Divacancies in diamond: a stepwise formation mechanism

Diffusion of monovacancies in diamond creates various trapped multivacancy clusters. The simplest diffusion process leads to a divacancy, V-2. The formation of V-2 is a critical step in the formation of larger vacancy clusters. We explored the relaxed potential energy surfaces in the formation of V-2 by ab initio density functional theory (DFT) obtaining structures, relative energies, diffusion barriers and reaction paths. The constricted environment of a diamond lattice leads to unexpected chemical bonding situations with unusually elongated carbon-carbon bonds that have multicenter character. Divacancies separated by one carbon are not stable. Even though the divacancy is the most stable final product, a novel isolated divacancy with two vacant sites separated by two bonded carbons [V-C=C-V] can be transformed into a divacancy only via a very large (>4.3 eV) barrier and therefore it should be a defect observable in irradiated diamonds. Trapping of a vacancy by another by forming a stable divacancy trap affects the development of NV-defects in nitrogen implanted diamonds that are subject to active current interest in a wide range of applications.