Abstract
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 P21/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 P21/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...
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