Structure of MF-AlF₃-ZrO₂(M = K, Na, Li) ionic melts

Journal of Molecular Liquids, 2020, Volume 299, № 112241

A.S. Vorob'ev, A.V. Suzdaltsev, P.S. Pershin, A.E. Galashev, Yu.P. Zaikov

Abstract — Quantum-mechanical calculations by the DFT method and a physical experiment were performed to clarify the type of complexes and their role when using ZrO2 as a source of zirconium to producing Al-Zr master alloys. The binding energy of complex anions formed from the components Zr-F, Al-O-F and Zr-O-F, whose formation is associated with the dissolution of ZrO₂ and Al₂O₃ oxides in fluoride melts MF-AlF₃ (M = K, Na, Li) was calculated using the program Siesta. The influence of the elemental composition of anions and cation from the second coordination sphere on the binding energy of complex anions is determined. It is shown that filling of the second coordination sphere with Na⁺ and K⁺ cations shifts the stability from the [Zr₂O₂F₆]²⁻ to [Zr₂O₂F₇]³⁻ complex compound. Replacement of the cation in the second coordination sphere in the row from K to Li leads to lower bond energy in all considered anions. Replacement of aluminium in [ZrF_x]^z- and [Zr₂O₂F_x]^z- complex groups leads to increasing of ion's size.

The available experimental data on the interaction of ZrO₂ with MF-AlF₃ (M = K, Na, Li) fluoride melts are presented and some new results have been obtained via Raman spectroscopy and XRD.

Based on the comparative analysis of calculations, available and new experimental data concerning interactions in MF-AlF₃-ZrO₂ (M = K, Na, Li) systems it is shown that ZrO₂ dissolution in the MF-AlF₃ melts significantly depends on the radius of the cation of the second coordination sphere. Thus, the ZrO₂ dissolution in the KF-AlF₃ melt takes place with the formation of the K₂ZrF₆ and Al₂O₃ compounds, while in the NaF-AlF₃-ZrO₂ system zirconium complexes and Al₂O₃ appears.

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