Mechanical, electronic, and optical properties of the A4B6 layered ferroelectrics: ab initio calculation

Abstract

We have performed a first principles study of the structural, elastic and electronic properties of orthorhombic SnS and GeS compounds using the density functional theory within the local density approximation. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, Debye temperature, and hardness have also been estimated in the present work. All of the calculated modulus and Poisson's ratio for SnS were less than the same parameters for GeS. Our calculations have discovered the large anisotropy of elastic parameters in the (100) and (010)-planes for both compounds. The band structures of orthorhombic SnS and GeS have been calculated along high symmetry directions in the first Brillouin zone (BZ). The calculation results for the band gap of Sn(Ge) S gave E-g=0.256 eV (0.852 eV) and has an indirect character for an interband transition. The real and imaginary parts of dielectric functions and (by using these results) the optical constant such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant were calculated. All of the principal features and singularities of the dielectric functions for both compounds were found in the energy region between 2 eV and 20 eV.