First principles search of hard materials within the Si-C-N ternary system

Starting from C3N4 and Si3N4 stoichiometries and from the pseudocubic model structure of the former, intermediate phases SiC2N4 and Si2CN4 are proposed and geometry optimised within density functional built pseudopotential method using both local density (LDA) and generalised gradient approximations (GGA). The ternary compounds are found to be less stable than the two binary systems but the trends in the calculated magnitudes of the bulk moduli B-0 from the fit of the E(V) curves with Birch equation of state: B-0 (SiC2N4) = 334.5 GPa and B-0 (Si2CN4) = 270.3 GPa can be interpolated from those of the two extreme compounds: B-0 (C3N4) = 424.1 GPa and B-0 (Si3N4) = 219.8 GPa. This translates the chemical role of the substituting element on one hand and allows validating Cohen's semiempirical law relating B-0 to the inverse powers of the average interatomic distances on the other hand. From a mismatch of the chemical bonding in Si(C)-N-C(Si) chain observed by the electron localisation function (ELF) plot we propose an interpretation for the instability of the intermediate ternary phases. The electronic structure (density of states and band structures) obtained from augmented spherical wave (ASW) calculations of the relaxed structures point to semiconducting behaviour with smaller band gaps for the intermediate phases (similar to 2 eV, compared with the similar to 4 eV gap of binaries). (C) 2004 Elsevier SAS. All rights reserved.