Quantitative studies on the microstructure evolution and its impact on the viscosity of a molten Al2O3-Na3AlF6 system by Raman spectroscopy and theoretical simulations.

[Display omitted] • A series of aluminum-oxygen-fluorine molecular model clusters were proposed in the molten Al 2 O 3 -Na 3 AlF 6 system. • The microstructure evolution of various Al-O-F species (AlF x 3-x, Al 2 OF y 4-y, and Al 2 O 2 F z 2-z) were quantitatively studied. • The correlation between microstructure and viscosity was investigated. • DFT was applied to simulate the spectra of solid crystalline α-Al 2 O 3 and Na 3 AlF 6. The microstructure of the molten alumina-cryolite system is an important aspect in the dissolution behavior of alumina (α-Al 2 O 3) in cryolite (Na 3 AlF 6) molten salt and impacts the physical properties of the molten system. In this work, the symmetric stretching vibrational wavenumbers of aluminum nonbridging fluorine (AlF x 3-x) and aluminum-oxygen-fluorine (Al 2 OF y 4-y, and Al 2 O 2 F z 2-z) bonds in the high wavenumber range, as well as their corresponding Raman scattering cross sections (RSCS), were first analyzed and determined by quantum chemistry (QC) ab initio calculations. The species in binary Al 2 O 3 -Na 3 AlF 6 melts with varying Al 2 O 3 contents from 0 to 6.0 wt% were then qualitatively and quantitatively analyzed using in situ high-temperature Raman spectroscopy in conjunction with first principles calculation from room temperature to melt. The species of AlF 4 -, AlF 5 2- and AlF 6 3- were found to be present and predominant in the melts with Al 2 O 3 contents ranging from 0 to 6.0 wt%. The content of Al 2 OF 6 2- species tended to increase with Al 2 O 3 content from 0 to 3.0 wt%, while Al 2 O 2 F 4 2- species only existed in the melts with an Al 2 O 3 concentration of more than 3.0 wt%. Finally, the abundance of various species in the molten Al 2 O 3 -Na 3 AlF 6 system was further correlated to the melt viscosity. The contribution to viscosity was primarily determined by the distribution of AlF x 3-x (x = 4, 5, and 6) and Al 2 OF 6 2- species, and the contribution ability of corresponding species to viscosity was Al 2 OF 6 2-, AlF 6 3-, AlF 5 2- and AlF 4 - from largest to smallest. [ABSTRACT FROM AUTHOR]