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

The structural and electronic properties of the recently synthesized ternary crystal Si 2 CN 4 are investigated by means of density functional calculations, in comparison with pure and C-defective β Si 3 N 4 . The theoretical equilibrium lattice parameters of Si 2 CN 4 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 2 CN 4 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 Si 3 N 4 , 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 Si 2 CN 4 . Copyright Società Italiana di Fisica, Springer-Verlag 1999