A first-principles study of the properties of P-43m-Si3x2 (X = N, P and As)

The new P-43m-Si3P2 and P-43m-Si3As2 structures are predicted using the first-principles approach based density functional theory (DFT). The elastic constants, structural stability, phonon dispersion spectra, band structures, density of states, and optical properties of P-43m-Si(3)x(2) (X = N, P and As) have been analyzed. The values of the elastic constants indicate that their structures are mechanically stable. Each elastic constant of the Si3N2 is greater than the corresponding elastic constants of Si3P2 and Si3As2. The Young's moduli, shear moduli, bulk moduli, Pugh ratios and Poisson's ratios of P-43m-Si(3)x(2) are calculated at 0 GPa. Si3N2 has a larger Young's modulus, so it has higher hardness and good resistance to deformation. The bulk moduli of P-43m-Si(3)x(2) are isotropic. The shear modulus of Si3As2 is anisotropic. The Pugh ratios of P-43m-Si(3)x(2) are 0.50, 0.49 and 0.39, respectively. Their Poisson's ratios are 0.28, 0.29 and 0.33, respectively. The results show that they are brittle materials at zero pressure. The calculated phonon spectra confirm that they are dynamically stable. The calculated enthalpy of formation indicates their thermodynamic stability. The energy band gaps of P-43m-Si(3)x(2) calculated by HSE06 hybrid function are 0.786, 0.955 and 0.343 eV, respectively. Si3N2 has a direct bandgap, Si3P2 and Si3As2 have indirect bandgaps. The dielectric functions, refractive indices, optical reflectance spectra, absorption coefficients, conductivities and loss functions of P-43m-Si(3)x(2) are calculated. The calculated static dielectric constants of P-43m-Si(3)x(2) are 5.207, 9.237 and 10.072, respectively. The maximum values of the loss functions of P-43m-Si(3)x(2) are 6.408, 5.672 and 5.276 eV, respectively.