Existence of Al2F7- in molten MF-AlF3 (M = K, Cs) systems as determined by Raman spectroscopy and structural simulation

The molten mixtures of alkali metal fluorides and aluminum fluoride are applied as aluminum electrolytes or brazing fluxes. However, the presence of Al2F7- in such molten systems is disputed. In the present study, MF-AlF3 (M = K, Cs) systems with molar ratios < 1 were studied by in-situ Raman spectroscopy and molecular simulation. The results show that, in addition to AlF63-, AlF52-, and AlF4-, the systems also contained Al2F7-. The characteristic bands in the Raman spectra belonging to Al2F7- were located at about 225 cm(-1), 315 cm(-1), 479 cm(-1), and 720 cm(-1). There are two possible structures of Al2F7-, which belong to the D-3d and D-3h point groups. Both of these structures are linear, and their single-point energies were found to differ by only 0.31 kcal/mol. (C) 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.