond (100) surfaces. These surfaces are nearly degenerate in energy at zero pressure, but their energy differences become larger under high external pressure. At finite temperatures, the increasing pressure induces graphitization of the surfaces. The transition pressures differ among the various surfaces. By calculating the free energies of the surfaces, we determined the most stable surfaces at various pressures and temperatures and constructed a schematic P-T "phase diagram" to illustrate the stability competition and structural transitions of the surfaces. This study provides a theoretical basis for the efficient utilization of diamond under high pressure and offers insights into the surface properties of materials under extreme conditions.