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.