First Principles Investigations in the Carbon-silicon System of Novel Tetragonal C8 and Si8 Allotropes, and Binary Si2C6 and Si4C4 Phases
Samir F Matar*
Lebanese German University (LGU), Sahel-Alma, Jounieh, Lebanon
*Corresponding Author: Samir F Matar, Lebanese German University (LGU), Sahel-Alma, Jounieh, Lebanon.
July 21, 2022; Published: July 28, 2022
Within the carbon-silicon system, novel tetragonal C8 and Si8 allotropes and two silicon carbides Si2C6 and Si4C4 are devised. The propositions are based on density functional theory (DFT) calculations of template structures, optimized to ground state energies and subsequently derived physical properties. All four phases belong to primitive tetragonal space group P-4m2 N°115 characterized by large c/a tetragonality ratio. The structures consist of corner sharing C4 and Si4 tetrahedra highlighting covalent (in C8) and polar covalent (in silicon carbides) chemical systems illustrated with charge density projections. C8 is identified as ultra-hard with a Vickers hardness (HV) amounting to 113 GPa, a result assigned to the large tetragonality ratio. Oppositely, Si8 allotrope is found soft with HV = 13 GPa alike cubic Si, and Si4C4 is identified with HV =33 GPa alike experimental SiC. Larger C-content Si2C6 is identified as super-hard with HV = 51 GPa. All new phases are mechanically (elastic constants with bulk and shear moduli) and dynamically (phonon band structures) stable. The electronic band structures are characteristic of insulating C8 with, a large band gap of about 5 eV like diamond, and semi-conducting Si2C6, Si4C4, and Si8 with band gaps of ~1 eV. The results are claimed as enriching further the Si-C system with novel materials aimed at diverse electronic and mechanic applications.
Keywords: DFT; Carbon; Silicon; Silicon Carbide; Super-hard; Elastic Constants; Phonons
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