First-principles characterisation of new ternary heterodiamond BC2N phases

In order to discover the existence of new ultrahard materials, we have perf ormed the substitution of some carbon atoms with boron and nitrogen in two different diamond forms: cubic and hexagonal. The number of substituted car bon atoms was fixed in order to obtain BC2N heterodiamond phases isoelectro nic with diamond, which is the hardest known material. After the carbon ato m replacement, a full geometry relaxation was performed with a first-princi ple pseudopotential (PP) method to find the fundamental electronic ground s tate. These hypothetical ternary compounds are expected to be more thermall y and chemically stable than diamond and harder than cubic boron nitride. T his possibility makes them the most interesting class of compounds that can replace the expensive diamond in many mechanical applications. In the pres ent work, we employ the PP method to predict the mechanical properties of t he new BC2N phases. An estimation of the hardness is given with the calcula tion of the bulk and shear moduli. The relative stability between the phase s has also been studied by using both the full potential linearised augment ed 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 d ensity-of-state (DOS) analysis. (C) 2001 Elsevier Science B.V. All rights r eserved.