Morphology of alumina in NaF-AlF3 systems determined by Raman spectroscopy and quantum mechanical calculations

Abstract

In-situ Raman spectroscopy and quantum mechanical calculations were used to study the forms of oxygen-containing complexes in molten NaF-AlF3 systems. The molar ratios of NaF to AlF3 were varied at 1.22, 1.9, and 2.7 at temperatures of 1023 K, 1223 K, and 1283 K, respectively. Upon adding alumina to molten NaF-AlF3 systems, oxygen-containing complexes, Al2OF4, Al3O2F83−, Al2OF84−, and Al2O2F42−, were formed. The structures of Al2OF4 and Na4Al2OF8 belong to the D2d and C2v point groups, respectively, while Na3Al3O2F8 and Na2Al2O2F4 structures belong to the C1 point group. The main bands in the experimental Raman spectra of Al2OF4, Al3O2F83−, Al2OF84−, and Al2O2F42− were located at 460 cm−1, 530 cm−1, 505 cm−1, and 400 cm−1, respectively. The types and contents of the oxygen-containing complex ions in the system were related to the NaF-to-AlF3 molar ratio and the concentration of alumina. In the molten NaF-AlF3-Al2O3 system, with a NaF-to-AlF3 molar ratio of 1.22, only Al2OF4 formed. When the molar ratio was increased to 1.9, the oxygen-containing entities generated were Al2OF4 and Al3O2F83−. The content of the two oxygen-containing entities was low when the alumina content was below 6 wt%. As the concentration of alumina was 6 wt% or more, Al3O2F83− gradually became the main oxygen-containing entity. For the system with a NaF-to-AlF3 molar ratio of 2.7, when the alumina concentration was less than 6 wt%, the main oxygen-containing entities were Al2OF4, Al3O2F83−, and Al2OF84−, but with low concentrations. The concentrations of Al3O2F83− and Al2OF84− ions increased considerably as the alumina content approached 6 wt%. Al2O2F42− was formed when the alumina concentration reached 10 wt%. The reaction equations occurring after adding alumina to molten NaF-AlF3 systems were also determined herein.