| Summary: | Cesium copper iodine perovskite is widely studied for applications in X-ray imaging and anti-counterfeiting fields based on its excellent photovoltaic properties. Uniquely, Cs3Cu2I5 undergoes a reversible phase transition to CsCu2I3 when induced by humidity, a phenomenon that is utilized in fluorescent anti-counterfeiting application. However, there is no relevant proof for the phase transition of CsCu2I3 upon humidity, which leads to an incomplete explanation of the water-induced phase transition of Cs3Cu2I5. Here, we illustrate the water-induced phase transition events of Cs3Cu2I5 and CsCu2I3 crystals that were made utilizing a straightforward anti-solvent approach. Interestingly, Cs3Cu2I5 can undergo a reversible phase transition based on humidity exposure and removal, whereas CsCu2I3 undergoes immediate phase decomposition and fluorescence burst upon humidity exposure, and recrystallization to low-formation-energy Cs3Cu2I5 is accompanied by fluorescence recurrence after humidity evaporation. Furthermore, Cs3Cu2I5 and CsCu2I3 crystals exhibit a fascinating X-ray light yield and a high spatial resolution of 16.6 and 12.5 lp mm−1, respectively. As a result, this research adds to our understanding of the phase transition between Cs3Cu2I5 and CsCu2I3 and also provides new candidates for advanced fluorescence anti-counterfeiting techniques and X-ray imaging scintillators.
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