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

The solubility of Nb(V) as a function of O2- composition in the ternary eut ectic LiF-NaF-KF (FLiNaK) melt at 700 degreesC has been determined. Raman s pectra at different oxide concentrations and temperatures were recorded. Ba sed on chemical analysis of melt samples and bands in the spectra suggestio ns related to the Nb-O-F complex formation are given. The addition of Na2O to FLiNaK containing 0.22 mole kg(-1) K2NbF7, show that at: n(O)/n(Nb) <2; there is no precipitation of Nb(V) compounds. The Raman spectra of the melt indicated the presence of monomeric NbF72- at low oxide concentrations, wh ich reacted with oxide to form monomeric NbOF52- when r was increasing. Fur ther additions of Na2O formed the NbO2F43- species having a maximum concent ration at n(O)/n(Nb) =2; when 2 <n(O)/n(Nb)<3, a solid of the type AlkNbO(3 ) was formed. At n(O)/n(Nb) =3, a minimum in the Nb(V) and O2- solubilities were observed, but the structure of the species in the melt were uncertain . At higher oxide content, 3 <n(O)/n(Nb)<4, the AlkNbO(3)(s) previously for med was dissolved, possibly with the formation of both corner and edge shar ing distorted NbO6 octahedra. The solubility of Nb2O5 in FLiNaK was measure d as a function of temperature (550-800 degreesC), and the dissolution mech anism seems to be given by the reaction: Alk(+) + Nb2O5(s) + 4F =NbO2F43- AlkNbO(3)(s). DeltaH degrees (sol) for the reaction (based on concentratio ns in mole kg(-1)) was found to be approximate to 90 kJ mol(-1) K-1. (C) 20 01 Elsevier Science B.V. All rights reserved.