Stability Analysis and Optoelectronic Properties of Mg3ZBr3 (Z = As, Sb, Bi) Perovskites for Evaluating the Performance in PIN Photo diode

The toxicity and stability issues of lead-based perovskites motivate non-toxic, durable alternatives. This work examines lead-free Mg3ZBr3 (Z = As, Sb, Bi) halide perovskites as optoelectronic materials, with emphasis on Mg3AsBr3 and Mg3SbBr3. Mg3AsBr3 and Mg3SbBr3 perovskites are predicted from first-principles calculations to crystallise in the cubic Pm¯3m phase, indirect gaps of 2.0645 eV for Mg3AsBr3, 1.6533 eV for Mg3SbBr3 and 1.5226 eV for Mg3BiBr3 were observed by hybrid functional. Optical spectra show a rise in absorption above the gap and an increasing static dielectric response along As→Sb→Bi. Phonon dispersion lacks imaginary branches for Mg3AsBr3 and Mg3SbBr3 , indicates the dynamical stability of these two materials under consideration, and exhibits large mode anharmonicity (Gr¨uneisen signatures), consistent with soft-lattice heat transport trends. It has been found, moving down the pnictogen series expands the lattice and lowers the Goldschmidt tolerance factor, which, together with enhanced pnictogen–Br p-orbital hybridization and stereochemically active ns2 lone pairs (Sb, Bi), narrows the band gap and elevates the optical dielectric response. Elastic analyses confirm Born stability and moderate stiffness, with Hillaveraged bulk moduli decreasing from ∼44 GPa (Mg3AsBr3) to ∼35 GPa (Mg3BiBr3). Drift–diffusion p–i–n simulations qualitatively track band-edge–limited spectra, aligning with the computed gaps. Together, these results position Mg3AsBr3 and Mg3SbBr3 as lead-free candidates for stable thin-film photodiodes and photovoltaics applications.