Study of fundamentals physicals properties of the layered BaFAgCh (Ch=S, Se and Te) by first principles methods

Abstract

In the present work, the structural, elastic, electronic, optical, thermoelectric and thermodynamic properties of the layered BaFAgCh (Ch = S, Se, Te) were investigated using the full potential linearized augmented plane wave (FP-LAPW) method in the framework of density functional theory (DFT). To evaluate the spin-orbit coupling (SOC) effect, both the scalar relativistic and full relativistic calculations were performed. The SOC effect is found to be not negligible in the title compounds. The calculated structural parameters are in good accordance with the existing experimental data. The monocrystalline and polycrystalline elastic modules of the title compounds were determined via the energy-strain method. The investigated compounds show a strong anisotropic behavior of the structural and elastic parameters. The calculated electronic band structures using the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential reveal that the three considered systems are large direct band gap semiconductors. The assignments of the energy band electronic states and chemical bonding characters were accomplished with the help of the l-decomposed atomic densities of states diagrams. Frequency-dependent polarized optical functions, including dielectric function, absorption coefficient, refraction index, extinction coefficient, reflectivity and energy loss function, were computed for an energy range from 0 to 30 eV. The microscopic origin of the electronic states that are responsible of the optical spectra structures were determined. The optical spectra exhibit a considerable anisotropy. The FP-LAPW band structure and the semi-classical Boltzmann transport theory were used to study the charge-carrier concentration and temperature dependences of the thermoelectric parameters, including Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit. Our results show that the values of the thermoelectric parameters of the p-type compounds are larger than that of the n-type ones. The optimal p-type doping concentrations and temperatures that yield the maximum values of the figure of merit of the title compounds were calculated. These are important parameters to guide experimental works. The quasiharmonic Debye model in combination with the FP-LAPW method was used to study the temperature and pressure dependences of the unit cell volume, bulk modulus, heat capacity, volume thermal expansion and Debye temperature