Metastable Inorganic Materials through Reactive Molten-Salt Ion Exchange

Abstract Material discovery is the foundational process for fundamental investigations and useful applications of functional and structural materials. It is the prerequisite for all downstream processes including property determination, synthetic optimization and scale-up, doping, defect engineering, morphology control, and surface and interfacial tuning. Through extensive experimental and, more recently, computational exploration, the number of yet-to-be-discovered thermodynamically stable compounds in low-order phase spaces has rapidly shrunk. However, new materials are needed to foster advances in established and emerging applications, particularly those related to energy storage, conversion, and efficiency. Thinking beyond equilibrium phase diagrams, a much larger phase space of metastable compounds exists if such phases can be synthetically accessed. This Perspective highlights the role of reactive molten-salt ion exchange (MSIX) as a technique for metastable inorganic material discovery. We discuss the thermodynamics, kinetics, crystal chemistry, and atomic-scale characterization of reactive molten-salt ion exchange. Emergent phenomena in ion exchange such as interdiffusion and the mixed-ion effect and challenges associated with characterizing mixed-ion and heterogeneous systems receive extra attention. In consideration of the breadth of MSIX, fundamental principles and general reaction schemes are used where possible with specific examples extensively referenced. MSIX is contextualized by comparisons to adjacent fields that involve molten salts, ion transport, and metastable materials. Opportunities and missed opportunities associated with computational approaches to molten-salt reactions and metastable materials are discussed. Unique prospects for in situ diffraction and NMR crystallography are highlighted along with future directions, including extensions to solid-state ion exchange (SSIX).