A defect molecule calculation for the vacancy in graphite

Abstract

The defect-molecule calculations of Coulson et al. [2] of the electronic structure of a vacancy in graphite have been improved and extended to allow for symmetric relaxation. Various problems encountered previously [1,2] have been shown to be due to the use of the Goeppert-Mayer and Sklar approximation. The rebonding forces on the nearest-neighbour atoms to the vacancy have been calculated and a perfect-lattice model was used to describe lattice relaxation effects; the difficulties experienced in work on diamond and silicon [5,6] using an imperfect-lattice model were avoided. For the neutral vacancy, the stabilization energy, E v , was found to be 1.44eV, and the (symmetric) relaxation energy, E R , to be 3.32 eV. The neutral vacancy formation energy has been re-calculated, and using the above values for E v and E R a value of 8.75 eV was obtained. This is somewhat higher than experimental estimates, but the difference may be explained by a high value for the formation entropy.