Unveiling the Impact of Nanoscale Cation Arrangement on Bandgap Tunability and Structural Stability in Cs0.5A0.5PbI3 (A = FA+, MA+) Perovskites

Nanoscale cation ordering in mixed-cation lead iodide perovskites (CsxA1–xPbI3; A = formamidinium: CH(NH2)2+, FA+; methylammonium: CH3NH3+, MA+) profoundly affects optoelectronic performance; however, the atomic-scale origins of this connection remain underexplored. Here, we combine first-principles density functional theory calculations and experimental observation to reveal that it is not the composition but the spatial arrangement of Cs+, FA+, and MA+ cations that governs lattice strain, bandgap dispersion, and stability. FA+ produces larger lattice expansion and Pb–I–Pb bond angle distortion minima (134°–171°) than MA+ (132°–173°), affecting octahedral tilting dynamics. Cation clustering (Type A) induces indirect bandgaps (1.37–2.11 eV), while uniform distributions (Type B) sustain direct gaps, justifying composition-independent photoluminescence redshifts (Δλ = 10–15 nm). Configuration-dependent formation energies reveal strain-mitigated configurations as thermodynamically preferred, yet localized lattice distortions (>2.8%) persist, buffered by iodine sublattice flexibility. Importantly, we reveal a nonmonotonic link between cation size disparity and octahedral distortion, contradicting the common assumption of entropy-driven homogeneous mixing in hybrid perovskites. By linking atomic-scale cation arrangement and macroscopic optoelectronic response, our results offer a predictive, design-oriented framework for producing stable, color-pure perovskite quantum dots. This discovery paves the way for a targeted cation ordering to unlock next-generation high-performance optoelectronic devices.