Alkali metal substitutional effect on the structural, mechanical, optoelectronic and transport properties of X

Double perovskites have attracted significant interest as promising candidates for energy harvesting applications and as potential alternatives to lead-based organic solar cells. In this study, we investigate the structural, mechanical, electronic, optical, and transport properties of X₂LaCuCl₆ (X = K, Rb, Cs) using first-principles calculations within the framework of the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in WIEN2k. Structural parameters optimized using the Birch-Murnaghan equation of state show a systematic increase in lattice constants and a corresponding decrease in bulk modulus with increasing ionic radius of the alkali metal (X = K, Rb, Cs) cation. The thermodynamic and dynamic stability of the compounds is confirmed through formation energy, cohesive energy calculations, Goldschmidt tolerance factors, and phonon dispersion analyses using the Phonopy package with ultrasoft pseudopotentials in Quantum ESPRESSO. Elastic constants satisfy the mechanical stability criteria, and derived mechanical parameters indicate ductile behaviour. Electronic band structures obtained via the modified Becke-Johnson (mBJ) potential reveal indirect band gaps of 1.83 eV (K), 1.81 eV (Rb), and 1.40 eV (Cs). Optical absorption spectra exhibit strong peaks in the far-ultraviolet region 112.41 cm⁻¹ at 11.41 eV (K), 118.56 cm⁻¹ at 11.36 eV (Rb), and 116.74 cm⁻¹ at 10.19 eV (Cs) with corresponding minimum reflectivity values of 19% for K and Rb compounds, and 22% for Cs, suggesting suitability for high-frequency optoelectronic applications. Transport properties, analysed using the BoltzTraP code, reveal Seebeck coefficients of 81.6 µV/K (K), 82.2 µV/K (Rb), and 66.7 µV/K (Cs), with dimensionless thermoelectric figure-of-merit (ZT) values of 0.17, 0.33, and 0.18, respectively. These findings suggest that X₂LaCuCl₆ compounds exhibit a favourable combination of optoelectronic and thermoelectric properties, making them promising candidates for integration into ultraviolet photonic and energy conversion devices.