First principles calculations of structural properties of β-Si3−nCnN4 (n=0, 1, 2, 3)

Abstract

First principles calculations were performed for the structural properties of superhard materials, hexagonal β-Si3N4, β-C3N4, and two model structures of β-Si2CN4 and β-SiC2N4. Bulk modulus and lattice constants a and c have been obtained by fitting the total energy calculations to the Birch equation of state. As more C atoms are substituted for Si in β-Si3N4, the bulk modulus is observed to progressively increase up to 4.44 Mbar, comparable to that of diamond (4.43 Mbar), and both lattice constants a and c are reduced. For β-Si3N4 and β-C3N4, the lowest energy geometries have been searched by optimizing the internal structural parameters at the theoretical equilibrium lattice constants. The results are discussed and compared with available experiments.