Density functional study of the ternary Si₂CN₄ and C_Si: Si₃N₄ compounds

Abstract

The structural and electronic properties of the recently synthesized ternary crystal Si₂CN₄ are investigated by means of density functional calculations, in comparison with pure and C-defective β Si₃N₄. The theoretical equilibrium lattice parameters of Si₂CN₄ 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 Si₂CN₄ 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_Si:β Si₃N₄, at low defect concentrations, either for scattered defect distributions or neighboring C_Si, 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 Si₂CN₄.