Insight into the structural, elastic and electronic properties of tetragonal inter-alkali metal chalcogenides CsNaX (X=S, Se, and Te) from first-principles calculations

To investigate the structural, elastic and electronic properties of the tetragonal inter-alkali metal chalcogenides CsNaX (X = S, Se, and Te), we have applied the full-potential linearized augmented plane-wave plus local orbital (FP-LAPW + lo) method within the density functional theory (DFT) framework. For the description of the exchange-correlation potential, the Wu-Cohen generalized gradient approximation (WC-GGA) was employed. Furthermore, the generalized gradient approximation of Engel and Vosko (EV-GGA) and Tran-Blaha modified Becke-Johnson (TB-mBJ) potential were also applied to obtain reliable results for the electronic properties of CsNaX compounds. Our optimized equilibrium structural parameters are in good agreement with experimental measurements where available. The Young's modulus, bulk modulus, shear modulus, Poisson's ratio, Pugh's ratio, acoustic velocities and Debye temperature were derived from our estimated elastic constants Cij. It is found that all the considered compounds are mechanically stable and ductile in nature. In addition, the elastic anisotropy of the examined compounds was studied by visualizing the directionally dependent Young's modulus, shear modulus and compressibility, as well as by calculating some elastic anisotropy indices. Electronic properties, such as the band structure, density of states and the charge density, were analyzed in detail. Calculated band structures show that the studied materials are semiconductors with indirect band gaps (Z-Γ).