Novel lead-free Cs₂NaFeBr₆ double perovskite exhibiting multifunctional energy-relevant properties: A comprehensive first-principles study

The rapid demand for high-efficiency and environmentally benign photovoltaic absorbers has intensified the search for lead-free halide perovskites that can simultaneously deliver strong optoelectronic response and long-term stability. In this context, double perovskites offer an attractive design space due to their compositional flexibility, suppressed Pb-toxicity concerns, and improved structural robustness compared to many Pb-based counterparts. Motivated by these considerations, we investigate the cubic lead-free double perovskite Cs \(_2\) NaFeBr \(_6\) using density functional theory and complementary stability and device-metric analyses. Geometric formability indicators confirm the feasibility of the cubic framework with \(t=0.9724\) , \(μ =0.425\) , and \(τ =1.865\) , while thermodynamic evaluation yields a negative formation energy and a positive decomposition energy, supporting energetic stability. Phonon dispersion shows no imaginary frequencies, establishing dynamical stability. The mechanical properties confirm a mechanically stable, relatively soft halide lattice, and the elastic constants further suggest ductile behavior. The Debye temperature is estimated to be \(Θ _D≈ 254\) K, consistent with the comparatively soft bonding environment of halide frameworks. Electronic-structure calculations reveal an indirect bandgap of 1.09 eV, with a closely spaced direct transition at \(Γ\) -point of 1.12 eV, where the energy difference is comparable to thermal energy at room temperature. Carrier-transport descriptors derived from band-edge curvature show moderate effective masses implying favorable electron transport and reasonably balanced carrier dynamics. Optical response exhibits strong dielectric screening with a static dielectric constant \(ε _1(0)≈ 6.5\) , supporting reduced Coulombic trapping and recombination. Finally, spectroscopic limited maximum efficiency analysis under AM 1.5G illumination predicts high photovoltaic performance, yielding a thickness-dependent power conversion efficiency (PCE) of 30.60% at 1 \(μ\) m and a maximum PCE of 32.09% at 5 \(μ\) m, approaching the Shockley-Queisser ceiling for this bandgap. Collectively, the stability signatures and optoelectronic metrics position Cs \(_2\) NaFeBr \(_6\) as a promising lead-free absorber for high-efficiency photovoltaic devices and motivate further experimental validation.