Ordering inBxC1−xcompounds with the graphite structure

We present a theoretical study of the structural stability of ordered configurations of ${\mathrm{B}}_{\mathit{x}}$ ${\mathrm{C}}_{1\mathrm{-}\mathit{x}}$ solid solutions with the graphite structure. The calculations were carried out self-consistently using norm-conserving pseudopotentials in the local-density approximation. The cohesive energies of a number of ordered structures at different compositions x have been subjected to a cluster expansion, which enables us to predict the cohesive energy of the random alloy at the corresponding compositions and to gain insight on the tendency of the system towards ordering. We find that, at T=0 K, some ordered configurations have a cohesive energy higher than the corresponding random distribution at the same compositions. The inspection of the cluster interactions leads moreover to the conclusion that segregation of boron in graphite is highly discouraged and long-range order is likely to occur.