Unveiling the Monoclinic Phase in CsPbBr_3-Cl Perovskite Crystals, Phase Transition Suppression and High Energy...

All-inorganic CsPbBr₃ and CsPbCl₃ perovskites are promising materials for high-performance solar cells and advanced radiation detection technologies with high stability. Here we report that CsPbBr_3-Cl (x = 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 CsPbBr₂Cl). We conducted an extensive crystallographic analysis on single crystals of 7 different compositions, including the end members CsPbBr₃ and CsPbCl₃. Contrary to previous beliefs, we discovered they exhibit a monoclinic structure with space group symmetry P2₁/m at room temperature, rather than the orthorhombic Pnma. 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 CsPbBr₂Cl, grown by the Bridgman method, we constructed γ-ray detectors achieving an energy resolution of 7.2% at 200 V for ⁵⁷Co radiation. Thermally stimulated current spectroscopy of the CsPbBr₂Cl samples revealed that the defect densities in crystals from different regions of the ingot were relatively uniform, with values of ∼4.72 × 10¹² and ∼5.09 × 10¹² cm^-3. These findings indicate that low deep-level defect densities can be achieved that are consistent with the notable performance of the CsPbBr₂Cl perovskite as a high-energy γ radiation detector.