Phase stability, elasticity, hardness and the minimum thermal conductivity of Si2N2O polymorphs from first principles calculations

Some fundamental properties of Si2N2O polymorphs are calculated using first principles calculations based on density functional theory. The results indicate that orthorhombic-Si2N2O is the most stable phase at ambient conditions; it transforms into tetragonal-Si2N2O at a relatively low pressure (10 GPa). PBEsol predicts lattice constants and mechanical properties better than PBE, but PBE gives better phase transition parameters. The mechanical properties, such as bulk modulus, Young׳s modulus and shear modulus, are evaluated by the Voigt–Reuss–Hill approach. The tetragonal-Si2N2O exhibits larger mechanical moduli than other phases. The obtained Vickers hardness of Si2N2O structures shows that the hardness of tetragonal-Si2N2O is slightly higher than those of monoclinic and orthorhombic phases. The minimum thermal conductivities of Si2N2O polymorphs in crystalline and amorphous states are estimated, and we conclude that the thermal conductivities of amorphous Si2N2O phases are comparable to typical thermal barrier coatings.