Silylated carbodiimides in molecular and extended structures

This work studies the ternary Si-C-N phases SiC₂N₄ and Si₂CN₄,exploiting an analogy between the NCN and O groups. Starting from the molecular model of N,N^'-bis(trimethylsilyl)-carbodiimide and proceeding to extended models, we calculate that the energy hypersurface associated with the Si-N=C bond angle φN is very shallow, for both molecular and extended structures. We propose a crystal structure for the low-temperature modification α-SiC₂N₄ in space group P4₃22 (95), which is 40meV(~4kJ/mol) lower in energy than an ideal cubic arrangement in space group Pn3m.A second structure, β-SiC₂N₄ [space group P4n2 (118)], is slightly higher in energy than α-SiC₂N₄,but still more stable than the cubic structure, and may be the high-temperature structure of SiC₂N₄.Both variants of SiC₂N₄ show a small bulk modulus of about 8GPa(~0.13Mbar),suggesting a high compressibility of these nonoxide covalently bonded materials. For Si₂CN₄ we refined the crystal structure of the compound within the experimentally determined space group Aba2 (41). We also found a second candidate nearly equal in energy, with space group Cmc2₁,differing only in the connection pattern of the SiN₂ layered sheets. Both ternary compounds appear to be thermodynamically unstable with respect to decomposition into Si₃N₄,C, and molecular N₂.