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

The local structure of molten LaF3-LiF-Li2O has been investigated by high temperature NMR spectroscopy. The (139)La and (19)F 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 (19)F 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 (139)La and (19)F chemical shifts suggests that several kinds of lanthanum oxyfluoride long-lived LaOxFy(3-x-y) units are present in the melt.