Design of Superhard Ternary Compounds under High Pressure: SiC2N4 and Si2CN4

Ab initio evolutionary methodology for crystal structure prediction is performed to explore the high-pressure structures of two ternary compounds, SiC2N4 and Si2CN4. For SiC2N4, we found intriguing high-pressure polymorphs with monoclinic C2/m and orthorhombic Cmmm symmetries containing tetrahedral CN4 and octahedral SiN6 units, respectively. For Si2CN4, two high-pressure monoclinic C2/m and P2(1)/m structures both consisting of octahedral SiN6 units were discovered. Thermodynamic study demonstrated that it is energetically desirable to synthesize the Cmmm structured SiC2N4 and P2(1)/m structured Si2CN4 at above 29 and 19 GPa, respectively. We have ruled out the earlier proposed high-pressure monoclinic structures for the two ternary compounds borrowed from known structural information. The newly predicted high-pressure phases of the two ternary compounds contain short, strong, and three-dimensional covalent bonding, which are responsible for the predicted superior mechanical properties, e.g., very large bulk and shear modulus. Hardness calculations suggest that Cmmm structured SiC2N4 and P2(1)/m (C2/m) structured Si2CN4 possess superhardness of 58.7 and 51.7 GPa (51.6 GPO, respectively. The underlying mechanism for superhardness of the predicted structures has been discussed and explained. Our current finding has demonstrated again the major role played by high pressure in search for new superhard materials.