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

Halide exchange is a popular strategy to tune the properties of CsPbX 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-CsPbI system, preventing the isolation of stable CsPb(ClI) nanocrystals. Yet, previous works have claimed that a full CsPbCl → CsPbI exchange can be achieved. Even more interestingly, interrupting the exchange prematurely yields a mixture of CsPbCl and CsPbI nanocrystals that coexist without undergoing further transformation. Here, we investigate the reaction mechanism of CsPbCl → CsPbI exchange in nanocrystals. We show that the reaction proceeds through the early formation of iodide-doped CsPbCl nanocrystals covered by a monolayer shell of CsI. These nanocrystals then leap over the miscibility gap between CsPbCl and CsPbI by briefly transitioning to short-lived and nonrecoverable CsPb(ClI) nanocrystals, which quickly expel the excess chloride and turn into the chloride-doped CsPbI nanocrystals found in the final product.