First-principles investigation of lead-free halide perovskites RbBCl <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si61.svg" display="inline" id="d1e1634"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msub

In this study, a comprehensive density functional theory (DFT) investigation was carried out to examine the structural, electronic, magnetic, optical, and thermoelectric properties of the lead-free halide perovskites RbBCl 3 (B= Fe, Co). The calculations were performed using the GGA+U and TB-mBJ+U approaches within the full-potential linearized augmented plane-wave (FP-LAPW) framework. The optimized structures confirm a stable cubic perovskite phase with space group Pm-3 m, supported by favorable tolerance factors and negative formation energies. The mechanical stability was verified through the Born–Huang criteria, indicating ductile behavior for RbFeCl 3 and higher rigidity for RbCoCl 3 . The spin-polarized electronic structures reveal half-metallic magnetic behavior, with complete spin polarization at the Fermi level. One spin channel exhibits metallic character, while the other shows a wide indirect band gap of 3.11–3.23 eV as obtained from the GGA+U approximation. The total magnetic moments of 4 μ B (Fe) and 3 μ B (Co) correspond to high-spin configurations stabilized by B–Cl–B superexchange interactions and p–d hybridization between metal and halogen orbitals. The optical spectra show strong spin-dependent anisotropy and high ultraviolet absorption, confirming their potential for spintronic and optoelectronic applications. Thermoelectric transport analysis based on the semiclassical Boltzmann theory indicates p-type conduction with positive Seebeck coefficients and Z T values approaching unity, suggesting promising energy-conversion efficiency. Overall, the combination of half-metallicity, magnetic spin-polarized stability, optical anisotropy, and efficient thermoelectric response positions RbFeCl 3 and RbCoCl 3 as multifunctional materials for next-generation spintronic, optoelectronic, and thermoelectric devices.