Transport properties of APdCu(Se2)(Se3) (A = K and Rb): new quaternary copper palladium polyselenides

The electronic structure, effective mass, optical properties and electrical transport coefficients of APdCu(Se-2)(Se-3) (where A = K and Rb), a new quaternary copper palladium polyselenide, were investigated using a density functional theory calculation within a generalized gradient approximation (GGA) plus the Hubbard term (U) (GGA + U). The electronic band structure shows that the calculated compounds have a direct band gap. From the partial density of states we found that, at an energy of -5.0 eV: (1) the Pd-s state strongly hybridizes with the Se-p state; (2) near the Fermi level the Se-p state hybridizes with the Cu-p state; and (3) at the lower conduction band the Pd-s state forms a strong hybridization with the Cu-s state. The investigation of electronic charge density shows that the Pd-Se and Cu-Se atoms form weak covalent bonds and have strong ionicity, whereas the K/Pd atoms exhibit pure ionic bonding. We also calculated the dielectric function, refractive index, extinction coefficient, absorption coefficient and reflectivity of the compounds. The calculated transport coefficients show the anisotropic nature of the compounds, in agreement with their electronic states. The transport properties reveal stronger carrier transport along the Cu-p/d and Pd-d orbitals, indicating that these orbitals are mainly responsible for the electrical transport. The maximum power factor values of the KPdCu(Se-2)(Se-3) (RbPdCu(Se-2)(Se-3)) compounds as a function of relaxation time reach 2.2 (1.8) x 10(11), 4.4 (3.5) x 10(11) and 1.3 (1.4) x 10(11) within P-xx, P-yy and P-zz components, respectively.