Mechanical anisotropy and electronic properties of X2N2 (XH2 ): first-principles calculations*

The mechanical anisotropy, structural properties, electronic band structures and thermal properties of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are detailed and investigated in this work. The novel silicon nitride phase Si₂ N₂ (SiH₂ ) and germanium nitride phase Ge₂ N₂ (GeH₂ ) in the Cmc 2₁ structure are proposed in this work. The novel proposed Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are both mechanically and dynamically stable. The electronic band calculation of the HSE06 hybrid functional shows that C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are all wide band gap semiconductor materials, and C₂ N₂ (CH₂ ) and Si₂ N₂ (SiH₂ ) are direct band gap semiconductor materials, while Ge₂ N₂ (GeH₂ ) is a quasi-direct band gap semiconductor material, the band gap of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are 5.634 eV, 3.013 eV, and 2.377 eV, respectively. The three-dimensional and plane distributions of Young’s modulus, shear modulus and Poisson’s ratio of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) show that these materials have different degrees of mechanical anisotropy. The order of Young’s modulus of Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) in different directions is different from that of C₂ N₂ (CH₂ ). When the tensile axis is in a particular direction, the order of the Young’s modulus of Si₂ N₂ (SiH₂ ): E _110] <E _120] <E _111] <E _101] <E _010] =E _100] <E _011] <E _001], and the order of the Young’s modulus of Ge₂ N₂ (GeH₂ ): E _110] <E _111] <E _101] <E _120] <E _100] <E _010] <E _011] <E _001] .