In-line tempering eliminates the domain boundary in perovskite single crystals for high-energy resolution ionizing radiation detectors.

Metal halide perovskite single crystals (SCs) emerge as a promising candidate for ionizing radiation detection. The realization of top-performing radiation detectors typically relies on careful crystal selection from broad candidate groups, as residual strain remains unavoidable during the SC growth process, which often leads to the formation of ferroelastic domains with varied orientations. Here, we introduce an in-line tempering strategy to alleviate microstrain and homogenize the domain orientation across methylammonium lead iodide (MAPbI) perovskite SCs. The progressive strain relief during the phase transition in situ, demonstrated by the removal of ferroelastic domain walls, substantially enhances the crystallinity and the optoelectronic properties of the MAPbI SCs. As a result, the gamma-ray energy spectrum detector leveraging these strain-relaxed SCs achieves an energy resolution of 7.2% at 59.5 keV for a Am gamma-ray source, and the 25-pixel device performs highly uniformly with concentrated current distribution, which paves the way for its implementation in high-resolution radiation spectroscopy.