Hardness and electronic properties of Si-C-N structures

Three novel hexagonal Si-C-N structures, namely SiC₃N₃, SiC₇N₆, and SiC₁₃N₁₄, were constructed on the basis of the α-Si₃N₄ crystal structure. The stability of the three structures is demonstrated by analyzing their elastic constants and phonon dispersion spectra and by calculating their formation energies. The calculated band structures and partial densities of states suggest that the SiC₃N₃ and SiC₇N₆ structures possess hole conductivity. The electron orbital analyses indicate that the SiC₃N₃ and SiC₇N₆ crystals possess three-dimensional and one-dimensional conductivity, respectively. SiC₁₃N₁₄ is a semiconductor with a wide bandgap of 4.39 eV. Based on two different hardness models and indentation shear stress calculations, the Vickers hardness values of SiC₃N₃, SiC₇N₆, and SiC₁₃N₁₄ are estimated to be 28.04/28.45/16.18 GPa, 31.17/34.19/20.24 GPa, and 40.60/41.59/36.40 GPa. This result indicates that SiC₃N₃ and SiC₇N₆ are conductive hard materials while SiC₁₃N₁₄ is a quasi superhard material.