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

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, μ_O ^o', and activity coefficient, γ_O , in NaCl-CaCl₂, NaCl-KCl-CaCl₂, and LiCl-KCl-CaCl₂ melts at 873 K, were evaluated by measuring the dependence of potential of O₂/O^2- on the oxygen partial pressure by using non-consumable ceramic electrodes; the μ_O ^o' was -527 ± 0.3, - 535 ± 0.1, and -538 ± 0.2 kJ mol^-1 and the γ_O 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.