First-principles calculations to investigate structural, electronic and optical properties of Cmc21-Ge2As2X (X = S, Se, Te and Po) under pressure effect

A group of new compound material Ge2As2X(X=S,Se,Te,Po) with Cmc21 space group is theoretically studied based on density functional theory (DFT) with GGA methods. By analyzing the phonon spectra and elastic constants, it is shown that these compounds have mechanical stability and dynamic stability at 0 GPa and 10 GPa. The lattice constants at different pressures indicate that these compounds are compressible. According to the elastic constants, the elastic (bulk, shear and Young's) moduli, Poisson's ratios, Pugh's ratios and elastic anisotropy indices of the material are calculated at 0 GPa and 10 GPa respectively. The calculation results show that the bulk modulus, B, of Ge2As2Se is the largest, the shear modulus, G, and Young's modulus, E, of Ge2As2S is the largest. The value of the B, G and E are increased due to pressure, indicating that their resistance to deformation is enhanced. According to Pugh ratio, K, except Ge2As2Te, the other compounds belong to brittle materials at 0 GPa, and all compounds belong to ductile materials at 10 GPa. Under pressure, the ductility of the compounds is enhanced. At 0 GPa and 10 GPa, the compounds possess elastic anisotropy. Under pressure, the elastic anisotropy of Ge2As2S and Ge2As2Se increase, while the elastic anisotropy of Ge2As2Te and Ge2As2Po decrease. The band structures and electronic densities of states of these compounds are calculated by the HSE06 method. The band gaps of the four compounds are 0.822eV, 0.725eV, 0.461eV and 0.331eV, at 0 GPa. In contrast, at pressurized 10 GPa, the band gaps of the compounds become zero, indicating their transition from semiconductor to metal. Finally, the optical properties of these compounds are calculated, including dielectric functions, refractive indices, absorption functions, loss functions, transmissivity function, reflectivity functions, conductivity functions and resistivity function. The results show that the characteristics for Ge2As2Po are significantly different from the other three compounds.