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

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 Na2O(s) was added t o melts containing a given amount of K2TaF7, and samples were withdrawn at different ratios of oxide to tantalum.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 ce ll technique. The solubility was studied at 700 degreesC. Solubility measur ements 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 AlkTaO(3), which may be a compound with Alk=Li, Na or K, was observed at higher ratios. A solubility minimum was o bserved 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 dioxof luoro complex, probably TaO2F43- (C-2v), was also present. At n(O)/n(Ta)>3 the solubility data were consistent with a dissolution of the AlkTaO(3) sol id, and formation of a TaO4Fx(x+3)- species. The tantalum tetraoxofluoride complex was possibly TaO4F25- (C-2v). The solubility data indicated precipi tation 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.