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

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-AIF(3) (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 [ZrFx](z-) and [Zr2O2Fx](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, white in the NaF-AlF3-ZrO2 system zirconium complexes and Al2O3 appears. (C) 2019 Elsevier B.V. All rights reserved.