Predicting Physical Properties of Tetragonal, Monoclinic and Orthorhombic M_3N_4 (M=C, Si, Sn) Polymorphs via First-Principles Calculations

The recently discovered tetragonal, monoclinic and orthorhombic polymorphs of M_3N_4 (M=C, Si, Sn) are investigated by using first-principles calculations. A set of anisotropic elastic quantities, i.e., the bulk and shear moduli, Young's modulus, Poisson ratio, B/G ratio and Vickers hardness of M_3N_4 (M=C, Si, Sn) are predicted. The quasi-harmonic Debye model, assuming that the solids are isotopic, may lead to large errors for the non-cubic crystals. The thermal effects are obtained by the traditional quasi-harmonic approach. The dependences of heat capacity, thermal expansion coefficient and Debye temperature on temperature and pressure are systematically discussed in the pressure range of 0–10 GPa and in the temperature range of 0–1100 K. More importantly, o-C_3N_4 is a negative thermal expansion material. Our results may have important consequences in shaping the understanding of the fundamental properties of these binary nitrides.