Thermodynamic Stability, Structure, and Optical Properties of Perovskite-Related CsPb2Br5 Single Crystals under Pressure

CsPb₂Br₅ belongs to all inorganic perovskite-related quasi-two-dimensional materials that have attracted considerable attention due to their potential for optoelectronic applications. In this study, we solve numerous controversies on the physical properties of this material. We show that optical absorption in the visible spectrum and green photoluminescence are due to microcrystallites of the three-dimensional CsPbBr₃ perovskite settled on the CsPb₂Br₅ plates and that carefully cleaned crystal plates are devoid of these features. The high-pressure structural and spectroscopic experiments, performed on the single crystals free of CsPbBr₃ impurities, evidenced that the layered tetragonal structure of CsPb₂Br₅ is stable at least up to 6 GPa. The absorption edge is located in the ultraviolet at around 350 nm and continuously red shifts under pressure. Moderate band gap narrowing is well correlated to the pressure-induced changes in the crystal structure. Although the compressibility of CsPb₂Br₅ is much higher than for CsPbBr₃, the response in optical properties is weaker because the Pb-Br layers responsible for the optical absorption are much less affected by hydrostatic pressure than those built of Cs⁺ cations. Our study clarifies the confusing data in the literature on the optical properties and thermodynamic stability of CsPb₂Br₅.