Synergistic Interaction Engineering Enables Homogeneous Mixed‐Cation Perovskite Single Crystals for High‐Performance X‐Ray Detection

Mixed A‐site compositions are an effective route to boost the x‐ray detection performance of perovskite single crystals, yet their growth is often accompanied by pronounced spatial compositional inhomogeneity. This macroscopic phase segregation originates from mismatched formation energies and crystallization kinetics among the different cation components, leading to internal composition gradients that hinder carrier transport but are rarely addressed in perovskite single‐crystal research. Here, we report an acetic acid‐enabled synergistic interaction‐engineering strategy for growing phase‐homogeneous mixed‐cation perovskite single crystals. Acetic acid simultaneously lowers the formation barrier of the formamidinium‐rich phase and suppresses the rapid assembly of the methylammonium‐rich phase, thereby balancing their crystallization rates and eliminating compositional gradients across the crystal. Owing to the improved uniformity and crystal quality, x‐ray detectors based on these single crystals deliver a sensitivity of 3.54 × 10 6 µC Gy −1 cm −2 , a low detection limit of 2.65 nGy s −1 , and excellent operational stability. This kinetic‐regulation concept offers a practical pathway to homogeneous mixed‐cation perovskite single crystals and supports their use in low‐dose, high‐performance optoelectronic devices.