Physical properties of Si₂Ge and SiGe₂ in hexagonal symmetry: First-principles calculations

We predict two novel group 14 element alloys Si2Ge and SiGe2 in P6222 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 P6222-Si2Ge and -SiGe2 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 P6222-Si2Ge and -SiGe2 are examined elaborately by illustrating the surface constructions of Young’s modulus, the contour surfaces of shear modulus, and the directional dependences of Poisson’s ratio, as well as discussing and comparing the differences with their corresponding group 14 element allotropes P6222-Si3 and -Ge3. Moreover, the Debye temperature and sound velocities are analyzed to study the thermodynamic properties of the proposed P6222-Si2Ge and -SiGe2.