First-principle investigation on the thermodynamics of X₂N₂O (X= C, Si, Ge) compounds

The structures under different pressures, elastic properties, electronic structures and lattice vibrations of the X₂N₂O (X= C, Si, Ge) compounds are investigated by using the first-principle method. Based on the phonon density of state, the thermodynamic properties of the present compounds are studied under different pressures and at different temperatures. The structural parameters including the bond lengths and bond angles are in agreement with available experimental measurements and theoretical calculations. We employ the elastic theory to calculate the nine independent elastic constants (C_ij) and the derived elastic moduli (B, G, E, v). Results indicate that these X₂N₂O (X= C, Si, Ge) compounds are mechanically stable and show the brittle behaviors. The electronic properties of the present compounds are analyzed by using the band structure and density of states. The phonon dispersion calculations imply that the present compounds are dynamically stable. Based on the quasi-harmonic approximation, the calculations of the specific heat indicate that the temperature in a range of 0 K-1500 K and pressure in a range of 0 GPa-40 GPa have a large effect on the thermal quantities of Ge₂N₂O, compared with on those of the C₂N₂O and Si₂N₂O compounds.