Screening the structural, dynamical, and mechanical stability, tuning band gaps, and optical properties of inorganic Cs₂RbABr₆ (A = Tl and Bi) double perovskites halide using DFT calculations

The substantial exploration of novel lead-free, non-toxic double perovskite halide (DPH) materials with suitable band gaps and high stability is desirable for modern perspective applications. In this work, we report density functional theory (DFT) calculations to screen the structural, dynamical, and mechanical stability, tuning the band gap, and optical properties of inorganic Cs₂RbABr₆ (A = Tl and Bi) DPH materials using WIEN2K quantum mechanical simulation code. The computation of tolerance factor, octahedral factor, and phonon band structures shows that the interested materials possess dynamic and structural stability in cubic symmetry. The computed cubic elastic constants C_ij, bulk modulus B, Young’s modulus Y, shear modulus G, Cauchy pressure C_P, Pugh ratio \(\frac{B}{G}\) $\frac{B}{G}$ , anisotropy factor A, and Poisson ratio ʋ confirm that both ternary Cs₂RbTlBr₆ and Cs₂RbBiBr₆ DPH materials are anisotropic, ductile, tough, and mechanically stable. Electronic properties depict that the tuning of the direct band gap (2.26 eV) from Γ–Γ in the Cs₂RbTlBr₆ to the direct band gap of 4.09 eV from L–L occurs in Cs₂RbBiBr₆ when the transition element Thallium (Tl) is replaced by the non-metallic Bismuth (Bi) element. The electronic properties show that Cs₂RbTlBr₆ is a semiconductor and Cs₂RbBiBr₆ is an insulator. Our investigations of optical properties show that inorganic Cs₂RbABr₆ (A = Tl and Bi) DPH possess better absorption coefficient, optical conductivity, and refractive index in the visible light spectrum, and thus Cs₂RbTlBr₆ and Cs₂RbBiBr₆ are proposed to be efficient materials for photosensitive and renewable energy applications.