First-principles insights into magneto-electronic and thermoelectric correlations in Fe- and Mn-doped Cs2SnI6 vacancy-ordered double perovskites.

Transition-metal doping in halide double perovskites provides an effective route to tune their electronic, optical, and transport properties for multifunctional device applications. In this work, we present a comprehensive density functional theory investigation of Fe- and Mn-doped Cs2SnI6 vacancy-ordered perovskites. Structural optimization confirms the thermodynamic stability of both doped systems and preserves the cubic-like A2BX6 lattice. Spin-resolved band structure and density of states analyses show that Fe substitution induces half-metallic ferromagnetism with a spin-asymmetric gap, whereas Mn substitution stabilizes a narrow-gap semiconducting state with enhanced rigidity. The calculated optical spectra, including the dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity, and energy loss function, indicate strong absorption in the visible to ultraviolet range, with distinct dopant-dependent features. Thermoelectric transport analysis further demonstrates that Fe doping enhances the Seebeck coefficient and power factor at elevated temperatures, while Mn doping alters the carrier type and conductivity trends. The combined results highlight clear magneto-electronic and magneto-elastic correlations, establishing Fe- and Mn-doped Cs2SnI6 as promising candidates for lead-free, environmentally stable materials in photovoltaics, spintronics, and thermoelectric energy conversion. The predictions stem from first-principles DFT+U calculations focusing on qualitative trends. The validation of these predictions experimentally, as well as the consideration of spin-orbit coupling, are, however, outside the present scope and will be looked at in the future.