Discovery of\na Thermodynamic-Control Two-Dimensional\nCs<sub>6</sub>Pb<sub>5</sub>I<sub>16</sub> Perovskite with a Unique\nGreen Emission Color via Dynamic Structural Transformation

New perovskite materials of two-dimensional (2D) all-inorganic\nRuddlesden–Popper (RP) perovskite Cs<sub>6</sub>Pb<sub>5</sub>I<sub>16</sub> nanosheets were successfully obtained from the structural\ntransformation of 2D PR-phase Cs<sub>7</sub>Pb<sub>6</sub>I<sub>19</sub> nanosheets. The 2D RP-phase Cs<sub>6</sub>Pb<sub>5</sub>I<sub>16</sub> perovskite nanosheets exhibited unique green emission with an emission\nwavelength of ∼500 nm. The crystal structure of the 2D RP-phase\nCs<sub>6</sub>Pb<sub>5</sub>I<sub>16</sub> perovskite nanosheets was\ndetermined by powder X-ray diffraction (XRD), high-resolution transmission\nelectron microscopy, and atomic force microscopy. The time-dependent\nphotoluminescence measurements and XRD spectra were used to observe\nthe optical and structure transformations from 2D Cs<sub>7</sub>Pb<sub>6</sub>I<sub>19</sub> (<i>n</i> = 6) to 2D Cs<sub>6</sub>Pb<sub>5</sub>I<sub>16</sub> (<i>n</i> = 5) perovskites.\nThe <i>in situ</i> XRD measurements confirmed that γ-phase\nCsPbI<sub>3</sub> was released during the structural transformation.\nMoreover, temperature-dependent <i>in situ</i> XRD measurements\nwere employed to examine the kinetic energy involved in the structural\ntransformation 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\nimportantly, 2D Cs<sub>6</sub>Pb<sub>5</sub>I<sub>16</sub> (<i>n</i> = 5) was more structurally thermodynamically stable than\n2D Cs<sub>7</sub>Pb<sub>6</sub>I<sub>19</sub> (<i>n</i> =\n6). This study provides an essential route for the discovery of new\ntypes of perovskite structures during structural transformation.