Theoretical Research Of Metasatable Phases In Si-C-N System By First Principles

The binary and ternary compounds with Si, C or N atoms represent excellent chemical and physical properties, thus they are widely used in industry. The experimental and theoretical studies have been always made in recent decades. However, the number of Si-C-N compounds with clearly stoichiometry is only three, and much fewer for Si3N4 than SiC and SiO2. Here we used CALYPSO code to predict the crystal structures of Si-C-N and Si3N4 compounds. Then we calculated the enthalpy differences, verified the structural stabilities of the compounds we predicted by CASTEP. And last, the electrical and physical properties for the stable compounds were further studied.The structures of 3C-SiC with C atoms replaced by N atoms, or Si atoms replaced by N atoms, or the first interstitial positions occupied by N atoms, or the second interstitial positions occupied by N atoms, were implemented by CASTEP code, the effect on the structural stabilities of 3C-SiC with N atoms were studied. And the effect of N content on electrical properties, elastic properties, and Vickers hardness of Si-C-N compounds were evaluated for the stable structures. Results show that the 3C-SiC with C atoms replaced by N, and the rate of the substituted equal to 1/4 or 1/8 are of mechanical and dynamic stability. The stable Si-C-N compounds have the quality of electron conductivity, which is due to the electron number of the out shell for N is one more than that for C. The Vickers hardness of the stable structures reduced with the increasing of N content.The compounds of SiCN have been extensive forecasted through CALYPSO combined with CASTEP code. By contrast, c-SiCN reported by Kawamura was studied here. Three SiCN crystal structures have been predicted, they are ambient pressure phase t-SiCN, high pressure phases o-SiCN and h-SiCN. The energy values of all three phases are much lower than that of c-SiCN. The t-SiCN is one superhard semiconductor with a narrow gap 0.89 eV, and the o-SiCN and h-SiCN are hard materials with hole-type conductivity.Exploration of novel phases of silicon nitride was performed using a combination of CALYPSO and CASTEP code. Two phases t-Si3N4 and m-Si3N4 with the energy lower than γ-Si3N4 have been found, and one further high pressure phase o-Si3N4 with transition pressure 198 GPa was predicted. Three compounds are all wide gap semiconductors. The transition from brittle manner to ductile manner occurred when the pressure increase. The coordination numbers get larger and the bond length become longer for high pressure phase than low pressure phase. Their Vickers hardness values are almost equal to α、β and γ phases, and the values change higher with the pressure increase.Similar Si-N layers exist in α, β and wII phases. Symmetry operations for the Si-N layers were used to build some like-β-Si3N4 crystal structures. Enthalpy differences, stability, electrical and physical properties were studied by CASTEP code. We found the compounds with low energy must meet the following requirements for the two adjacent Si-N layers: only one mirror layers and the center N atoms at different c axis direction. In the like--β-Si3N4 crystal structures, we found one potential δ phase, which is part of α phase. That is the adjacent Si-N layers of two β phases in α phase.