Estimation of the proportions of aluminum-fluorine species in NaF-AlF3 mixtures by in situ Raman spectroscopy and theoretical simulations

The quantitative distribution of different types of clusters in a binary NaF-AlF3 molten salt system with a series of cryolite ratios (CR, NaF/AlF3 molar ratio) was explored by in situ high-temperature Raman spectroscopy in conjunction with quantum chemistry (QC) ab initio calculations. Density functional theory (DFT) was used to simulate the Raman spectra of solid crystalline Na5Al3F14 (CR = 1.67) and Na3AlF6 (CR = 3.00). Aluminum-fluorine molecular model clusters containing species with typical characteristic microstructures were built in a molten NaF-AlF3 system. Deconvolution of the major stretching vibrational bands of Al-F-Al and Al-F bonds was carried out by using the Voigt function. The anionic fractions of different species present in the molten NaF-AlF3 system were quantitatively analyzed by a function of the microstructure dependent on Raman scattering cross sections (RSCS). AlF63-, AlF52-, and AlF4- were the dominant species in the aluminum-fluorine molten salt system. As the NaF in the melt increased, the content of AlF52- reached the maximum concentration when CR = 2.00. Apart from the recognized existence of both AlF4- and AlF63-, the aluminum bridging fluorine species of Al2F7- has been proven to exist in molten systems when CR ≤ 1.00. The relationship between the viscosity and various (n-3)Na+·AlFn(n-3)- species distributions has been investigated. The results showed that the contribution of different (n-3)Na+·AlFn(n-3)- to viscosity was essentially the same, and simply depended on the number of different species.