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Theoretical study on elastic properties of Si2N2O by ab initio calculation

The elastic constants of crystalline Si2N2O remain unknown since it was discovered in the 1960s. We determine the nine independent elastic constants of orthorhombic Si2N2O by ab initio calculations. We applied various deformation modes with strains up to ±0.01 to a unit cell, calculated the energy-strain relationships, and deduced all the elastic constants by fitting the harmonic-oscillation function. Our results are as follows: C11 = 311.1, C22 = 238.5, C33 = 317.9, C44 = 136.1, C55 = 57.6, C66 = 73.9, C12 = 79.6, C13 = 52.2, and C23 = 33.6 GPa. Despite the different crystal structures and symmetries, the direction-over-averaged Young’s modulus of Si2N2O is well explained by the nitrogen content and Young’s moduli of α-SiO2 and β-Si3N4. The anisotropy of sound-wave velocity was investigated, and its origin was examined on the basis of the crystallographic structure. The quasi-isotropic plane for the longitudinal-wave propagation was identified.