Solubility and Raman spectra of Ta(V) in LiF–NaF–KF–Na2O melts

Abstract

Raman spectroscopy and solubility measurements have been used to study Ta(V) complexes in the molten LiF–NaF–KF eutectic (FLiNaK). Na₂O(s) was added to melts containing a given amount of K₂TaF₇, and samples were withdrawn at different ratios of oxide to tantalum. The samples were analysed for oxygen and Ta, and studied by Raman spectroscopy using the windowless graphite cell technique. The solubility was studied at 700 °C. Solubility measurements in a FLiNaK melt containing 0.0956 mol kg⁻¹ K₂TaF₇ indicated that all the added amounts of oxide and tantalum dissolved in melts with n_O/n_Ta < 1.8, and precipitation of solid AlkTaO₃, which may be a compound with Alk = Li, Na or K, was observed at higher ratios. A solubility minimum was observed at n_O/n_Ta = 3. From the Raman data, the existence of the TaF₇²⁻ (C_s) and TaOF₅²⁻ (C_4v) species at ratios up to 2 was established. A dioxofluoro complex, probably TaO₂F₄³⁻ (C_2v), was also present. At n_O/n_Ta > 3 the solubility data were consistent with a dissolution of the AlkTaO₃ solid, and formation of a TaO₄F_x^(x + 3)− species. The tantalum tetraoxofluoride complex was possibly TaO₄F₂⁵⁻ (C_2v). The solubility data indicated precipitation of an oxide rich solid at n_O/n_Ta > 4. The Raman spectra supported the existence of TaO₄F₂⁵⁻ (C_2v) species as well as (TaO₅)_n or (TaO₆)_n network structures in these melts.