Raman spectroscopy and quantum theory calculations on complexes in the KF-AlF3-Al2O3 system

The ionic structure of the KF-AlF3-Al2O3 system was studied using in situ high-temperature Raman spectroscopy and quantum mechanics calculations. The complexes in the KF-AlF3-Al2O3 system with different molar ratios of KF to AlF3 were AlF63-, AlF52-, AlF4-, Al2OF4, Al2OF84-, Al2OF62-, and Al2O2F42-. The structures of Al2OF4, Al2OF84-, Al2OF62-, and Al2O2F42- were optimized considering that the counterion K+, Al2OF4 exhibits D-2d point group symmetry, K4Al2OF8 and K2Al2OF6 exhibit C-2v point group symmetry, and K2Al2O2F4 exhibits C-2h point group symmetry. The experimental main Raman bands of Al2OF84-, Al2OF62-, and Al2O2F42- were at 495 cm(-1), 465 cm(-1), and 410 cm-1, respectively, after comparing their theoretical Raman spectra. The forms and contents of the oxygen-containing complex ions in the system were related to the molar ratio of KF to AlF3 and to the concentration of alumina. AlF63-, AlF52-, AlF4-, and Al2OF4 were found in the molten KF-AlF3-Al2O3 system with a KF-to-AlF3 molar ratio of 1.22 at a temperature of 1003 K. In the KF-AlF3-Al2O3 system with a KF-to-AlF3 molar ratio of 2 at a temperature of 1273 K, when the alumina concentration was less than or equal to 10 wt%, it contained AlF63-, AlF52-, AlF4-, Al2OF4, and Al2OF84-, and when the alumina concentration reached 12 wt%, Al2OF62- and Al2O2F42 appeared. In the KF-AlF3-Al2O3 system with a KF-to-AlF3 molar ratio of 3 at a temperature of 1273 K, when the alumina concentration was less than or equal to 10 wt%, the system contained AlF63-, AlF52-, AlF4-, Al2OF4, and Al2OF84-. When the alumina concentration increased to 14 wt%, Al2OF62- and Al2O2F42- began to appear but AlF4- disappeared. As the concentration of alumina increased, the contents of Al2OF62- and Al2O2F42- increased. The ionic reactions that formed the various oxygen-containing complexes were obtained. The occurrence of these chemical reactions was related to the molar ratio of KF to AlF3 in the system and the alumina concentration. When the KF-AlF3-Al2O3 molten system with KF-to-AlF3 molar ratios of 1. 22, 2 and 3 were used as the aluminum electrolytes, the ion discharged at the anode was Al2OF84- (except for the system with a KF-to-AlF3 molar ratio of 1.22 in which the Al2OF4 was discharged at the anode), and the ion discharged at the cathode was Al2OF4 according to wave function analyses. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The ionic structure of the KF-AlF3-Al2O3 system was studied through in situ high-temperature Raman spectroscopy and quantum mechanics calculations, revealing different oxygen-containing complex ions formed with various molar ratios of KF to AlF3. The forms and contents of these complex ions were found to be influenced by the molar ratio of KF to AlF3 and the concentration of alumina in the system. The experimental findings provided insights into the ionic reactions and discharge behaviors of the system when used as aluminum electrolytes with different molar ratios of KF to AlF3.