Diverse forms of $σ$ bonding in two-dimensional Si allotropes: Nematic orbitals in the $MoS_2$ structure

The interplay of $sp^2$- and $sp^3$-type bonding defines silicon allotropes in two- and three-dimensional forms. A novel two-dimensional phase bearing structural resembleance to a single MoS$_2$ layer is found to possess a lower total energy than low-buckled silicene and to be stable in terms of its phonon dispersion relations. A new set of cigar-shaped, nematic orbitals originating from the Si $sp^2$ orbitals realizes bonding with a 6-fold coordination of the inner Si atoms of the layer. The identification of these nematic orbitals advocates diverse Si bonding configurations different from those of C atoms.

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