Evaluation of NaCl-KCl-Al₂S₃ Molten Salts by Raman Spectroscopy and Quantum Chemical Calculations

The complex ions in NaCl-KCl-Al₂S₃ molten salts at temperatures of 923-1003 K were investigated via Raman spectroscopy and quantum chemistry calculations. The results showed the presence of Al₂S₃Cl^-, AlSCl₃^2-, and AlSCl₂^- in the molten salts. Besides that, there are two types of dissolution reactions for Al₂S₃: one is the reaction with Cl^-, generating AlSCl₃^2- and AlSCl₂^-, and the other is the reaction with Cl^-, forming Al₂S₃Cl^-. Once the temperature increases, the reaction between Al₂S₃Cl^- and Cl^- occurs, promoting the formation of AlSCl₃^2- and AlSCl₂^-. Meanwhile, the reaction between AlSCl₂^- and Cl^- occurs with the formation of AlSCl₃^2-. With the increase in Al₂S₃ content, Al₂S₃ participates in the reaction with Cl^-, producing Al₂S₃Cl^-, which also leads to the formation of AlSCl₂^- and AlSCl₃^2-. Meanwhile, the reaction of Al₂S₃Cl^- with Cl^- results in the generation of AlSCl₃^2- and AlSCl₂^-. The non-bridging S atom in Al₂S₃Cl^- and the S atoms in AlSCl₃^2- and AlSCl₂^- are the easiest to be attacked by electrophilic molecules. In turn, the Al atom bonded to one Cl atom and two S atoms in Al₂S₃Cl^- and the Al atoms in AlSCl₃^2- and AlSCl₂^- are the primary sites reacting with nucleophilic molecules.