Bond Energy of Vacancies and Interstitial Atoms in Graphite

Abstract

Baker and Kelly¹ have estimated that the energy to form a vacancy in graphite (that is, the energy to remove an atom from the lattice and place it on the free surface) is U_f = 3.3±0.9 eV. (The symbol ± refers to maximum limit of values.) They comment that this is less than the heat of sublimation, U_s = 7.5 eV per atom, indicating an energy-relaxation of 4.2 ± 0.9 eV at the vacancy. Equality should occur² if the broken lattice bonds around the vacancy remain broken. In graphite, however, all the lattice bonds are unsaturated, and we would expect that the unpaired electrons of the three atoms around the vacancy would form covalent bonds with electrons derived from unsaturated bonds of adjacent atoms (cf. ref. 3). These we will call partial bonds. The net energy associated with their formation (denoted by 3U₁ per vacancy) we identify with the required energy relaxation. Thus: