First-Principles Studies on Properties of Boron-Related Impurities in c-BN

We investigate, by first-principles calculations, the pressure dependence of formation enthalpies and defective geometry and bulk modulus of boron-related impurities (V_B, C_B, N_B, and O_B) with different charged states in cubic boron nitride (c-BN) using a supercell approach. It is found that the nitrogen atoms surrounding the defect relax inward in the case of C_B, while the nitrogen atoms relax outward in the other cases. These boron-related impurities become much more stable and have larger concentration with increasing pressure. The impurity C_B⁺¹ is found to have the lowest formation enthalpy, make the material exhibit semiconductor characters and have the bulk modulus higher than ideal c-BN and than those in the cases of other impurities. Our results suggest that the hardness of c-BN may be strengthened when a carbon atom substitutes at a B site.