Structural, Electronic, and Thermodynamic Properties of Tetragonal t-SixGe3−xN4

The structural, mechanical, anisotropic, electronic, and thermal properties of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ in the tetragonal phase are systematically investigated in the present work. The mechanical stability is proved by the elastic constants of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄. Moreover, they all demonstrate brittleness, because B/G < 1.75, and v < 0.26. The elastic anisotropy of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ is characterized by Poisson's ratio, Young's modulus, the percentage of elastic anisotropy for bulk modulus AB, the percentage of elastic anisotropy for shear modulus AG, and the universal anisotropic index AU. The electronic structures of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ are all wide band gap semiconductor materials, with band gaps of 4.26 eV, 3.94 eV, 3.83 eV, and 3.25 eV, respectively, when using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. Moreover, t-Ge₃N₄ is a quasi-direct gap semiconductor material. The thermodynamic properties of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ are investigated utilizing the quasi-harmonic Debye model. The effects of temperature and pressure on the thermal expansion coefficient, heat capacity, Debye temperature, and Grüneisen parameters are discussed in detail.