Density functional study of the ternary Si<Subscript>2</Subscript>CN<Subscript>4</Subscript> and C<Subscript>Si</Subscript>: Si<Subscript>3</Subscript>N<Subscript>4</Subscript> compounds

The structural and electronic properties of the recently synthesized ternary crystal Si<Subscript>2</Subscript>CN<Subscript>4</Subscript> are investigated by means of density functional calculations, in comparison with pure and C-defective β Si<Subscript>3</Subscript>N<Subscript>4</Subscript>. The theoretical equilibrium lattice parameters of Si<Subscript>2</Subscript>CN<Subscript>4</Subscript> well agree with experimental results, and the optimized atomic positions refine those extracted from diffraction data, permitting a precise description of the atomic structure. According to our calculations, the enthalpy of the reaction of dissociation of crystalline Si<Subscript>2</Subscript>CN<Subscript>4</Subscript> into silicon nitride, silicon carbide and molecular nitrogen is positive, suggesting that the novel compound should be relatively stable at normal conditions, consistently with the experimental observation. The analysis of C<Subscript>Si</Subscript>:β Si<Subscript>3</Subscript>N<Subscript>4</Subscript>, at low defect concentrations, either for scattered defect distributions or neighboring C<Subscript>Si</Subscript>, reveals the presence of many dilated bonds. The microscopic stress is mainly responsible for the lower stability of carbon defective silicon nitride with respect to Si<Subscript>2</Subscript>CN<Subscript>4</Subscript>. Copyright Società Italiana di Fisica, Springer-Verlag 1999