Exploring the Optoelectronic Potential of K ₃ AlX ₆ (X = Br, I) Halide Compounds via First-Principles Calculations

Lead-free halide perovskites are promising eco-friendly materials for optoelectronic and photovoltaic applications, yet balancing stability, band gap, and optical performance remains challenging. Here, density functional theory is used to systematically investigate vacancy-ordered cubic K 3 AlX 6 (X = Br, I). Both compounds crystallize in the stable Fm–3m phase with negative formation energies (–3.13 eV/atom for Br and –2.66 eV/atom for I), confirming thermodynamic stability. Phonon spectra show no imaginary modes, and elastic constants satisfy Born–Huang criteria, indicating dynamical and mechanical stability. Spin–orbit–coupled calculations reveal direct Γ-point band gaps of 3.16 eV for K 3 AlBr 6 and 1.55 eV for K 3 AlI 6 , demonstrating halide-driven tunability from ultraviolet to visible regions. K 3 AlBr 6 exhibits greater stiffness and higher Debye temperature, while K 3 AlI 6 shows improved ductility and lattice softness. Optical analysis indicates strong UV absorption, low reflectivity, and moderate dielectric constants. Overall, halide substitution induces lattice expansion, band-gap narrowing, and enhanced dielectric response, highlighting K 3 AlX 6 as stable and tunable lead-free candidates for advanced photovoltaic and photonic technologies.