Quantum-chemical study of the structure and properties of hypothetical superhard materials based on the cubic silicon-carbon nitrides

Density functional B3LYP/6-31G(d,p) calculations have been carried out to s tudy the structural peculiarities and physical properties of the series of cubic (defect zinc-blende) silicon-carbon nitrides with composition SixC3-x N4 (x = 0, 1, 2, 3). As model systems, we have considered six clusters with the structure of the adamantane molecule (CH)(4)(CH2)(6) (I), hexamethylen etetramine-like molecules N-4(CH2)(6-n)(SiH2)(n) (II-V) (n = 0, 2, 4, 6), a nd silicon-substituted adamantane molecule (SiH)(4)(SiH2)(6) (VI). These 10 heavy-atom clusters have been used to simulate the crystalline fragments o f diamond (I), cubic (defect zinc-blende) silicon-carbon nitrides (II-V), a nd cubic (zinc-blende) silicon solid (VI). It was found that the full B3LYP /6-31G(d,p) geometry optimization of these clusters allow us to reproduce t he structures, unit cell parameters, and bulk modulus (hardness) of real cr ystals (I and VI) quite well and to predict the structural and mechanical p roperties of the hypothetical crystalline compounds (II-V).