Physical Properties of Group 14 in P6222 Phase: First-Principles Calculations**Supported by the National Natural Science Foundation of China under Grant No. 61804120, and the Talent Science and Technology Foundation of Xi’an University of Architecture and Technology under Grant No. RC1612

Abstract

Two new Group IV element allotropes Si3 and Ge3 in P6222 phase are predicted in this work and their physical properties are investigated using the density functional theory. Each of the newly predicted allotropes has a superdense structure, which is mechanically, dynamically, and thermodynamically stable, as verified by elastic constants, phonon dispersion spectra and relative enthalpies, respectively. The mechanical anisotropy properties are studied in detail by illustrating the directional dependence of Young’s modulus, discussing the universal anisotropic index, and calculating shear anisotropy factors together with bulk moduli. It shows that P6222–Si3 exhibits the greater anisotropy than P6222–Ge3, and interestingly both of the newly predicted crystals appear to be isotropic in the (001) plane. Additionally, the Debye temperature, sound velocities, and the minimum thermal conductivity are examined to evaluate the thermodynamic properties of C3, Si3, and Ge3 in P6222 phase, and the electronic band structures are achieved by HSE06 hybrid functional, which indicate that P6222–C3 and –Si3 are indirect band gap semiconductors and P6222–Ge3 exhibits the metallic feature.