Unveiling the Monoclinic\nPhase in CsPbBr<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> Perovskite Crystals,\nPhase Transition Suppression and High Energy Resolution γ‑Ray\nDetection

All-inorganic CsPbBr<sub>3</sub> and CsPbCl<sub>3</sub> perovskites\nare promising materials for high-performance solar cells and advanced\nradiation detection technologies with high stability. Here we report\nthat CsPbBr<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> (<i>x</i> = 0–3) crystals exhibit eutectoid\nbehavior for the melting points and phase transition temperatures.\nThe well-known halide perovskite cubic phase transition temperature\nshifts near room temperature (∼37 °C for CsPbBr<sub>2</sub>Cl). We conducted an extensive crystallographic analysis on single\ncrystals of 7 different compositions, including the end members CsPbBr<sub>3</sub> and CsPbCl<sub>3</sub>. Contrary to previous beliefs, we\ndiscovered they exhibit a monoclinic structure with space group symmetry <i>P</i>2<sub>1</sub>/<i>m</i> at room temperature, rather\nthan the orthorhombic <i>Pnma</i>. This new structural model\nis more precise and features a unit cell volume that is four times\nlarger than that of the orthorhombic model. From high-quality single\ncrystals of CsPbBr<sub>2</sub>Cl, grown by the Bridgman method, we\nconstructed γ-ray detectors achieving an energy resolution of\n7.2% at 200 V for <sup>57</sup>Co radiation. Thermally stimulated\ncurrent spectroscopy of the CsPbBr<sub>2</sub>Cl samples revealed\nthat the defect densities in crystals from different regions of the\ningot were relatively uniform, with values of ∼4.72 ×\n10<sup>12</sup> and ∼5.09 × 10<sup>12</sup> cm<sup>–3</sup>. These findings indicate that low deep-level defect densities can\nbe achieved that are consistent with the notable performance of the\nCsPbBr<sub>2</sub>Cl perovskite as a high-energy γ radiation\ndetector.