Mechanochemical Route to High-Entropy Three-Dimensional Halide Perovskites

High-entropy alloying has been demonstrated as a powerful approach to obtaining materials with exceptional structural and functional properties. The presence of several atomic species in the same crystallographic site alters short-range order compared to nonalloyed phases and results in different mechanical, chemical, and electronic behavior with emergent features. While the synthesis of high-entropy alloys (HEAs) is established in metallurgy, their formation as halide perovskites has been so far limited by synthetic bottlenecks, including different solubilities of the binary salt precursors and the complexity of crystallization pathways in solution. Herein, we report a solvent-free mechanochemical synthesis route for high-entropy double perovskites, yielding simultaneous near-stoichiometric alloying at two and three crystal sublattices. Detailed structural characterization reveals the atomic mechanisms involved in the formation of the HEAs through asymmetric ion-exchange rates from parent nonalloyed perovskites, as well as the formation of inorganic layered nanoheterostructures. Strong nonlinear behavior is observed in the optical properties (bandgap bowing), leading to full visible-range light absorption for the chloride perovskites. Theoretical calculations at the density functional theory provide a framework for the process, shedding light into the electronic structure of the complex alloys. As a proof of concept, the alloys are implemented in a standard electrochemical cell fitted with a quartz window for photoelectrochemical oxygen evolution testing, showing a clear photocatalytic response.