Predicted structural evolution and detailed insight into configuration correlation, mechanical properties of silicon–boron binary compounds

Abstract

The crystal structures, phase stability, mechanical and electronic structures of silicon–boron binaries have been investigated systematically using first-principles of pseudopotential calculations. The calculated formation enthalpies of α-SiB₃, SiB₆ and SiB_n follow the sequence: ΔH_f (α-SiB₃) > ΔH_f (SiB₆) > ΔH_f (SiB_n), which is in good agreement with the previous experimental results. Monoclinic SiB₆ with P2₁/m symmetry and hexagonal SiB₃₆ with a B₈₀ unit (a ring linked by diametrically located 2 × B₂₈, 2 × B₁₂) are suggested as the more energetically and mechanically favorable phases by our calculations. The predicted bulk modulus of Si–B binaries is between 120 and 180 GPa, while there is a dramatic drop for two kinds of α-SiB₃ in the shear modulus from 130.3 GPa (SiB₃) to 71.1 GPa (SiB₄). We infer that the additional centered boron atoms, located long the spatial diagonal in SiB₄, is responsible for the weakness along this direction. In addition to β-SiB₃, the new proposed P2₁/m-SiB₆ and R3m-SiB₆ is found to be semiconducting with 0.41 eV indirect and 1.654 eV direct band gap, respectively. There is no band gap provided by band structures of SiB₄ and SiB₃₆, as well as their DOS values are quite large at Fermi level, indicating they are energetically unstable under 0 K and GPa.