Structural, elastic, electronic and optical properties of novel antiferroelectric KNaX (X = S, Se, and Te) compounds: First principles study

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This work deals with the investigation of structural, elastic, electronic and optical properties of ternary compounds belonging to the inter-alkali metal chalcogenide family KNaS, KNaSe, and KNaTe by using density functional theory (DFT) based calculations. From the structural properties, it is found that the lattice parameters and the atomic coordinates of KNaX (X = S, Se, and Te) are in good agreement with the experimental results stated in the literature. The compounds are considered as soft materials since their bulk modulus values are small. In addition, the elastic properties indicate that KNaS possesses the largest parameters of elastic constant Cij compared with other compounds, thereby revealing that KNaX compounds exhibit an elastic anisotropy. Meanwhile, the bulk modulus, shear and Young moduli, and Poisson ratio are also calculated. It is observed that all compounds are brittle and that the ionic character is dominant. In order to confirm the anisotropic character of the mechanical properties, several parameters such as universal, bulk and shear anisotropic indexes are investigated. For instance, a novel fascinating approach has been proposed by using the 3D-surfaces. The electronic properties demonstrate their semiconductor nature with a direct wide band gap of 2.61, 2.25 and 2.00 eV for KNaS, KNaSe and KNaTe respectively, as well as confirm their ionic behavior by checking the charge density distributions. Furthermore, since the optical properties of our compounds have not been yet reported in previous works, the dielectric function, refraction index, extinction index, reflectivity, loss energy function, optical conductivity and absorption spectra have been studied in details.