Discovery of a Thermodynamic-Control Two-Dimensional Cs₆Pb₅I₁₆ Perovskite with a Unique Green Emission Color via Dynamic Structural Transformation

New perovskite materials of two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs₆Pb₅I₁₆ nanosheets were successfully obtained from the structural transformation of 2D PR-phase Cs₇Pb₆I₁₉ nanosheets. The 2D RP-phase Cs₆Pb₅I₁₆ perovskite nanosheets exhibited unique green emission with an emission wavelength of ∼500 nm. The crystal structure of the 2D RP-phase Cs₆Pb₅I₁₆ perovskite nanosheets was determined by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy, and atomic force microscopy. The time-dependent photoluminescence measurements and XRD spectra were used to observe the optical and structure transformations from 2D Cs₇Pb₆I₁₉ (<i>n</i> = 6) to 2D Cs₆Pb₅I₁₆ (<i>n</i> = 5) perovskites. The <i>in situ</i> XRD measurements confirmed that γ-phase CsPbI₃ was released during the structural transformation. Moreover, temperature-dependent <i>in situ</i> XRD measurements were employed to examine the kinetic energy involved in the structural transformation from the <i>n</i> = 6 form to the <i>n</i> = 5 form. Specifically, an intermediate structure from <i>n</i> = 6 to <i>n</i> = 5 was also identified. Most importantly, 2D Cs₆Pb₅I₁₆ (<i>n</i> = 5) was more structurally thermodynamically stable than 2D Cs₇Pb₆I₁₉ (<i>n</i> = 6). This study provides an essential route for the discovery of new types of perovskite structures during structural transformation.