An in situ spectroscopic study of the local structure of oxyfluoride melts: NMR insights into the speciation in molten LiF–LaF3–Li2O systems

Abstract

The local structure of molten LaF₃–LiF–Li₂O has been investigated by high temperature NMR spectroscopy. The ¹³⁹La and ¹⁹F chemical shifts have been measured as a function of temperature and composition. The NMR spectra show that Li₂O reacts completely with LaF₃ to form a LaOF compound in the solid state below the melting temperature of the sample. LaOF is not completely dissolved in the fluoride melt and solid LaOF is observed in the ¹⁹F spectra for Li₂O concentrations above 10 mol%. We discuss the local environment of lanthanum ions in molten LaF₃–LiF–Li₂O and compare the results to those with the LaF₃–LiF–CaO system. The analysis of the temperature and Li₂O concentration dependences of the ¹³⁹La and ¹⁹F chemical shifts suggests that several kinds of lanthanum oxyfluoride long-lived LaO_xF_y^3−x−y units are present in the melt.