RbNbX₃ (X = cl, Br, I) lead-free halide perovskites: A DFT study of structural, elastic, electronic, and thermoelectric properties for energy applications

The search for stable lead-free perovskites has gained significant attention as a promising solution to address the toxicity and instability challenges of lead-based counterparts. This investigation comprehensively examines rubidium-based cubic halide perovskites RbNbX₃ (X = Cl, Br, I) using first-principles calculations within the density functional theory framework. Through the full-potential linearized augmented-plane-wave (FP-LAPW) method implemented in WIEN2k, we systematically analyze the structural, elastic, mechanical, electronic, magnetic, and thermoelectric properties. Structural optimization via the Birch-Murnaghan equation of state confirms cubic symmetry with lattice parameters of 5.2049 Å, 5. 4918 Å and 5.8665 Å for RbNbX₃ (X = Cl, Br, I), respectively, and reveals a stable ferromagnetic ground state. Electronic structure calculations using the modified Becke-Johnson (mBJ) potential demonstrate semiconducting behavior with indirect band gaps of 1.023 eV (Cl), 0.866 eV (Br), and 0.710 eV (I), accompanied by a consistent total magnetic moment of ∼3 μB predominantly originating from Nb-4d orbitals. Densities of states are calculated to predict the interaction of orbitals of distinct atoms in the compounds. Mechanical stability is confirmed through elastic constant analysis, with RbNbCl₃ and RbNbBr₃ exhibiting brittle characteristics while RbNbI₃ shows ductile behavior based on Pugh's ratio evaluation. Thermoelectric analysis confirms that RbNbX₃ perovskites exhibit excellent potential for energy conversion applications, demonstrating peak ZT values of 0.99 (100K) for RbNbI₃, 0.86 (500 K) for RbNbBr₃, and 0.8 (900 K) for RbNbCl₃. These high figures of merit arise from their superior Seebeck coefficients (up to 2917 μV/K) and low thermal conductivity. All these findings establish RbNbX₃ perovskites as promising candidates for spintronic and other advanced energy technologies, all within an environmentally friendly framework.