Ground-state properties of cubic C-BN solid solutions

The ab initio calculations of the ground-state properties of cubic C-BN solid solutions are presented for the first time to our knowledge. The Löwdin perturbation technique has been used in the ab initio LMTO-ASA method to calculate the total energy, the equilibrium lattice constant, the bulk modulus and its pressure derivative and the formation energy of mixed crystals of cubic BN and diamond. The calculated results indicate that the solid solution between C (diamond) and cubic BN is nonideal, with the equilibrium lattice constants larger than the predicted values of ideal mixing (Vegard's law) and the positive energies of formation. The bulk moduli of BN-rich (x > 0.5) are lower than those of diamond, the value of ideal mixing and even cubic BN. This anomalous behaviour is consistent with the nonideal expansion of the equilibrium lattice constant. The calculated results are in good agreement with the recent experimental measurements by Knittle et al 1995 Phys. Rev. B 51 12149.