Structural, elastic constants, hardness, and optical properties of pyrite-type dinitrides (CN2, SiN2, GeN2)

The crystal structures, band structures, elastic constants, hardness, and optical properties of pyrite-type dinitrides (CN2, SiN2, and GeN2) are obtained from the density functional theory using the plane-wave pseudopotential (PWP) method within the local density and generalized gradient approximations. The formation enthalpies for AN2 (A=C, Si, and Ge) compounds suggest the three structures that are stable. The calculated band structures show the indirect gaps (Γ-R) in CN2, SiN2, and GeN2. The intrinsic hardnesses of AN2 (A=C, Si, and Ge ) compounds are calculated. Our results show that the cubic CN2 and SiN2 are superhard materials. Furthermore, we studied the optical properties such as the complex dielectric function and the electron energy loss spectra.