Relationship between the Thermodynamic Quantities and Coordination Structure of Oxide Ions in CaCl₂-Based Chloride Melts

Measuring the thermodynamic quantities and coordination structure of oxide ions in CaCl2-based melts is essential for comprehensively understanding the relationship between the thermodynamic and microscopic behaviors of high-temperature molten salts. In this study, the standard formal chemical potentials of oxide ion, μO2–o′, and activity coefficient, γO2–, in NaCl–CaCl2, NaCl–KCl–CaCl2, and LiCl–KCl–CaCl2 melts at 873 K, were evaluated by measuring the dependence of potential of O2/O2– on the oxygen partial pressure by using non-consumable ceramic electrodes; the μO2–o′ was −527 ± 0.3, – 535 ± 0.1, and −538 ± 0.2 kJ mol–1 and the γO2– was 0.10, 0.30, and 0.45 for each melt. In addition, the coordination structure of oxide ions in each melt was investigated by combining high-temperature Raman spectroscopy and density functional theory calculations. The coordination structures of oxide ions were identified as [NaO2]3– and [CaOCl3]3– in the NaCl–CaCl2 melt, [CaOCl2]2– in the NaCl–KCl–CaCl2 melt, and [LiCaO]+ and [Li3KO]2+ in the LiCl–KCl–CaCl2 melt, revealing that the stable structure was significantly different depending on the melt composition. The activity coefficients showed a tendency to depend on the nearest-neighbor cation coordinated with the oxide ion. The reported data will provide insights into the physicochemical properties of high-temperature melts and contribute to controlling the compatibility of materials and melts in pyrochemical engineering processes.