Halide-Engineered Multifunctionality in Lead-Free Cs2AlTlX6 (X = I, F) Double Perovskites: A First-Principles Study of Structural, Optoelectronic, Elastic, and Thermoelectric Properties

Abstract

Lead-free double perovskites have emerged as promising materials for next-generation optoelectronic and energy-conversion technologies because of their chemical tunability, structural stability, and multifunctional physical properties. In this study, the structural, elastic, electronic, optical, and thermoelectric properties of cubic Cs2AlTlX6 (X = I, F) double perovskites were scientifically explored using density functional theory. Structural optimization within the PBE-GGA framework confirms that both compounds are stable in the cubic Fm-3m phase, with Cs2AlTlI6 exhibiting a larger lattice constant and unit-cell volume, whereas Cs2AlTlF6 shows a higher bulk modulus and stronger mechanical rigidity. Elastic-constant analysis satisfies the Born–Huang stability criteria for both systems, indicating mechanical stability; Cs2AlTlI6 displays brittle behavior, while Cs2AlTlF6 exhibits comparatively greater stiffness and marginal ductile character. Electronic properties were analyzed using the TB-mBJ potential reveal indirect band gaps of 2.97 eV for Cs2AlTlI6 and 7.8 eV for Cs2AlTlF6, classifying the iodide compound as a semiconductor and the fluoride analogue as a wide-band-gap insulator. Optical analysis over the 0–40 eV photon-energy range demonstrates pronounced dielectric response, optical conductivity, absorption, refractive-index variation, reflectivity, extinction, and energy-loss features, indicating potential for visible–UV and ultraviolet photonic applications. Thermoelectric transport calculations further show that Cs2AlTlI6 possesses a large positive Seebeck coefficient, an enhanced power factor, and an almost temperature-independent figure of merit close to unity over 100–1000 K, whereas Cs2AlTlF6 exhibits weak thermoelectric efficiency despite its higher electrical conductivity because of its negligible thermopower. Overall, halide substitution strongly governs the multifunctional response of Cs2AlTlX6, with Cs2AlTlI6 emerging as the more favorable candidate for thermoelectric and optoelectronic applications, while Cs2AlTlF6 appears more suitable for ultraviolet optical environments requiring greater mechanical robustness.