The Lead-free Double Perovskites K2LiTlX6 (X = Cl, Br, I) as an Emerging Aspirant for Solar Cells and Thermoelectric Energy Applications

Double perovskites present promising opportunities for harnessing energy from both sunlight and wasted heat. In this study, we conducted a comprehensive theoretical analysis to evaluate the mechanical, electronic, optical, and transport properties of K2LiTlX6 (X = Cl, Br, I) employing the WEIN2K code. The optimized FCC unit cell's lattice parameters exhibited an increase from 10.34 to 11.98 & Aring; when substituting Cl with Br/I. To ascertain thermodynamic and structural stability, we computed the enthalpy of formation (-2.64 to -1.89 eV) and the tolerance factor (0.97 - 0.93). The results provide evidence for the thermodynamic and structural stability of the compounds. Mechanical parameter calculations, comprising bulk, Young, and shear moduli, along with Poisson's ratio in three dimensions, reveal their anisotropic nature, with greater significance observed in K2LiTlBr6 and K2LiTlI6. Moreover, the numerical values of Poisson and Pugh ratios indicated their ductile characteristics. The materials also exhibit noteworthy properties such as hardness, ultra-low lattice thermal conductivities, and greater melting and Debye temperatures, enhancing their potential applications. The direct bandgap values underwent a transformation from the visible to the infrared region (2.57 - 0.14 eV) as the halogen size increased. Using the BoltzTraP code, we calculated temperature-dependent thermodynamic parameters, showing high electrical conductivity related to thermal conductivity. The combination of a higher Seebeck coefficient and ZT highlighted the appropriateness of these compounds for future energy harvesting applications.