Hybridization-exchange interplay governs defect tolerance in halide perovskites

The role of partially filled 3d states in governing defect tolerance in halide perovskites remains unclear, particularly in the presence of strong intra-atomic exchange and metal-halide covalency. Using first-principles calculations on substitutional 3d transition metals in CsPbI₃, we show that defect tolerance is not governed by the strength of d-p hybridization alone, but by its interplay with exchange splitting and crystal-field alignment. Early 3d dopants illustrate this balance: insufficient hybridization combined with low filling promotes localized deep states, whereas an optimal hybridization window stabilizes resonant states. In contrast, increasing d-shell occupancy enhances intra-atomic exchange, progressively localizing spin density and driving a crossover to exchange-dominated behavior in the late 3d regime. The resulting defect character reflects a competition between covalent delocalization and exchange stabilization, which together determine orbital ordering, magnetic polarization, and the spatial extent of defect states. These results establish a chemically transparent descriptor framework based on hybridization-exchange balance for integrating spin-active dopants without necessarily compromising electronic quality.