Search for ultra-hard materials:: theoretical characterisation of novel orthorhombic BC2N crystals

By replacing of the carbon atoms with boron and nitrogen in the crystalline phase of fcc diamond, new candidates for ultra-hard materials have been proposed in the BC2N stoichiometry. Their characterisation is here performed using methods based on density functional theory in its local density approximation. Full geometry relaxation of the substituted diamond gives the possibility to discover metastable systems which are isoelectronic with diamond and cubic boron nitride (c-BN). Such a kind of alloys are expected to be thermally and chemically (i.e. versus oxidation) more stable than diamond and harder than c-BN, That is, ternary B-C-N compounds could likely supersede the expensive diamond in various applications. In the present work we also employ first-principles methods to predict the mechanical properties of the achieved three-dimensional BC2N phases. Calculations of the cohesive properties, bulk and elastic moduli are presented. Considering the correlation between the hardness and the shear modulus and the values of the bulk moduli, we predict that the phases I and II of BC2N could be harder than c-BN. Electronic properties were studied in detail by means of density of states and band structure analysis. The calculation of the C, N and B K ELNES spectra is also shown for the presented phases. (C) 2001 Elsevier Science Ltd. All rights reserved.