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

We predict two novel group 14 element alloys Si2Ge and SiGe2 in P6(2)22 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(2)22-Si2Ge and P6(2)22-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 P6(2)22-Si2Ge and P6(2)22-SiGe2 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(2)22Si 3 and P6(2)22-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(2)22-Si2Ge and P6(2)22-SiGe2.