A comprehensive study of Lanthanides based Halide Double Perovskites A₂MTlX₆ for high frequency optical and thermoelectric applications

The design of stable, nontoxic, and efficient halide double perovskites has emerged as a strategic pathway toward next-generation optoelectronic and energy conversion devices. In this work, we present a comprehensive first-principles investigation of A₂MTlX₆ (A = K, Rb, Cs; M = Sc, Y, La; X = Cl, Br, I) compounds using the FP-LAPW method with local orbitals, incorporating PBEsol-GGA and the TB-mBJ exchange–correlation schemes for accurate property prediction. Structural stability was confirmed through formation energies, tolerance and octahedral factors, and Born stability criteria. Elastic constant analysis revealed predominantly ductile behavior, with only Cs₂ScTlCl₆ and Rb₂LaTlCl₆ showing brittleness, while high melting points and moderate anisotropy underline their robustness. Electronic calculations demonstrate wide band gaps (2.79–4.66 eV), with most compounds exhibiting direct transitions suitable for visible–UV absorption. Optical spectra reveal transparency in the low-energy regime and strong absorption and conductivity peaks in the visible–ultraviolet range, suggesting excellent light-harvesting potential. Among them, K₂ScTlI₆, Rb₂ScTlI₆, Cs₂ScTlI₆, and Rb₂LaTlI₆ emerge as particularly promising candidates for photovoltaic and optoelectronic applications. Thermoelectric properties, evaluated via Seebeck coefficient, electrical and thermal conductivities, and power factor, indicate ultralow lattice thermal conductivity from Slack’s model and figure of merit (ZT) values of 0.66–0.85, approaching the benchmark for practical devices. The synergistic combination of optical absorption, mechanical resilience, and thermoelectric efficiency positions A₂MTlX₆ halide double perovskites as highly versatile and sustainable materials for future renewable energy technologies.