Prediction of Novel SiCN Compounds: First-Principles Calculations

Abstract

A stable ambient pressure phase of t-SiCN was determined by extensive forecasting through crystal structure analysis by particle swarm optimization algorithm. The energy of t-SiCN was much lower than that of c-SiCN, a structure proposed 40 years ago. Two high-pressure phases of o-SiCN and h-SiCN were also predicted. Transformations from ambient pressure phase t-SiCN to o-SiCN and h-SiCN occurred at 21.6 and 21.9 GPa, respectively. It is likely to be quenched to ambient conditions for h-SiCN and o-SiCN, as the energy values of h-SiCN and o-SiCN are very close at the range of pressures we calculated. The three novel phases of t-SiCN, o-SiCN, and h-SiCN are all mechanically and dynamically stable at ambient pressure as determined by their elastic constants and phonon dispersions. At ambient pressure, the densities of t-SiCN, o-SiCN, and h-SiCN crystals were calculated to be 3.20, 3.71, and 3.75 g/cm3, respectively. On the basis of the density of states of these compounds, t-SiCN was a narrow gap semiconductor with a band gap of 0.89 eV, whereas o-SiCN and h-SiCN were of hole-type conductivity. The hardness of t-SiCN was 41.5 GPa, which indicates that it is a superhard material. At ambient pressure, o-SiCN and h-SiCN exhibited hardness values of 30.0 and 30.2 GPa, respectively.