Ab Initio Investigation of Cooperativity in Ion Exchange

Ion exchange reactions are being increasingly employed for the templated synthesis of inorganic nanomaterials for plasmonics, topological quantum computing, and solid-state electrolytes. Some cation exchange reactions have been found to be cooperative akin to the nature of ligand binding processes in enzymes. What is the structural origin of this cooperative behavior? This question is answered here through computational simulations of Cd2+-to-Cu+ replacement reactions in a CdSe crystal. Density functional theory simulations show that the incorporation of Cu+, at the initial stages of exchange, induces a translocation of Cd2+ ions from their ideal tetrahedral sites to the otherwise unfavored octahedral sites. This reorganized structure has a lower energy cost for Cd2+-to-Cu+ replacement and is therefore much more amenable to further cation exchange. Thus, Cu+-insertion-induced reorganization of the cationic sublattice is at the origin of the cooperativity exhibited by the cation exchange reaction. This atomic-level understanding establishes a close link between solid-state chemical reactions and phase transitions and will lead to more precise transformations for accessing unusual material phases.