CsPbCl(3) → CsPbI(3) Exchange in Perovskite Nanocrystals Proceeds through a Jump-the-Gap Reaction Mechanism

[Image: see text] Halide exchange is a popular strategy to tune the properties of CsPbX(3) nanocrystals after synthesis. However, while Cl → Br and Br → I exchanges proceed through the formation of stable mixed-halide nanocrystals, the Cl ⇌ I exchange is more elusive. Indeed, the large size difference between chloride and iodide ions causes a miscibility gap in the CsPbCl(3)–CsPbI(3) system, preventing the isolation of stable CsPb(Cl(x)I(1–x))(3) nanocrystals. Yet, previous works have claimed that a full CsPbCl(3) → CsPbI(3) exchange can be achieved. Even more interestingly, interrupting the exchange prematurely yields a mixture of CsPbCl(3) and CsPbI(3) nanocrystals that coexist without undergoing further transformation. Here, we investigate the reaction mechanism of CsPbCl(3) → CsPbI(3) exchange in nanocrystals. We show that the reaction proceeds through the early formation of iodide-doped CsPbCl(3) nanocrystals covered by a monolayer shell of CsI. These nanocrystals then leap over the miscibility gap between CsPbCl(3) and CsPbI(3) by briefly transitioning to short-lived and nonrecoverable CsPb(Cl(x)I(1–x))(3) nanocrystals, which quickly expel the excess chloride and turn into the chloride-doped CsPbI(3) nanocrystals found in the final product.