Research Advances in Understanding the Correlations between Fluoroacidity and Physicochemical Properties of Molten Fluorides

Abstract

Molten fluorides, recognized as a vital class of high-temperature functional materials, have attracted considerable interest in advanced nuclear energy systems and high-temperature metallurgical processes due to their outstanding thermal stability, broad electrochemical window, and low neutron absorption cross-section. The physicochemical properties of these melts are primarily dictated by the dynamic coordination equilibrium between fluoride ions (F^-) and metal ions. This review provides a comprehensive overview of recent progress in understanding the structure-activity relationship between Lewis acidity/basicity (quantified by the F^- activity) and the macroscopic physicochemical behavior of molten fluorides. Advanced characterization techniques, such as nuclear magnetic resonance (NMR) and electrochemical methods, have played a critical role in uncovering the dynamic interplay between microscopic coordination environments and acid-base characteristics. Key findings highlight that the acid-base properties of molten fluorides significantly influence essential thermophysical parameters, including viscosity, thermal conductivity, and thermal stability. Moreover, these properties critically affect the corrosion behavior of structural materials and the dissolution mechanisms of oxides through coordination chemistry. By integrating insights across multiple scales, this review establishes a micro-to-macro correlation framework that offers both fundamental understanding and practical guidance for the design and optimization of molten fluoride systems in nuclear and metallurgical applications.