Designing Topotactic Ion-Exchange Reactions in Solid-State Oxides Through First-Principles Calculations

Topotactic ion-exchange reactions between inorganic compounds (precursors) and ionic crystals (ion sources) at low temperatures provide a promising approach for accessing metastable compounds that are absent from the equilibrium phase diagrams. These reactions typically require a negative Gibbs free energy change from the reactant to the product. However, owing to the limited experimental thermodynamic data, chloride and nitrate have been conventionally selected as ion sources in previous studies, which restrict material exploration. In this study, we focused on ternary wurtzite-type oxides, β-MIGaO2 (MI = Na, Ag, Cu, Li), where monovalent ions exchange with each other, as a model system. We computationally calculated the entropy change of the reaction for 42 different combinations of β-MIGaO2 with ionic crystals and experimentally examined the feasibility of ion exchange to assess their consistency. Our findings suggest that, based on the calculated enthalpies, it is possible to predict with high precision whether complete ion exchange will occur or if a solid solution will form. In addition, it is indicated that the composition of the solid solution can be roughly predicted by estimating the mixing enthalpy and the entropic gain of ion mixing. This study demonstrates that the design of ion-exchange reactions can be facilitated by screening suitable combinations of precursors and ion sources by using simple first-principles calculations before experimentation.