Full ab initio geometry optimization of all known crystalline phases of Si3N4

The crystal structures of the four experimentally observed phases of Si3N4 with space groups P6(3) / m, P6(3), P3(1) / c, and Fd(3) / m are fully optimized by means of first-principles total-energy calculations. The relaxation of all internal parameters is achieved by using a simple finite difference method. For beta-Si3N4, the structure P6(3) without the mirror symmetry has a lower energy than the P6(3) / m with mirror symmetry by 0.238 eV per formula unit. The calculated ground-state properties are found to be in good agreement with experimental values.