Physical Properties of Si₂Ge and SiGe₂ in Hexagonal Symmetry: First-Principles Calculations

We predict two novel group 14 element alloys Si_2Ge and SiGe_2 in P6_222 phase in this work through first-principles calculations. The structures, stability, elastic anisotropy, electronic and thermodynamic properties of these two proposed alloys are investigated systematically. The proposed P6_222-Si_2Ge and P6_222-SiGe_2 have a hexagonal symmetry structure, and the phonon dispersion spectra and elastic constants indicate that these two alloys are dynamically and mechanically stable at ambient pressure. The elastic anisotropy properties of P6_222-Si_2Ge and P6_222-SiGe_2 are examined elaborately by illustrating the surface constructions of Young's modulus, the contour surfaces of shear modulus, and the directional dependence of Poisson's ratio; the differences with their corresponding group 14 element allotropes P6_222-Si_3 and P6_222-Ge_3 are also discussed and compared. Moreover, the Debye temperature and sound velocities are analyzed to study the thermodynamic properties of the proposed P6_222-Si_2Ge and P6_222-SiGe_2.