Unveiling the Monoclinic Phase in CsPbBrCl Perovskite Crystals, Phase Transition Suppression and High Energy Resolution γ-Ray Detection.

All-inorganic CsPbBr and CsPbCl perovskites are promising materials for high-performance solar cells and advanced radiation detection technologies with high stability. Here we report that CsPbBrCl ( = 0-3) crystals exhibit eutectoid behavior for the melting points and phase transition temperatures. The well-known halide perovskite cubic phase transition temperature shifts near room temperature (∼37 °C for CsPbBrCl). We conducted an extensive crystallographic analysis on single crystals of 7 different compositions, including the end members CsPbBr and CsPbCl. Contrary to previous beliefs, we discovered they exhibit a monoclinic structure with space group symmetry 2/ at room temperature, rather than the orthorhombic . This new structural model is more precise and features a unit cell volume that is four times larger than that of the orthorhombic model. From high-quality single crystals of CsPbBrCl, grown by the Bridgman method, we constructed γ-ray detectors achieving an energy resolution of 7.2% at 200 V for Co radiation. Thermally stimulated current spectroscopy of the CsPbBrCl samples revealed that the defect densities in crystals from different regions of the ingot were relatively uniform, with values of ∼4.72 × 10 and ∼5.09 × 10 cm. These findings indicate that low deep-level defect densities can be achieved that are consistent with the notable performance of the CsPbBrCl perovskite as a high-energy γ radiation detector.