First-principles characterisation of new ternary heterodiamond BC2N phases

In order to discover the existence of new ultrahard materials, we have performed the substitution of some carbon atoms with boron and nitrogen in two different diamond forms: cubic and hexagonal. The number of substituted carbon atoms was fixed in order to obtain BC2N heterodiamond phases isoelectronic with diamond, which is the hardest known material. After the carbon atom replacement, a full geometry relaxation was performed with a first-principle pseudopotential (PP) method to find the fundamental electronic ground state. These hypothetical ternary compounds are expected to be more thermally and chemically stable than diamond and harder than cubic boron nitride. This possibility makes them the most interesting class of compounds that can replace the expensive diamond in many mechanical applications. In the present work, we employ the PP method to predict the mechanical properties of the new BC2N phases. An estimation of the hardness is given with the calculation of the bulk and shear moduli. The relative stability between the phases has also been studied by using both the full potential linearised augmented plane waves (FP-LAPW) and the PP methods. Further, with the help of the FP-LAPW approach, the electronic properties are discussed by means of the density-of-state (DOS) analysis. (C) 2001 Elsevier Science B.V. All rights reserved.