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

Abstract

Measuring the thermodynamic quantities and coordination structure of oxide ions in CaCl 2 -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, μ O 2− o ′ , and activity coefficient, γ O 2− , in NaCl−CaCl 2 , NaCl−KCl−CaCl 2 , and LiCl−KCl−CaCl 2 melts at 873 K, were evaluated by measuring the dependence of potential of O 2 /O 2− on the oxygen partial pressure by using non-consumable ceramic electrodes; the μ O 2− o ′ was −527 ± 0.3, − 535 ± 0.1, and −538 ± 0.2 kJ mol −1 and the γ O 2− 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 [NaO 2 ] 3− and [CaOCl 3 ] 3− in the NaCl−CaCl 2 melt, [CaOCl 2 ] 2− in the NaCl−KCl−CaCl 2 melt, and [LiCaO] + and [Li 3 KO] 2+ in the LiCl− KCl−CaCl 2 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.