First-principles calculation of Si-C-N structures with metallicity and high hardness

Stable hard and conductive ceramics under ambient conditions are vital for many industrial applications. However, maintaining high hardness while achieving semiconducting-metallic transition in Si3N4 materials is greatly challenging both experimentally and theoretically. Herein, two hexagonal Si-C-N structures, denoted SiC6N7-1 and SiC6N7-2, were constructed using alpha-Si3N4 as a matrix model. Calculations of elastic constants and phonon spectra showed stable structures under ambient pressure. Both SiC6N7-1 and SiC6N7-2 can possibly be synthesized at the pressures attainable by large-volume press technology. Compared to a-Si3N4, both Si-C-N ternary compounds achieved a semiconductive-metallic transition owing to the elemental substitution. Property calculations showed conductive Si-C-N compounds with high hardness values of 52 GPa for SiC6N7-1 and 31 GPa for SiC6N7-2. Therefore, the introduction of C atoms tuned the intrinsic properties of both structures, achieving high hardness along with good electrical conductivity. Overall, these findings look promising for future design and synthesis paths of novel conductive ceramics with prospective applications.