Mechanical anisotropy and electronic properties of X2N2(XH2): first-principles calculations**Project supported by the National Natural Science Foundation of China (grant numbers 61864004 and 61564005).

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The mechanical anisotropy, structural properties, electronic band structures and thermal properties of C2N2(CH2), Si2N2(SiH2) and Ge2N2(GeH2) are detailed and investigated in this work. The novel silicon nitride phase Si2N2(SiH2) and germanium nitride phase Ge2N2(GeH2) in the Cmc21 structure are proposed in this work. The novel proposed Si2N2(SiH2) and Ge2N2(GeH2) are both mechanically and dynamically stable. The electronic band calculation of the HSE06 hybrid functional shows that C2N2(CH2), Si2N2(SiH2) and Ge2N2(GeH2) are all wide band gap semiconductor materials, and C2N2(CH2) and Si2N2(SiH2) are direct band gap semiconductor materials, while Ge2N2(GeH2) is a quasi-direct band gap semiconductor material, the band gap of C2N2(CH2), Si2N2(SiH2) and Ge2N2(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 C2N2(CH2), Si2N2(SiH2) and Ge2N2(GeH2) show that these materials have different degrees of mechanical anisotropy. The order of Young’s modulus of Si2N2(SiH2) and Ge2N2(GeH2) in different directions is different from that of C2N2(CH2). When the tensile axis is in a particular direction, the order of the Young’s modulus of Si2N2(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 Ge2N2(GeH2): E[110] < E[111] < E[101] < E[120] < E[100] < E[010] < E[011] < E[001].